From 0f6d1beccf6cffee75acef741329bb80831b69f6 Mon Sep 17 00:00:00 2001
From: Megan Marsh <megan@hashicorp.com>
Date: Mon, 16 Dec 2019 21:23:05 -0800
Subject: [PATCH] add an extra string array to the Prepare() return values in
 the builder interfaces; this sets up the ability for builders to give the
 provisioners custom user-accessible build-time variables.

---
 builder/alicloud/ecs/builder.go               |  8 +--
 builder/alicloud/ecs/builder_test.go          | 22 +++---
 builder/amazon/chroot/builder.go              |  8 +--
 builder/amazon/chroot/builder_test.go         | 26 +++----
 builder/amazon/ebs/builder.go                 |  8 +--
 builder/amazon/ebs/builder_test.go            | 16 ++---
 builder/amazon/ebssurrogate/builder.go        |  8 +--
 builder/amazon/ebssurrogate/builder_test.go   |  4 +-
 builder/amazon/ebsvolume/builder.go           |  8 +--
 builder/amazon/ebsvolume/builder_test.go      | 10 +--
 builder/amazon/instance/builder.go            |  8 +--
 builder/amazon/instance/builder_test.go       | 36 +++++-----
 builder/azure/arm/builder.go                  |  6 +-
 builder/azure/arm/builder_test.go             |  2 +-
 builder/azure/chroot/builder.go               | 10 +--
 builder/azure/chroot/builder_test.go          |  2 +-
 builder/cloudstack/builder.go                 |  6 +-
 builder/cloudstack/builder_test.go            |  2 +-
 builder/digitalocean/builder.go               |  6 +-
 builder/digitalocean/builder_test.go          | 46 ++++++-------
 builder/docker/builder.go                     |  6 +-
 builder/docker/communicator_test.go           |  6 +-
 builder/file/builder.go                       |  6 +-
 builder/googlecompute/builder.go              |  6 +-
 builder/hcloud/builder.go                     |  6 +-
 builder/hyperone/builder.go                   |  6 +-
 builder/hyperv/iso/builder.go                 |  8 +--
 builder/hyperv/iso/builder_test.go            | 64 ++++++++---------
 builder/hyperv/vmcx/builder.go                |  8 +--
 builder/hyperv/vmcx/builder_test.go           | 42 ++++++------
 builder/jdcloud/builder.go                    |  8 +--
 builder/linode/builder.go                     |  6 +-
 builder/linode/builder_test.go                | 30 ++++----
 builder/lxc/builder.go                        |  6 +-
 builder/lxc/builder_test.go                   |  4 +-
 builder/lxd/builder.go                        |  6 +-
 builder/lxd/builder_test.go                   |  8 +--
 builder/ncloud/builder.go                     |  6 +-
 builder/null/builder.go                       |  6 +-
 builder/oneandone/builder.go                  |  6 +-
 builder/oneandone/builder_test.go             |  4 +-
 builder/openstack/builder.go                  | 10 +--
 builder/openstack/builder_test.go             |  2 +-
 builder/oracle/classic/builder.go             |  8 +--
 builder/oracle/oci/builder.go                 |  6 +-
 builder/osc/bsu/builder.go                    |  8 +--
 builder/osc/bsu/builder_test.go               | 16 ++---
 builder/osc/bsusurrogate/builder.go           |  8 +--
 builder/osc/bsuvolume/builder.go              |  8 +--
 builder/osc/bsuvolume/builder_test.go         | 10 +--
 builder/osc/chroot/builder.go                 |  8 +--
 builder/parallels/iso/builder.go              |  8 +--
 builder/parallels/iso/builder_test.go         | 28 ++++----
 builder/parallels/pvm/builder.go              |  6 +-
 builder/profitbricks/builder.go               |  6 +-
 builder/profitbricks/builder_test.go          |  4 +-
 builder/proxmox/builder.go                    |  6 +-
 builder/proxmox/config_test.go                |  4 +-
 builder/qemu/builder.go                       |  8 +--
 builder/qemu/builder_test.go                  | 68 +++++++++----------
 builder/scaleway/builder.go                   |  6 +-
 builder/scaleway/builder_test.go              | 30 ++++----
 builder/tencentcloud/cvm/builder.go           |  8 +--
 builder/triton/builder.go                     |  4 +-
 builder/ucloud/uhost/builder.go               |  8 +--
 builder/ucloud/uhost/builder_test.go          | 12 ++--
 builder/vagrant/builder.go                    |  8 +--
 builder/vagrant/builder_test.go               |  2 +-
 builder/virtualbox/iso/builder.go             |  8 +--
 builder/virtualbox/iso/builder_test.go        | 44 ++++++------
 builder/virtualbox/ovf/builder.go             |  6 +-
 builder/virtualbox/vm/builder.go              |  6 +-
 builder/vmware/iso/builder.go                 |  6 +-
 builder/vmware/iso/builder_test.go            | 60 ++++++++--------
 builder/vmware/vmx/builder.go                 |  6 +-
 builder/vmware/vmx/builder_test.go            |  6 +-
 builder/yandex/builder.go                     |  6 +-
 command/build_parallel_test.go                |  6 +-
 common/shell-local/config.go                  |  8 +--
 helper/config/decode.go                       | 42 +++++++++++-
 packer/build.go                               | 18 ++++-
 packer/build_test.go                          |  4 +-
 packer/builder.go                             |  7 +-
 packer/builder_mock.go                        |  4 +-
 packer/plugin/builder.go                      |  2 +-
 packer/provisioner.go                         | 44 ++++++------
 packer/rpc/builder.go                         | 18 ++---
 packer/rpc/builder_test.go                    |  4 +-
 .../checksum/post-processor_test.go           |  2 +-
 .../compress/post-processor_test.go           |  2 +-
 provisioner/ansible-local/provisioner_test.go |  2 +-
 provisioner/ansible/provisioner.go            |  3 -
 template/interpolate/funcs.go                 | 21 +++---
 93 files changed, 592 insertions(+), 533 deletions(-)

diff --git a/builder/alicloud/ecs/builder.go b/builder/alicloud/ecs/builder.go
index 5b593cee9..44eab7fbd 100644
--- a/builder/alicloud/ecs/builder.go
+++ b/builder/alicloud/ecs/builder.go
@@ -41,7 +41,7 @@ const (
 	ALICLOUD_DEFAULT_LONG_TIMEOUT  = 3600
 )
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &b.config.ctx,
@@ -53,7 +53,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}, raws...)
 	b.config.ctx.EnableEnv = true
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	if b.config.PackerConfig.PackerForce {
@@ -68,11 +68,11 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	errs = packer.MultiErrorAppend(errs, b.config.RunConfig.Prepare(&b.config.ctx)...)
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return nil, errs
+		return nil, nil, errs
 	}
 
 	packer.LogSecretFilter.Set(b.config.AlicloudAccessKey, b.config.AlicloudSecretKey)
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/alicloud/ecs/builder_test.go b/builder/alicloud/ecs/builder_test.go
index e67861f5d..9a691de77 100644
--- a/builder/alicloud/ecs/builder_test.go
+++ b/builder/alicloud/ecs/builder_test.go
@@ -35,7 +35,7 @@ func TestBuilder_Prepare_BadType(t *testing.T) {
 		"access_key": []string{},
 	}
 
-	warnings, err := b.Prepare(c)
+	_, warnings, err := b.Prepare(c)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -50,7 +50,7 @@ func TestBuilderPrepare_ECSImageName(t *testing.T) {
 
 	// Test good
 	config["image_name"] = "ecs.n1.tiny"
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -61,7 +61,7 @@ func TestBuilderPrepare_ECSImageName(t *testing.T) {
 	// Test bad
 	config["ecs_image_name"] = "foo {{"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -72,7 +72,7 @@ func TestBuilderPrepare_ECSImageName(t *testing.T) {
 	// Test bad
 	delete(config, "image_name")
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -87,7 +87,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -120,7 +120,7 @@ func TestBuilderPrepare_Devices(t *testing.T) {
 			"disk_device": "/dev/xvdc",
 		},
 	}
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -160,7 +160,7 @@ func TestBuilderPrepare_IgnoreDataDisks(t *testing.T) {
 	var b Builder
 	config := testBuilderConfig()
 
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -173,7 +173,7 @@ func TestBuilderPrepare_IgnoreDataDisks(t *testing.T) {
 	}
 
 	config["image_ignore_data_disks"] = "false"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -186,7 +186,7 @@ func TestBuilderPrepare_IgnoreDataDisks(t *testing.T) {
 	}
 
 	config["image_ignore_data_disks"] = "true"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -203,7 +203,7 @@ func TestBuilderPrepare_WaitSnapshotReadyTimeout(t *testing.T) {
 	var b Builder
 	config := testBuilderConfig()
 
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -219,7 +219,7 @@ func TestBuilderPrepare_WaitSnapshotReadyTimeout(t *testing.T) {
 	}
 
 	config["wait_snapshot_ready_timeout"] = ALICLOUD_DEFAULT_TIMEOUT
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/amazon/chroot/builder.go b/builder/amazon/chroot/builder.go
index f1cf7432a..2a0331265 100644
--- a/builder/amazon/chroot/builder.go
+++ b/builder/amazon/chroot/builder.go
@@ -182,7 +182,7 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	b.config.ctx.Funcs = awscommon.TemplateFuncs
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
@@ -201,7 +201,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	if b.config.Architecture == "" {
@@ -319,11 +319,11 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return warns, errs
+		return nil, warns, errs
 	}
 
 	packer.LogSecretFilter.Set(b.config.AccessKey, b.config.SecretKey, b.config.Token)
-	return warns, nil
+	return nil, warns, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/amazon/chroot/builder_test.go b/builder/amazon/chroot/builder_test.go
index e6999bff6..b181b878f 100644
--- a/builder/amazon/chroot/builder_test.go
+++ b/builder/amazon/chroot/builder_test.go
@@ -31,7 +31,7 @@ func TestBuilderPrepare_AMIName(t *testing.T) {
 	// Test good
 	config["ami_name"] = "foo"
 	config["skip_region_validation"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -42,7 +42,7 @@ func TestBuilderPrepare_AMIName(t *testing.T) {
 	// Test bad
 	config["ami_name"] = "foo {{"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -53,7 +53,7 @@ func TestBuilderPrepare_AMIName(t *testing.T) {
 	// Test bad
 	delete(config, "ami_name")
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -67,7 +67,7 @@ func TestBuilderPrepare_ChrootMounts(t *testing.T) {
 	config := testConfig()
 
 	config["chroot_mounts"] = nil
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -83,7 +83,7 @@ func TestBuilderPrepare_ChrootMountsBadDefaults(t *testing.T) {
 	config["chroot_mounts"] = [][]string{
 		{"bad"},
 	}
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -96,7 +96,7 @@ func TestBuilderPrepare_SourceAmi(t *testing.T) {
 	config := testConfig()
 
 	config["source_ami"] = ""
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -105,7 +105,7 @@ func TestBuilderPrepare_SourceAmi(t *testing.T) {
 	}
 
 	config["source_ami"] = "foo"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -119,7 +119,7 @@ func TestBuilderPrepare_CommandWrapper(t *testing.T) {
 	config := testConfig()
 
 	config["command_wrapper"] = "echo hi; {{.Command}}"
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -132,7 +132,7 @@ func TestBuilderPrepare_CopyFiles(t *testing.T) {
 	b := &Builder{}
 	config := testConfig()
 
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -150,7 +150,7 @@ func TestBuilderPrepare_CopyFilesNoDefault(t *testing.T) {
 	config := testConfig()
 
 	config["copy_files"] = []string{}
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -171,7 +171,7 @@ func TestBuilderPrepare_RootDeviceNameAndAMIMappings(t *testing.T) {
 	config["root_device_name"] = "/dev/sda"
 	config["ami_block_device_mappings"] = []interface{}{map[string]string{}}
 	config["root_volume_size"] = 15
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) == 0 {
 		t.Fatal("Missing warning, stating block device mappings will be overwritten")
 	} else if len(warnings) > 1 {
@@ -187,7 +187,7 @@ func TestBuilderPrepare_AMIMappingsNoRootDeviceName(t *testing.T) {
 	config := testConfig()
 
 	config["ami_block_device_mappings"] = []interface{}{map[string]string{}}
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -201,7 +201,7 @@ func TestBuilderPrepare_RootDeviceNameNoAMIMappings(t *testing.T) {
 	config := testConfig()
 
 	config["root_device_name"] = "/dev/sda"
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/amazon/ebs/builder.go b/builder/amazon/ebs/builder.go
index f8648e204..08d93119c 100644
--- a/builder/amazon/ebs/builder.go
+++ b/builder/amazon/ebs/builder.go
@@ -72,7 +72,7 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	b.config.ctx.Funcs = awscommon.TemplateFuncs
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
@@ -89,7 +89,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	if b.config.PackerConfig.PackerForce {
@@ -123,11 +123,11 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return warns, errs
+		return nil, warns, errs
 	}
 
 	packer.LogSecretFilter.Set(b.config.AccessKey, b.config.SecretKey, b.config.Token)
-	return warns, nil
+	return nil, warns, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/amazon/ebs/builder_test.go b/builder/amazon/ebs/builder_test.go
index a8200b5a8..4ea816130 100644
--- a/builder/amazon/ebs/builder_test.go
+++ b/builder/amazon/ebs/builder_test.go
@@ -32,7 +32,7 @@ func TestBuilder_Prepare_BadType(t *testing.T) {
 		"access_key": []string{},
 	}
 
-	warnings, err := b.Prepare(c)
+	_, warnings, err := b.Prepare(c)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -48,7 +48,7 @@ func TestBuilderPrepare_AMIName(t *testing.T) {
 	// Test good
 	config["ami_name"] = "foo"
 	config["skip_region_validation"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -59,7 +59,7 @@ func TestBuilderPrepare_AMIName(t *testing.T) {
 	// Test bad
 	config["ami_name"] = "foo {{"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -70,7 +70,7 @@ func TestBuilderPrepare_AMIName(t *testing.T) {
 	// Test bad
 	delete(config, "ami_name")
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -85,7 +85,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -101,7 +101,7 @@ func TestBuilderPrepare_InvalidShutdownBehavior(t *testing.T) {
 	// Test good
 	config["shutdown_behavior"] = "terminate"
 	config["skip_region_validation"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -111,7 +111,7 @@ func TestBuilderPrepare_InvalidShutdownBehavior(t *testing.T) {
 
 	// Test good
 	config["shutdown_behavior"] = "stop"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -121,7 +121,7 @@ func TestBuilderPrepare_InvalidShutdownBehavior(t *testing.T) {
 
 	// Test bad
 	config["shutdown_behavior"] = "foobar"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/amazon/ebssurrogate/builder.go b/builder/amazon/ebssurrogate/builder.go
index abf159fad..835d86cdc 100644
--- a/builder/amazon/ebssurrogate/builder.go
+++ b/builder/amazon/ebssurrogate/builder.go
@@ -70,7 +70,7 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	b.config.ctx.Funcs = awscommon.TemplateFuncs
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
@@ -87,7 +87,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	if b.config.PackerConfig.PackerForce {
@@ -145,12 +145,12 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		errs = packer.MultiErrorAppend(errs, errors.New(`The only valid ami_architecture values are "x86_64" and "arm64"`))
 	}
 	if errs != nil && len(errs.Errors) > 0 {
-		return warns, errs
+		return nil, warns, errs
 	}
 
 	packer.LogSecretFilter.Set(b.config.AccessKey, b.config.SecretKey, b.config.Token)
 
-	return warns, nil
+	return nil, warns, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/amazon/ebssurrogate/builder_test.go b/builder/amazon/ebssurrogate/builder_test.go
index 94fdacd6b..37490bc61 100644
--- a/builder/amazon/ebssurrogate/builder_test.go
+++ b/builder/amazon/ebssurrogate/builder_test.go
@@ -31,7 +31,7 @@ func TestBuilder_Prepare_BadType(t *testing.T) {
 		"access_key": []string{},
 	}
 
-	warnings, err := b.Prepare(c)
+	_, warnings, err := b.Prepare(c)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -46,7 +46,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/amazon/ebsvolume/builder.go b/builder/amazon/ebsvolume/builder.go
index b3d05299d..fc9305983 100644
--- a/builder/amazon/ebsvolume/builder.go
+++ b/builder/amazon/ebsvolume/builder.go
@@ -79,7 +79,7 @@ type EngineVarsTemplate struct {
 	SourceAMI   string
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	b.config.ctx.Funcs = awscommon.TemplateFuncs
 	// Create passthrough for {{ .BuildRegion }} and {{ .SourceAMI }} variables
 	// so we can fill them in later
@@ -92,7 +92,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		InterpolateContext: &b.config.ctx,
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	// Accumulate any errors
@@ -129,11 +129,11 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return warns, errs
+		return nil, warns, errs
 	}
 
 	packer.LogSecretFilter.Set(b.config.AccessKey, b.config.SecretKey, b.config.Token)
-	return warns, nil
+	return nil, warns, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/amazon/ebsvolume/builder_test.go b/builder/amazon/ebsvolume/builder_test.go
index 85c8ae833..0d8b27ac1 100644
--- a/builder/amazon/ebsvolume/builder_test.go
+++ b/builder/amazon/ebsvolume/builder_test.go
@@ -31,7 +31,7 @@ func TestBuilder_Prepare_BadType(t *testing.T) {
 		"access_key": []string{},
 	}
 
-	warnings, err := b.Prepare(c)
+	_, warnings, err := b.Prepare(c)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -46,7 +46,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -62,7 +62,7 @@ func TestBuilderPrepare_InvalidShutdownBehavior(t *testing.T) {
 	// Test good
 	config["shutdown_behavior"] = "terminate"
 	config["skip_region_validation"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -72,7 +72,7 @@ func TestBuilderPrepare_InvalidShutdownBehavior(t *testing.T) {
 
 	// Test good
 	config["shutdown_behavior"] = "stop"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -82,7 +82,7 @@ func TestBuilderPrepare_InvalidShutdownBehavior(t *testing.T) {
 
 	// Test bad
 	config["shutdown_behavior"] = "foobar"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/amazon/instance/builder.go b/builder/amazon/instance/builder.go
index 92a761459..79a688ec4 100644
--- a/builder/amazon/instance/builder.go
+++ b/builder/amazon/instance/builder.go
@@ -93,7 +93,7 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	configs := make([]interface{}, len(raws)+1)
 	configs[0] = map[string]interface{}{
 		"bundle_prefix": "image-{{timestamp}}",
@@ -118,7 +118,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, configs...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	if b.config.PackerConfig.PackerForce {
@@ -218,10 +218,10 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return warns, errs
+		return nil, warns, errs
 	}
 	packer.LogSecretFilter.Set(b.config.AccessKey, b.config.SecretKey, b.config.Token)
-	return warns, nil
+	return nil, warns, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/amazon/instance/builder_test.go b/builder/amazon/instance/builder_test.go
index 0c2a05145..21d4d4030 100644
--- a/builder/amazon/instance/builder_test.go
+++ b/builder/amazon/instance/builder_test.go
@@ -47,7 +47,7 @@ func TestBuilderPrepare_AccountId(t *testing.T) {
 	defer tempfile.Close()
 
 	config["account_id"] = ""
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -56,7 +56,7 @@ func TestBuilderPrepare_AccountId(t *testing.T) {
 	}
 
 	config["account_id"] = "foo"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -65,7 +65,7 @@ func TestBuilderPrepare_AccountId(t *testing.T) {
 	}
 
 	config["account_id"] = "0123-0456-7890"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -87,7 +87,7 @@ func TestBuilderPrepare_AMIName(t *testing.T) {
 	// Test good
 	config["ami_name"] = "foo"
 	config["skip_region_validation"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -98,7 +98,7 @@ func TestBuilderPrepare_AMIName(t *testing.T) {
 	// Test bad
 	config["ami_name"] = "foo {{"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -109,7 +109,7 @@ func TestBuilderPrepare_AMIName(t *testing.T) {
 	// Test bad
 	delete(config, "ami_name")
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -126,7 +126,7 @@ func TestBuilderPrepare_BundleDestination(t *testing.T) {
 	defer tempfile.Close()
 
 	config["bundle_destination"] = ""
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -146,7 +146,7 @@ func TestBuilderPrepare_BundlePrefix(t *testing.T) {
 	defer os.Remove(tempfile.Name())
 	defer tempfile.Close()
 
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -167,7 +167,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -184,7 +184,7 @@ func TestBuilderPrepare_S3Bucket(t *testing.T) {
 	defer tempfile.Close()
 
 	config["s3_bucket"] = ""
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -193,7 +193,7 @@ func TestBuilderPrepare_S3Bucket(t *testing.T) {
 	}
 
 	config["s3_bucket"] = "foo"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -210,7 +210,7 @@ func TestBuilderPrepare_X509CertPath(t *testing.T) {
 	defer tempfile.Close()
 
 	config["x509_cert_path"] = ""
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -219,7 +219,7 @@ func TestBuilderPrepare_X509CertPath(t *testing.T) {
 	}
 
 	config["x509_cert_path"] = "i/am/a/file/that/doesnt/exist"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -235,7 +235,7 @@ func TestBuilderPrepare_X509CertPath(t *testing.T) {
 	defer tf.Close()
 
 	config["x509_cert_path"] = tf.Name()
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -252,7 +252,7 @@ func TestBuilderPrepare_X509KeyPath(t *testing.T) {
 	defer tempfile.Close()
 
 	config["x509_key_path"] = ""
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -261,7 +261,7 @@ func TestBuilderPrepare_X509KeyPath(t *testing.T) {
 	}
 
 	config["x509_key_path"] = "i/am/a/file/that/doesnt/exist"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -277,7 +277,7 @@ func TestBuilderPrepare_X509KeyPath(t *testing.T) {
 	defer tf.Close()
 
 	config["x509_key_path"] = tf.Name()
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -294,7 +294,7 @@ func TestBuilderPrepare_X509UploadPath(t *testing.T) {
 	defer tempfile.Close()
 
 	config["x509_upload_path"] = ""
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/azure/arm/builder.go b/builder/azure/arm/builder.go
index 0a087c6fd..f1e6259c9 100644
--- a/builder/azure/arm/builder.go
+++ b/builder/azure/arm/builder.go
@@ -34,10 +34,10 @@ const (
 	DefaultSecretName       = "packerKeyVaultSecret"
 )
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := newConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 
 	b.config = c
@@ -47,7 +47,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	b.setTemplateParameters(b.stateBag)
 	b.setImageParameters(b.stateBag)
 
-	return warnings, errs
+	return nil, warnings, errs
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/azure/arm/builder_test.go b/builder/azure/arm/builder_test.go
index a71f92874..e9b781259 100644
--- a/builder/azure/arm/builder_test.go
+++ b/builder/azure/arm/builder_test.go
@@ -8,7 +8,7 @@ import (
 
 func TestStateBagShouldBePopulatedExpectedValues(t *testing.T) {
 	var testSubject = &Builder{}
-	_, err := testSubject.Prepare(getArmBuilderConfiguration(), getPackerConfiguration())
+	_, _, err := testSubject.Prepare(getArmBuilderConfiguration(), getPackerConfiguration())
 	if err != nil {
 		t.Fatalf("failed to prepare: %s", err)
 	}
diff --git a/builder/azure/chroot/builder.go b/builder/azure/chroot/builder.go
index 91ceca991..c5fbefc9d 100644
--- a/builder/azure/chroot/builder.go
+++ b/builder/azure/chroot/builder.go
@@ -116,7 +116,7 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	b.config.ctx.Funcs = azcommon.TemplateFuncs
 	b.config.ctx.Funcs["vm"] = CreateVMMetadataTemplateFunc()
 	err := config.Decode(&b.config, &config.DecodeOpts{
@@ -134,7 +134,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	var errs *packer.MultiError
@@ -143,7 +143,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	// Defaults
 	err = b.config.ClientConfig.SetDefaultValues()
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	if b.config.ChrootMounts == nil {
@@ -254,11 +254,11 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil {
-		return warns, errs
+		return nil, warns, errs
 	}
 
 	packer.LogSecretFilter.Set(b.config.ClientConfig.ClientSecret, b.config.ClientConfig.ClientJWT)
-	return warns, nil
+	return nil, warns, nil
 }
 
 func checkDiskCacheType(s string) interface{} {
diff --git a/builder/azure/chroot/builder_test.go b/builder/azure/chroot/builder_test.go
index dd40f7737..f84dd9ccc 100644
--- a/builder/azure/chroot/builder_test.go
+++ b/builder/azure/chroot/builder_test.go
@@ -55,7 +55,7 @@ func TestBuilder_Prepare(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			b := &Builder{}
 
-			_, err := b.Prepare(tt.config)
+			_, _, err := b.Prepare(tt.config)
 
 			if (err != nil) != tt.wantErr {
 				t.Errorf("Builder.Prepare() error = %v, wantErr %v", err, tt.wantErr)
diff --git a/builder/cloudstack/builder.go b/builder/cloudstack/builder.go
index dc1dace69..138943956 100644
--- a/builder/cloudstack/builder.go
+++ b/builder/cloudstack/builder.go
@@ -21,14 +21,14 @@ type Builder struct {
 }
 
 // Prepare implements the packer.Builder interface.
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	config, errs := NewConfig(raws...)
 	if errs != nil {
-		return nil, errs
+		return nil, nil, errs
 	}
 	b.config = config
 
-	return nil, nil
+	return nil, nil, nil
 }
 
 // Run implements the packer.Builder interface.
diff --git a/builder/cloudstack/builder_test.go b/builder/cloudstack/builder_test.go
index 6d7ab1444..0b61dfd20 100644
--- a/builder/cloudstack/builder_test.go
+++ b/builder/cloudstack/builder_test.go
@@ -43,7 +43,7 @@ func TestBuilder_Prepare(t *testing.T) {
 	b := &Builder{}
 
 	for desc, tc := range cases {
-		_, errs := b.Prepare(tc.Config)
+		_, _, errs := b.Prepare(tc.Config)
 
 		if tc.Err {
 			if errs == nil {
diff --git a/builder/digitalocean/builder.go b/builder/digitalocean/builder.go
index 661e2c2a4..baaeffce1 100644
--- a/builder/digitalocean/builder.go
+++ b/builder/digitalocean/builder.go
@@ -25,14 +25,14 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = *c
 
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/digitalocean/builder_test.go b/builder/digitalocean/builder_test.go
index 70ec6fea9..5113495cf 100644
--- a/builder/digitalocean/builder_test.go
+++ b/builder/digitalocean/builder_test.go
@@ -32,7 +32,7 @@ func TestBuilder_Prepare_BadType(t *testing.T) {
 		"api_key": []string{},
 	}
 
-	warnings, err := b.Prepare(c)
+	_, warnings, err := b.Prepare(c)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -47,7 +47,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -62,7 +62,7 @@ func TestBuilderPrepare_Region(t *testing.T) {
 
 	// Test default
 	delete(config, "region")
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -75,7 +75,7 @@ func TestBuilderPrepare_Region(t *testing.T) {
 	// Test set
 	config["region"] = expected
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -94,7 +94,7 @@ func TestBuilderPrepare_Size(t *testing.T) {
 
 	// Test default
 	delete(config, "size")
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -107,7 +107,7 @@ func TestBuilderPrepare_Size(t *testing.T) {
 	// Test set
 	config["size"] = expected
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -126,7 +126,7 @@ func TestBuilderPrepare_Image(t *testing.T) {
 
 	// Test default
 	delete(config, "image")
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -139,7 +139,7 @@ func TestBuilderPrepare_Image(t *testing.T) {
 	// Test set
 	config["image"] = expected
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -157,7 +157,7 @@ func TestBuilderPrepare_StateTimeout(t *testing.T) {
 	config := testConfig()
 
 	// Test default
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -172,7 +172,7 @@ func TestBuilderPrepare_StateTimeout(t *testing.T) {
 	// Test set
 	config["state_timeout"] = "5m"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -183,7 +183,7 @@ func TestBuilderPrepare_StateTimeout(t *testing.T) {
 	// Test bad
 	config["state_timeout"] = "tubes"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -197,7 +197,7 @@ func TestBuilderPrepare_SnapshotTimeout(t *testing.T) {
 	config := testConfig()
 
 	// Test default
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -212,7 +212,7 @@ func TestBuilderPrepare_SnapshotTimeout(t *testing.T) {
 	// Test set
 	config["snapshot_timeout"] = "15m"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -223,7 +223,7 @@ func TestBuilderPrepare_SnapshotTimeout(t *testing.T) {
 	// Test bad
 	config["snapshot_timeout"] = "badstring"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -237,7 +237,7 @@ func TestBuilderPrepare_PrivateNetworking(t *testing.T) {
 	config := testConfig()
 
 	// Test default
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -252,7 +252,7 @@ func TestBuilderPrepare_PrivateNetworking(t *testing.T) {
 	// Test set
 	config["private_networking"] = true
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -270,7 +270,7 @@ func TestBuilderPrepare_SnapshotName(t *testing.T) {
 	config := testConfig()
 
 	// Test default
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -285,7 +285,7 @@ func TestBuilderPrepare_SnapshotName(t *testing.T) {
 	// Test set
 	config["snapshot_name"] = "foobarbaz"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -296,7 +296,7 @@ func TestBuilderPrepare_SnapshotName(t *testing.T) {
 	// Test set with template
 	config["snapshot_name"] = "{{timestamp}}"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -316,7 +316,7 @@ func TestBuilderPrepare_DropletName(t *testing.T) {
 	config := testConfig()
 
 	// Test default
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -331,7 +331,7 @@ func TestBuilderPrepare_DropletName(t *testing.T) {
 	// Test normal set
 	config["droplet_name"] = "foobar"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -342,7 +342,7 @@ func TestBuilderPrepare_DropletName(t *testing.T) {
 	// Test with template
 	config["droplet_name"] = "foobar-{{timestamp}}"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -353,7 +353,7 @@ func TestBuilderPrepare_DropletName(t *testing.T) {
 	// Test with bad template
 	config["droplet_name"] = "foobar-{{"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/docker/builder.go b/builder/docker/builder.go
index 63cb8f55d..f8b2bbf96 100644
--- a/builder/docker/builder.go
+++ b/builder/docker/builder.go
@@ -20,14 +20,14 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/docker/communicator_test.go b/builder/docker/communicator_test.go
index b028d367a..afaa5e60f 100644
--- a/builder/docker/communicator_test.go
+++ b/builder/docker/communicator_test.go
@@ -35,7 +35,7 @@ func TestUploadDownload(t *testing.T) {
 
 	// Setup the builder
 	builder := &Builder{}
-	warnings, err := builder.Prepare(tpl.Builders["docker"].Config)
+	_, warnings, err := builder.Prepare(tpl.Builders["docker"].Config)
 	if err != nil {
 		t.Fatalf("Error preparing configuration %s", err)
 	}
@@ -118,7 +118,7 @@ func TestLargeDownload(t *testing.T) {
 
 	// Setup the builder
 	builder := &Builder{}
-	warnings, err := builder.Prepare(tpl.Builders["docker"].Config)
+	_, warnings, err := builder.Prepare(tpl.Builders["docker"].Config)
 	if err != nil {
 		t.Fatalf("Error preparing configuration %s", err)
 	}
@@ -222,7 +222,7 @@ func TestFixUploadOwner(t *testing.T) {
 
 	// Setup the builder
 	builder := &Builder{}
-	warnings, err := builder.Prepare(tpl.Builders["docker"].Config)
+	_, warnings, err := builder.Prepare(tpl.Builders["docker"].Config)
 	if err != nil {
 		t.Fatalf("Error preparing configuration %s", err)
 	}
diff --git a/builder/file/builder.go b/builder/file/builder.go
index 0defa6049..a318e87eb 100644
--- a/builder/file/builder.go
+++ b/builder/file/builder.go
@@ -23,14 +23,14 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 // Run is where the actual build should take place. It takes a Build and a Ui.
diff --git a/builder/googlecompute/builder.go b/builder/googlecompute/builder.go
index 2a30d39c5..803970972 100644
--- a/builder/googlecompute/builder.go
+++ b/builder/googlecompute/builder.go
@@ -23,13 +23,13 @@ type Builder struct {
 }
 
 // Prepare processes the build configuration parameters.
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 // Run executes a googlecompute Packer build and returns a packer.Artifact
diff --git a/builder/hcloud/builder.go b/builder/hcloud/builder.go
index 92f9f474b..3514d74ba 100644
--- a/builder/hcloud/builder.go
+++ b/builder/hcloud/builder.go
@@ -22,13 +22,13 @@ type Builder struct {
 
 var pluginVersion = "1.0.0"
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	config, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = *config
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/hyperone/builder.go b/builder/hyperone/builder.go
index d9e85e5f0..fcaf50490 100644
--- a/builder/hyperone/builder.go
+++ b/builder/hyperone/builder.go
@@ -20,10 +20,10 @@ type Builder struct {
 	client *openapi.APIClient
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	config, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 
 	b.config = *config
@@ -43,7 +43,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 
 	b.client = openapi.NewAPIClient(cfg)
 
-	return nil, nil
+	return nil, nil, nil
 }
 
 type wrappedCommandTemplate struct {
diff --git a/builder/hyperv/iso/builder.go b/builder/hyperv/iso/builder.go
index 1b40fac84..bde46d478 100644
--- a/builder/hyperv/iso/builder.go
+++ b/builder/hyperv/iso/builder.go
@@ -85,7 +85,7 @@ type Config struct {
 }
 
 // Prepare processes the build configuration parameters.
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &b.config.ctx,
@@ -96,7 +96,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	// Accumulate any errors and warnings
@@ -164,10 +164,10 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 // Run executes a Packer build and returns a packer.Artifact representing
diff --git a/builder/hyperv/iso/builder_test.go b/builder/hyperv/iso/builder_test.go
index 640221de2..b3c4736f3 100644
--- a/builder/hyperv/iso/builder_test.go
+++ b/builder/hyperv/iso/builder_test.go
@@ -42,7 +42,7 @@ func TestBuilderPrepare_Defaults(t *testing.T) {
 	var b Builder
 	config := testConfig()
 
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -60,7 +60,7 @@ func TestBuilderPrepare_DiskSize(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "disk_size")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -74,7 +74,7 @@ func TestBuilderPrepare_DiskSize(t *testing.T) {
 
 	config["disk_size"] = 256
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -96,7 +96,7 @@ func TestBuilderPrepare_DiskBlockSize(t *testing.T) {
 
 	// Test default with empty disk_block_size
 	delete(config, "disk_block_size")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -112,7 +112,7 @@ func TestBuilderPrepare_DiskBlockSize(t *testing.T) {
 	for _, test_size := range test_sizes {
 		config["disk_block_size"] = test_size
 		b = Builder{}
-		warns, err = b.Prepare(config)
+		_, warns, err = b.Prepare(config)
 		if test_size > expected_max_block_size || test_size < expected_min_block_size {
 			if len(warns) > 0 {
 				t.Fatalf("bad, should have no warns: %#v", warns)
@@ -153,7 +153,7 @@ func TestBuilderPrepare_FixedVHDFormat(t *testing.T) {
 
 	// use_fixed_vhd_format should work with generation = 1, skip_compaction
 	// = true, and differencing_disk = false
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -164,7 +164,7 @@ func TestBuilderPrepare_FixedVHDFormat(t *testing.T) {
 	//use_fixed_vhd_format should not work with differencing_disk = true
 	config["differencing_disk"] = true
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -176,7 +176,7 @@ func TestBuilderPrepare_FixedVHDFormat(t *testing.T) {
 	//use_fixed_vhd_format should not work with skip_compaction = false
 	config["skip_compaction"] = false
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -188,7 +188,7 @@ func TestBuilderPrepare_FixedVHDFormat(t *testing.T) {
 	//use_fixed_vhd_format should not work with generation = 2
 	config["generation"] = 2
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -202,7 +202,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "floppy_files")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -217,7 +217,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	floppiesPath := "../../../common/test-fixtures/floppies"
 	config["floppy_files"] = []string{fmt.Sprintf("%s/bar.bat", floppiesPath), fmt.Sprintf("%s/foo.ps1", floppiesPath)}
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -236,7 +236,7 @@ func TestBuilderPrepare_InvalidFloppies(t *testing.T) {
 	config := testConfig()
 	config["floppy_files"] = []string{"nonexistent.bat", "nonexistent.ps1"}
 	b = Builder{}
-	_, errs := b.Prepare(config)
+	_, _, errs := b.Prepare(config)
 	if errs == nil {
 		t.Fatalf("Nonexistent floppies should trigger multierror")
 	}
@@ -252,7 +252,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -267,7 +267,7 @@ func TestBuilderPrepare_ISOChecksum(t *testing.T) {
 
 	// Test bad
 	config["iso_checksum"] = ""
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -278,7 +278,7 @@ func TestBuilderPrepare_ISOChecksum(t *testing.T) {
 	// Test good
 	config["iso_checksum"] = "FOo"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -294,7 +294,7 @@ func TestBuilderPrepare_ISOChecksumType(t *testing.T) {
 
 	// Test bad
 	config["iso_checksum_type"] = ""
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -305,7 +305,7 @@ func TestBuilderPrepare_ISOChecksumType(t *testing.T) {
 	// Test good
 	config["iso_checksum_type"] = "mD5"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -320,7 +320,7 @@ func TestBuilderPrepare_ISOChecksumType(t *testing.T) {
 	// Test unknown
 	config["iso_checksum_type"] = "fake"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -331,7 +331,7 @@ func TestBuilderPrepare_ISOChecksumType(t *testing.T) {
 	// Test none
 	config["iso_checksum_type"] = "none"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) == 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -353,7 +353,7 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	// Test both empty
 	config["iso_url"] = ""
 	b = Builder{}
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -364,7 +364,7 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	// Test iso_url set
 	config["iso_url"] = "http://www.packer.io"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -381,7 +381,7 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	config["iso_url"] = "http://www.packer.io"
 	config["iso_urls"] = []string{"http://www.packer.io"}
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -397,7 +397,7 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	}
 
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -431,7 +431,7 @@ func TestBuilderPrepare_SizeNotRequiredWhenUsingExistingHarddrive(t *testing.T)
 	}
 
 	b = Builder{}
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -455,7 +455,7 @@ func TestBuilderPrepare_SizeNotRequiredWhenUsingExistingHarddrive(t *testing.T)
 	}
 
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -489,7 +489,7 @@ func TestBuilderPrepare_SizeIsRequiredWhenNotUsingExistingHarddrive(t *testing.T
 	}
 
 	b = Builder{}
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -517,7 +517,7 @@ func TestBuilderPrepare_MaximumOfSixtyFourAdditionalDisks(t *testing.T) {
 	config["disk_additional_size"] = disks
 
 	b = Builder{}
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -537,7 +537,7 @@ func TestBuilderPrepare_CommConfig(t *testing.T) {
 		config["winrm_host"] = "1.2.3.4"
 
 		var b Builder
-		warns, err := b.Prepare(config)
+		_, warns, err := b.Prepare(config)
 		if len(warns) > 0 {
 			t.Fatalf("bad: %#v", warns)
 		}
@@ -565,7 +565,7 @@ func TestBuilderPrepare_CommConfig(t *testing.T) {
 		config["ssh_host"] = "1.2.3.4"
 
 		var b Builder
-		warns, err := b.Prepare(config)
+		_, warns, err := b.Prepare(config)
 		if len(warns) > 0 {
 			t.Fatalf("bad: %#v", warns)
 		}
@@ -593,7 +593,7 @@ func TestUserVariablesInBootCommand(t *testing.T) {
 	config[packer.UserVariablesConfigKey] = map[string]string{"test-variable": "test"}
 	config["boot_command"] = []string{"blah {{user `test-variable`}} blah"}
 
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -634,7 +634,7 @@ func TestBuilderPrepare_UseLegacyNetworkAdapter(t *testing.T) {
 	config["use_legacy_network_adapter"] = true
 
 	b = Builder{}
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -646,7 +646,7 @@ func TestBuilderPrepare_UseLegacyNetworkAdapter(t *testing.T) {
 	config["generation"] = 2
 
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
diff --git a/builder/hyperv/vmcx/builder.go b/builder/hyperv/vmcx/builder.go
index af93007e5..17baf0b75 100644
--- a/builder/hyperv/vmcx/builder.go
+++ b/builder/hyperv/vmcx/builder.go
@@ -78,7 +78,7 @@ type Config struct {
 }
 
 // Prepare processes the build configuration parameters.
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &b.config.ctx,
@@ -89,7 +89,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	// Accumulate any errors and warnings
@@ -204,10 +204,10 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 // Run executes a Packer build and returns a packer.Artifact representing
diff --git a/builder/hyperv/vmcx/builder_test.go b/builder/hyperv/vmcx/builder_test.go
index 9f268aab4..2fa7ba3cf 100644
--- a/builder/hyperv/vmcx/builder_test.go
+++ b/builder/hyperv/vmcx/builder_test.go
@@ -49,7 +49,7 @@ func TestBuilderPrepare_Defaults(t *testing.T) {
 	defer os.RemoveAll(td)
 	config["clone_from_vmcx_path"] = td
 
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -76,7 +76,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -90,7 +90,7 @@ func TestBuilderPrepare_CloneFromExistingMachineOrImportFromExportedMachineSetti
 	config := testConfig()
 	delete(config, "clone_from_vmcx_path")
 
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -114,7 +114,7 @@ func TestBuilderPrepare_ExportedMachinePathDoesNotExist(t *testing.T) {
 
 	config["clone_from_vmcx_path"] = td
 
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -138,7 +138,7 @@ func TestBuilderPrepare_ExportedMachinePathExists(t *testing.T) {
 
 	config["clone_from_vmcx_path"] = td
 
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -154,7 +154,7 @@ func disabled_TestBuilderPrepare_CloneFromVmSettingUsedSoNoCloneFromVmcxPathRequ
 
 	config["clone_from_vm_name"] = "test_machine_name_that_does_not_exist"
 
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -184,7 +184,7 @@ func TestBuilderPrepare_ISOChecksum(t *testing.T) {
 
 	// Test bad
 	config["iso_checksum"] = ""
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -195,7 +195,7 @@ func TestBuilderPrepare_ISOChecksum(t *testing.T) {
 	// Test good
 	config["iso_checksum"] = "FOo"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -219,7 +219,7 @@ func TestBuilderPrepare_ISOChecksumType(t *testing.T) {
 
 	// Test bad
 	config["iso_checksum_type"] = ""
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -230,7 +230,7 @@ func TestBuilderPrepare_ISOChecksumType(t *testing.T) {
 	// Test good
 	config["iso_checksum_type"] = "mD5"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -245,7 +245,7 @@ func TestBuilderPrepare_ISOChecksumType(t *testing.T) {
 	// Test none
 	config["iso_checksum_type"] = "none"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) == 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -276,7 +276,7 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	// Test both empty (should be allowed, as we cloning a vm so we probably don't need an ISO file)
 	config["iso_url"] = ""
 	b = Builder{}
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -287,7 +287,7 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	// Test iso_url set
 	config["iso_url"] = "http://www.packer.io"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -304,7 +304,7 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	config["iso_url"] = "http://www.packer.io"
 	config["iso_urls"] = []string{"http://www.packer.io"}
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -320,7 +320,7 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	}
 
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -350,7 +350,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	config["clone_from_vmcx_path"] = td
 
 	delete(config, "floppy_files")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -365,7 +365,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	floppies_path := "../../../common/test-fixtures/floppies"
 	config["floppy_files"] = []string{fmt.Sprintf("%s/bar.bat", floppies_path), fmt.Sprintf("%s/foo.ps1", floppies_path)}
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -393,7 +393,7 @@ func TestBuilderPrepare_InvalidFloppies(t *testing.T) {
 
 	config["floppy_files"] = []string{"nonexistent.bat", "nonexistent.ps1"}
 	b = Builder{}
-	_, errs := b.Prepare(config)
+	_, _, errs := b.Prepare(config)
 	if errs == nil {
 		t.Fatalf("Nonexistent floppies should trigger multierror")
 	}
@@ -422,7 +422,7 @@ func TestBuilderPrepare_CommConfig(t *testing.T) {
 		config["winrm_host"] = "1.2.3.4"
 
 		var b Builder
-		warns, err := b.Prepare(config)
+		_, warns, err := b.Prepare(config)
 		if len(warns) > 0 {
 			t.Fatalf("bad: %#v", warns)
 		}
@@ -459,7 +459,7 @@ func TestBuilderPrepare_CommConfig(t *testing.T) {
 		config["ssh_host"] = "1.2.3.4"
 
 		var b Builder
-		warns, err := b.Prepare(config)
+		_, warns, err := b.Prepare(config)
 		if len(warns) > 0 {
 			t.Fatalf("bad: %#v", warns)
 		}
@@ -494,7 +494,7 @@ func TestUserVariablesInBootCommand(t *testing.T) {
 	config[packer.UserVariablesConfigKey] = map[string]string{"test-variable": "test"}
 	config["boot_command"] = []string{"blah {{user `test-variable`}} blah"}
 
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
diff --git a/builder/jdcloud/builder.go b/builder/jdcloud/builder.go
index 4b3700f11..4f6a4c0ba 100644
--- a/builder/jdcloud/builder.go
+++ b/builder/jdcloud/builder.go
@@ -11,7 +11,7 @@ import (
 	"github.com/hashicorp/packer/template/interpolate"
 )
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &b.config.ctx,
@@ -22,19 +22,19 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, fmt.Errorf("[ERROR] Failed in decoding JSON->mapstructure")
+		return nil, nil, fmt.Errorf("[ERROR] Failed in decoding JSON->mapstructure")
 	}
 
 	errs := &packer.MultiError{}
 	errs = packer.MultiErrorAppend(errs, b.config.JDCloudCredentialConfig.Prepare(&b.config.ctx)...)
 	errs = packer.MultiErrorAppend(errs, b.config.JDCloudInstanceSpecConfig.Prepare(&b.config.ctx)...)
 	if errs != nil && len(errs.Errors) != 0 {
-		return nil, errs
+		return nil, nil, errs
 	}
 
 	packer.LogSecretFilter.Set(b.config.AccessKey, b.config.SecretKey)
 
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/linode/builder.go b/builder/linode/builder.go
index 3bc3dfb3f..ce6850f72 100644
--- a/builder/linode/builder.go
+++ b/builder/linode/builder.go
@@ -25,13 +25,13 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (ret packer.Artifact, err error) {
diff --git a/builder/linode/builder_test.go b/builder/linode/builder_test.go
index c80d1c76c..8176114a4 100644
--- a/builder/linode/builder_test.go
+++ b/builder/linode/builder_test.go
@@ -31,7 +31,7 @@ func TestBuilder_Prepare_BadType(t *testing.T) {
 		"linode_token": []string{},
 	}
 
-	warnings, err := b.Prepare(c)
+	_, warnings, err := b.Prepare(c)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -46,7 +46,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -61,7 +61,7 @@ func TestBuilderPrepare_Region(t *testing.T) {
 
 	// Test default
 	delete(config, "region")
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -74,7 +74,7 @@ func TestBuilderPrepare_Region(t *testing.T) {
 	// Test set
 	config["region"] = expected
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -93,7 +93,7 @@ func TestBuilderPrepare_Size(t *testing.T) {
 
 	// Test default
 	delete(config, "instance_type")
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -106,7 +106,7 @@ func TestBuilderPrepare_Size(t *testing.T) {
 	// Test set
 	config["instance_type"] = expected
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -125,7 +125,7 @@ func TestBuilderPrepare_Image(t *testing.T) {
 
 	// Test default
 	delete(config, "image")
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -138,7 +138,7 @@ func TestBuilderPrepare_Image(t *testing.T) {
 	// Test set
 	config["image"] = expected
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -156,7 +156,7 @@ func TestBuilderPrepare_ImageLabel(t *testing.T) {
 	config := testConfig()
 
 	// Test default
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -171,7 +171,7 @@ func TestBuilderPrepare_ImageLabel(t *testing.T) {
 	// Test set
 	config["image_label"] = "foobarbaz"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -182,7 +182,7 @@ func TestBuilderPrepare_ImageLabel(t *testing.T) {
 	// Test set with template
 	config["image_label"] = "{{timestamp}}"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -202,7 +202,7 @@ func TestBuilderPrepare_Label(t *testing.T) {
 	config := testConfig()
 
 	// Test default
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -217,7 +217,7 @@ func TestBuilderPrepare_Label(t *testing.T) {
 	// Test normal set
 	config["instance_label"] = "foobar"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -228,7 +228,7 @@ func TestBuilderPrepare_Label(t *testing.T) {
 	// Test with template
 	config["instance_label"] = "foobar-{{timestamp}}"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -239,7 +239,7 @@ func TestBuilderPrepare_Label(t *testing.T) {
 	// Test with bad template
 	config["instance_label"] = "foobar-{{"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/lxc/builder.go b/builder/lxc/builder.go
index 5c8747c7e..f5dfd4cbb 100644
--- a/builder/lxc/builder.go
+++ b/builder/lxc/builder.go
@@ -23,14 +23,14 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, errs := NewConfig(raws...)
 	if errs != nil {
-		return nil, errs
+		return nil, nil, errs
 	}
 	b.config = c
 
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/lxc/builder_test.go b/builder/lxc/builder_test.go
index 4eeb27594..20108202d 100644
--- a/builder/lxc/builder_test.go
+++ b/builder/lxc/builder_test.go
@@ -25,7 +25,7 @@ func TestBuilderPrepare_ConfigFile(t *testing.T) {
 	var b Builder
 	// Good
 	config := testConfig()
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -37,7 +37,7 @@ func TestBuilderPrepare_ConfigFile(t *testing.T) {
 	config = testConfig()
 	delete(config, "config_file")
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/lxd/builder.go b/builder/lxd/builder.go
index ce1656217..a4a1d4863 100644
--- a/builder/lxd/builder.go
+++ b/builder/lxd/builder.go
@@ -21,14 +21,14 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, errs := NewConfig(raws...)
 	if errs != nil {
-		return nil, errs
+		return nil, nil, errs
 	}
 	b.config = c
 
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/lxd/builder_test.go b/builder/lxd/builder_test.go
index b2ea70e55..a2277932c 100644
--- a/builder/lxd/builder_test.go
+++ b/builder/lxd/builder_test.go
@@ -24,7 +24,7 @@ func TestBuilderPrepare_ConfigFile(t *testing.T) {
 	var b Builder
 	// Good
 	config := testConfig()
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -35,7 +35,7 @@ func TestBuilderPrepare_ConfigFile(t *testing.T) {
 	// Good, remote image
 	config = testConfig()
 	config["image"] = "remote:bar"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -46,7 +46,7 @@ func TestBuilderPrepare_ConfigFile(t *testing.T) {
 	// Good, remote output image
 	config = testConfig()
 	config["output_image"] = "remote:foo"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -58,7 +58,7 @@ func TestBuilderPrepare_ConfigFile(t *testing.T) {
 	config = testConfig()
 	delete(config, "image")
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/ncloud/builder.go b/builder/ncloud/builder.go
index 36702c933..1205573fd 100644
--- a/builder/ncloud/builder.go
+++ b/builder/ncloud/builder.go
@@ -17,16 +17,16 @@ type Builder struct {
 	runner   multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
 
 	b.stateBag = new(multistep.BasicStateBag)
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/null/builder.go b/builder/null/builder.go
index 5939cd024..5bba58bc2 100644
--- a/builder/null/builder.go
+++ b/builder/null/builder.go
@@ -16,14 +16,14 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/oneandone/builder.go b/builder/oneandone/builder.go
index 5ea7674bc..785d9fdbd 100644
--- a/builder/oneandone/builder.go
+++ b/builder/oneandone/builder.go
@@ -18,14 +18,14 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/oneandone/builder_test.go b/builder/oneandone/builder_test.go
index daa8536e3..46045bf9d 100644
--- a/builder/oneandone/builder_test.go
+++ b/builder/oneandone/builder_test.go
@@ -30,7 +30,7 @@ func TestBuilder_Prepare_BadType(t *testing.T) {
 		"api_key": []string{},
 	}
 
-	warns, err := b.Prepare(c)
+	_, warns, err := b.Prepare(c)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -46,7 +46,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 	config := testConfig()
 
 	config["i_should_not_be_valid"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/openstack/builder.go b/builder/openstack/builder.go
index 8411f803d..d8a3ae66f 100644
--- a/builder/openstack/builder.go
+++ b/builder/openstack/builder.go
@@ -35,13 +35,13 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &b.config.ctx,
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	// Accumulate any errors
@@ -51,11 +51,11 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	errs = packer.MultiErrorAppend(errs, b.config.RunConfig.Prepare(&b.config.ctx)...)
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return nil, errs
+		return nil, nil, errs
 	}
 
 	if b.config.ImageConfig.ImageDiskFormat != "" && !b.config.RunConfig.UseBlockStorageVolume {
-		return nil, fmt.Errorf("use_blockstorage_volume must be true if image_disk_format is specified.")
+		return nil, nil, fmt.Errorf("use_blockstorage_volume must be true if image_disk_format is specified.")
 	}
 
 	// By default, instance name is same as image name
@@ -64,7 +64,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	packer.LogSecretFilter.Set(b.config.Password)
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/openstack/builder_test.go b/builder/openstack/builder_test.go
index 5fa296d30..0a9ac3288 100644
--- a/builder/openstack/builder_test.go
+++ b/builder/openstack/builder_test.go
@@ -20,7 +20,7 @@ func TestBuilder_Prepare_BadType(t *testing.T) {
 		"password": []string{},
 	}
 
-	warns, err := b.Prepare(c)
+	_, warns, err := b.Prepare(c)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
diff --git a/builder/oracle/classic/builder.go b/builder/oracle/classic/builder.go
index d97b3be10..0905e4e84 100644
--- a/builder/oracle/classic/builder.go
+++ b/builder/oracle/classic/builder.go
@@ -24,10 +24,10 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(rawConfig ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(rawConfig ...interface{}) ([]string, []string, error) {
 	config, err := NewConfig(rawConfig...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 	b.config = config
 
@@ -36,9 +36,9 @@ func (b *Builder) Prepare(rawConfig ...interface{}) ([]string, error) {
 	errs = packer.MultiErrorAppend(errs, b.config.PVConfig.Prepare(&b.config.ctx))
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return nil, errs
+		return nil, nil, errs
 	}
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/oracle/oci/builder.go b/builder/oracle/oci/builder.go
index a4b31b8e6..e54c4000b 100644
--- a/builder/oracle/oci/builder.go
+++ b/builder/oracle/oci/builder.go
@@ -26,14 +26,14 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(rawConfig ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(rawConfig ...interface{}) ([]string, []string, error) {
 	config, err := NewConfig(rawConfig...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 	b.config = config
 
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/osc/bsu/builder.go b/builder/osc/bsu/builder.go
index ab4280417..e96011190 100644
--- a/builder/osc/bsu/builder.go
+++ b/builder/osc/bsu/builder.go
@@ -42,7 +42,7 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	b.config.ctx.Funcs = osccommon.TemplateFuncs
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
@@ -59,7 +59,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	if b.config.PackerConfig.PackerForce {
@@ -75,11 +75,11 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	errs = packer.MultiErrorAppend(errs, b.config.RunConfig.Prepare(&b.config.ctx)...)
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return nil, errs
+		return nil, nil, errs
 	}
 
 	packer.LogSecretFilter.Set(b.config.AccessKey, b.config.SecretKey, b.config.Token)
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/osc/bsu/builder_test.go b/builder/osc/bsu/builder_test.go
index 1736044df..c700ccc06 100644
--- a/builder/osc/bsu/builder_test.go
+++ b/builder/osc/bsu/builder_test.go
@@ -32,7 +32,7 @@ func TestBuilder_Prepare_BadType(t *testing.T) {
 		"access_key": []string{},
 	}
 
-	warnings, err := b.Prepare(c)
+	_, warnings, err := b.Prepare(c)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -48,7 +48,7 @@ func TestBuilderPrepare_OMIName(t *testing.T) {
 	// Test good
 	config["omi_name"] = "foo"
 	config["skip_region_validation"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -59,7 +59,7 @@ func TestBuilderPrepare_OMIName(t *testing.T) {
 	// Test bad
 	config["omi_name"] = "foo {{"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -70,7 +70,7 @@ func TestBuilderPrepare_OMIName(t *testing.T) {
 	// Test bad
 	delete(config, "omi_name")
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -85,7 +85,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -101,7 +101,7 @@ func TestBuilderPrepare_InvalidShutdownBehavior(t *testing.T) {
 	// Test good
 	config["shutdown_behavior"] = "terminate"
 	config["skip_region_validation"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -111,7 +111,7 @@ func TestBuilderPrepare_InvalidShutdownBehavior(t *testing.T) {
 
 	// Test good
 	config["shutdown_behavior"] = "stop"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -121,7 +121,7 @@ func TestBuilderPrepare_InvalidShutdownBehavior(t *testing.T) {
 
 	// Test bad
 	config["shutdown_behavior"] = "foobar"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/osc/bsusurrogate/builder.go b/builder/osc/bsusurrogate/builder.go
index 1186a5bdc..ed8b7cdf2 100644
--- a/builder/osc/bsusurrogate/builder.go
+++ b/builder/osc/bsusurrogate/builder.go
@@ -39,7 +39,7 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 
 	b.config.ctx.Funcs = osccommon.TemplateFuncs
 
@@ -58,7 +58,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	if b.config.PackerConfig.PackerForce {
@@ -90,11 +90,11 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return nil, errs
+		return nil, nil, errs
 	}
 
 	packer.LogSecretFilter.Set(b.config.AccessKey, b.config.SecretKey, b.config.Token)
-	return nil, nil
+	return nil, nil, nil
 
 }
 
diff --git a/builder/osc/bsuvolume/builder.go b/builder/osc/bsuvolume/builder.go
index e0b5a8fa7..7ab76aa88 100644
--- a/builder/osc/bsuvolume/builder.go
+++ b/builder/osc/bsuvolume/builder.go
@@ -44,7 +44,7 @@ type EngineVarsTemplate struct {
 	SourceOMI   string
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	b.config.ctx.Funcs = osccommon.TemplateFuncs
 	// Create passthrough for {{ .BuildRegion }} and {{ .SourceOMI }} variables
 	// so we can fill them in later
@@ -57,7 +57,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		InterpolateContext: &b.config.ctx,
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	// Accumulate any errors
@@ -78,11 +78,11 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return nil, errs
+		return nil, nil, errs
 	}
 
 	packer.LogSecretFilter.Set(b.config.AccessKey, b.config.SecretKey, b.config.Token)
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/osc/bsuvolume/builder_test.go b/builder/osc/bsuvolume/builder_test.go
index 956fba60f..27c4a80c0 100644
--- a/builder/osc/bsuvolume/builder_test.go
+++ b/builder/osc/bsuvolume/builder_test.go
@@ -31,7 +31,7 @@ func TestBuilder_Prepare_BadType(t *testing.T) {
 		"access_key": []string{},
 	}
 
-	warnings, err := b.Prepare(c)
+	_, warnings, err := b.Prepare(c)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -46,7 +46,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -62,7 +62,7 @@ func TestBuilderPrepare_InvalidShutdownBehavior(t *testing.T) {
 	// Test good
 	config["shutdown_behavior"] = "terminate"
 	config["skip_region_validation"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -72,7 +72,7 @@ func TestBuilderPrepare_InvalidShutdownBehavior(t *testing.T) {
 
 	// Test good
 	config["shutdown_behavior"] = "stop"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -82,7 +82,7 @@ func TestBuilderPrepare_InvalidShutdownBehavior(t *testing.T) {
 
 	// Test bad
 	config["shutdown_behavior"] = "foobar"
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/osc/chroot/builder.go b/builder/osc/chroot/builder.go
index e6813d263..ac506aa24 100644
--- a/builder/osc/chroot/builder.go
+++ b/builder/osc/chroot/builder.go
@@ -63,7 +63,7 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	b.config.ctx.Funcs = osccommon.TemplateFuncs
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
@@ -82,7 +82,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	if b.config.PackerConfig.PackerForce {
@@ -178,11 +178,11 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return warns, errs
+		return nil, warns, errs
 	}
 
 	packer.LogSecretFilter.Set(b.config.AccessKey, b.config.SecretKey, b.config.Token)
-	return warns, nil
+	return nil, warns, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/parallels/iso/builder.go b/builder/parallels/iso/builder.go
index b7ea4788f..8764ad36a 100644
--- a/builder/parallels/iso/builder.go
+++ b/builder/parallels/iso/builder.go
@@ -82,7 +82,7 @@ type Config struct {
 	ctx interpolate.Context
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &b.config.ctx,
@@ -96,7 +96,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	// Accumulate any errors and warnings
@@ -169,10 +169,10 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/parallels/iso/builder_test.go b/builder/parallels/iso/builder_test.go
index 88a28144e..e131bc137 100644
--- a/builder/parallels/iso/builder_test.go
+++ b/builder/parallels/iso/builder_test.go
@@ -32,7 +32,7 @@ func TestBuilder_ImplementsBuilder(t *testing.T) {
 func TestBuilderPrepare_Defaults(t *testing.T) {
 	var b Builder
 	config := testConfig()
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -54,7 +54,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "floppy_files")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -69,7 +69,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	floppies_path := "../../../common/test-fixtures/floppies"
 	config["floppy_files"] = []string{fmt.Sprintf("%s/bar.bat", floppies_path), fmt.Sprintf("%s/foo.ps1", floppies_path)}
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -88,7 +88,7 @@ func TestBuilderPrepare_InvalidFloppies(t *testing.T) {
 	config := testConfig()
 	config["floppy_files"] = []string{"nonexistent.bat", "nonexistent.ps1"}
 	b = Builder{}
-	_, errs := b.Prepare(config)
+	_, _, errs := b.Prepare(config)
 	if errs == nil {
 		t.Fatalf("Nonexistent floppies should trigger multierror")
 	}
@@ -103,7 +103,7 @@ func TestBuilderPrepare_DiskSize(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "disk_size")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -117,7 +117,7 @@ func TestBuilderPrepare_DiskSize(t *testing.T) {
 
 	config["disk_size"] = 60000
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -136,7 +136,7 @@ func TestBuilderPrepare_DiskType(t *testing.T) {
 
 	// Test a default disk_type
 	delete(config, "disk_type")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -151,7 +151,7 @@ func TestBuilderPrepare_DiskType(t *testing.T) {
 	// Test with a bad
 	config["disk_type"] = "fake"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -163,7 +163,7 @@ func TestBuilderPrepare_DiskType(t *testing.T) {
 	config["disk_type"] = "plain"
 	config["skip_compaction"] = false
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) == 0 {
 		t.Fatalf("should have warning")
 	}
@@ -175,7 +175,7 @@ func TestBuilderPrepare_DiskType(t *testing.T) {
 	config["disk_type"] = "plain"
 	config["skip_compaction"] = true
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -191,7 +191,7 @@ func TestBuilderPrepare_HardDriveInterface(t *testing.T) {
 
 	// Test a default boot_wait
 	delete(config, "hard_drive_interface")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -206,7 +206,7 @@ func TestBuilderPrepare_HardDriveInterface(t *testing.T) {
 	// Test with a bad
 	config["hard_drive_interface"] = "fake"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -217,7 +217,7 @@ func TestBuilderPrepare_HardDriveInterface(t *testing.T) {
 	// Test with a good
 	config["hard_drive_interface"] = "scsi"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -232,7 +232,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
diff --git a/builder/parallels/pvm/builder.go b/builder/parallels/pvm/builder.go
index a199650d1..10296f4cc 100644
--- a/builder/parallels/pvm/builder.go
+++ b/builder/parallels/pvm/builder.go
@@ -20,14 +20,14 @@ type Builder struct {
 }
 
 // Prepare processes the build configuration parameters.
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 // Run executes a Packer build and returns a packer.Artifact representing
diff --git a/builder/profitbricks/builder.go b/builder/profitbricks/builder.go
index 3ab9d18fe..01bfeb1f5 100644
--- a/builder/profitbricks/builder.go
+++ b/builder/profitbricks/builder.go
@@ -17,14 +17,14 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/profitbricks/builder_test.go b/builder/profitbricks/builder_test.go
index 98252b410..f16953a33 100644
--- a/builder/profitbricks/builder_test.go
+++ b/builder/profitbricks/builder_test.go
@@ -31,7 +31,7 @@ func TestBuilder_Prepare_BadType(t *testing.T) {
 		"api_key": []string{},
 	}
 
-	warns, err := b.Prepare(c)
+	_, warns, err := b.Prepare(c)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -47,7 +47,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 	config := testConfig()
 
 	config["i_should_not_be_valid"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/proxmox/builder.go b/builder/proxmox/builder.go
index 083cfeb3d..59038f6c2 100644
--- a/builder/proxmox/builder.go
+++ b/builder/proxmox/builder.go
@@ -27,13 +27,13 @@ var _ packer.Builder = &Builder{}
 
 var pluginVersion = "1.0.0"
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	config, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = *config
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/proxmox/config_test.go b/builder/proxmox/config_test.go
index 84dd13c6d..6733eda17 100644
--- a/builder/proxmox/config_test.go
+++ b/builder/proxmox/config_test.go
@@ -81,7 +81,7 @@ func TestBasicExampleFromDocsIsValid(t *testing.T) {
 	}
 
 	b := &Builder{}
-	warn, err := b.Prepare(tpl.Builders["proxmox"].Config)
+	_, warn, err := b.Prepare(tpl.Builders["proxmox"].Config)
 	if err != nil {
 		t.Fatal(err, warn)
 	}
@@ -149,7 +149,7 @@ func TestAgentSetToFalse(t *testing.T) {
 	}
 
 	b := &Builder{}
-	warn, err := b.Prepare(tpl.Builders["proxmox"].Config)
+	_, warn, err := b.Prepare(tpl.Builders["proxmox"].Config)
 	if err != nil {
 		t.Fatal(err, warn)
 	}
diff --git a/builder/qemu/builder.go b/builder/qemu/builder.go
index 6d85d2654..a8093f2fe 100644
--- a/builder/qemu/builder.go
+++ b/builder/qemu/builder.go
@@ -336,7 +336,7 @@ type Config struct {
 	ctx interpolate.Context
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &b.config.ctx,
@@ -348,7 +348,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	var errs *packer.MultiError
@@ -569,10 +569,10 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/qemu/builder_test.go b/builder/qemu/builder_test.go
index 4616ed3e2..4eea7174e 100644
--- a/builder/qemu/builder_test.go
+++ b/builder/qemu/builder_test.go
@@ -62,7 +62,7 @@ func TestBuilder_ImplementsBuilder(t *testing.T) {
 func TestBuilderPrepare_Defaults(t *testing.T) {
 	var b Builder
 	config := testConfig()
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -101,7 +101,7 @@ func TestBuilderPrepare_VNCBindAddress(t *testing.T) {
 
 	// Test a default boot_wait
 	delete(config, "vnc_bind_address")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -122,7 +122,7 @@ func TestBuilderPrepare_DiskCompaction(t *testing.T) {
 	config["skip_compaction"] = false
 	config["disk_compression"] = true
 	config["format"] = "img"
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -141,7 +141,7 @@ func TestBuilderPrepare_DiskCompaction(t *testing.T) {
 	config["disk_compression"] = true
 	config["format"] = "qcow2"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -178,7 +178,7 @@ func TestBuilderPrepare_DiskSize(t *testing.T) {
 		delete(config, "disk_size")
 		config["disk_size"] = tc.InputSize
 
-		warns, err := b.Prepare(config)
+		_, warns, err := b.Prepare(config)
 		if len(warns) > 0 {
 			t.Fatalf("bad: %#v", warns)
 		}
@@ -198,7 +198,7 @@ func TestBuilderPrepare_AdditionalDiskSize(t *testing.T) {
 
 	config["disk_additional_size"] = []string{"1M"}
 	config["disk_image"] = true
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -209,7 +209,7 @@ func TestBuilderPrepare_AdditionalDiskSize(t *testing.T) {
 	delete(config, "disk_image")
 	config["disk_additional_size"] = []string{"1M"}
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -228,7 +228,7 @@ func TestBuilderPrepare_Format(t *testing.T) {
 
 	// Bad
 	config["format"] = "illegal value"
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -239,7 +239,7 @@ func TestBuilderPrepare_Format(t *testing.T) {
 	// Good
 	config["format"] = "qcow2"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -250,7 +250,7 @@ func TestBuilderPrepare_Format(t *testing.T) {
 	// Good
 	config["format"] = "raw"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -269,7 +269,7 @@ func TestBuilderPrepare_UseBackingFile(t *testing.T) {
 	config["disk_image"] = false
 	config["format"] = "qcow2"
 	b = Builder{}
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -281,7 +281,7 @@ func TestBuilderPrepare_UseBackingFile(t *testing.T) {
 	config["disk_image"] = true
 	config["format"] = "raw"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -293,7 +293,7 @@ func TestBuilderPrepare_UseBackingFile(t *testing.T) {
 	config["disk_image"] = true
 	config["format"] = "qcow2"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -307,7 +307,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "floppy_files")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -322,7 +322,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	floppies_path := "../../common/test-fixtures/floppies"
 	config["floppy_files"] = []string{fmt.Sprintf("%s/bar.bat", floppies_path), fmt.Sprintf("%s/foo.ps1", floppies_path)}
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -341,7 +341,7 @@ func TestBuilderPrepare_InvalidFloppies(t *testing.T) {
 	config := testConfig()
 	config["floppy_files"] = []string{"nonexistent.bat", "nonexistent.ps1"}
 	b = Builder{}
-	_, errs := b.Prepare(config)
+	_, _, errs := b.Prepare(config)
 	if errs == nil {
 		t.Fatalf("Nonexistent floppies should trigger multierror")
 	}
@@ -357,7 +357,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -379,7 +379,7 @@ func TestBuilderPrepare_OutputDir(t *testing.T) {
 
 	config["output_directory"] = dir
 	b = Builder{}
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -390,7 +390,7 @@ func TestBuilderPrepare_OutputDir(t *testing.T) {
 	// Test with a good one
 	config["output_directory"] = "i-hope-i-dont-exist"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -405,7 +405,7 @@ func TestBuilderPrepare_ShutdownTimeout(t *testing.T) {
 
 	// Test with a bad value
 	config["shutdown_timeout"] = "this is not good"
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -416,7 +416,7 @@ func TestBuilderPrepare_ShutdownTimeout(t *testing.T) {
 	// Test with a good one
 	config["shutdown_timeout"] = "5s"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -433,7 +433,7 @@ func TestBuilderPrepare_SSHHostPort(t *testing.T) {
 	config["ssh_host_port_min"] = 1000
 	config["ssh_host_port_max"] = 500
 	b = Builder{}
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -444,7 +444,7 @@ func TestBuilderPrepare_SSHHostPort(t *testing.T) {
 	// Bad
 	config["ssh_host_port_min"] = -500
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -456,7 +456,7 @@ func TestBuilderPrepare_SSHHostPort(t *testing.T) {
 	config["ssh_host_port_min"] = 500
 	config["ssh_host_port_max"] = 1000
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -471,7 +471,7 @@ func TestBuilderPrepare_SSHPrivateKey(t *testing.T) {
 
 	config["ssh_private_key_file"] = ""
 	b = Builder{}
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -481,7 +481,7 @@ func TestBuilderPrepare_SSHPrivateKey(t *testing.T) {
 
 	config["ssh_private_key_file"] = "/i/dont/exist"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -503,7 +503,7 @@ func TestBuilderPrepare_SSHPrivateKey(t *testing.T) {
 
 	config["ssh_private_key_file"] = tf.Name()
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -517,7 +517,7 @@ func TestBuilderPrepare_SSHPrivateKey(t *testing.T) {
 	tf.Write([]byte(testPem))
 	config["ssh_private_key_file"] = tf.Name()
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -532,7 +532,7 @@ func TestBuilderPrepare_SSHWaitTimeout(t *testing.T) {
 
 	// Test a default boot_wait
 	delete(config, "ssh_wait_timeout")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -543,7 +543,7 @@ func TestBuilderPrepare_SSHWaitTimeout(t *testing.T) {
 	// Test with a bad value
 	config["ssh_wait_timeout"] = "this is not good"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -554,7 +554,7 @@ func TestBuilderPrepare_SSHWaitTimeout(t *testing.T) {
 	// Test with a good one
 	config["ssh_wait_timeout"] = "5s"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -569,7 +569,7 @@ func TestBuilderPrepare_QemuArgs(t *testing.T) {
 
 	// Test with empty
 	delete(config, "qemuargs")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -587,7 +587,7 @@ func TestBuilderPrepare_QemuArgs(t *testing.T) {
 	}
 
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -611,7 +611,7 @@ func TestBuilderPrepare_VNCPassword(t *testing.T) {
 	config["vnc_use_password"] = true
 	config["output_directory"] = "not-a-real-directory"
 	b = Builder{}
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
diff --git a/builder/scaleway/builder.go b/builder/scaleway/builder.go
index 47ddc8376..b1086955a 100644
--- a/builder/scaleway/builder.go
+++ b/builder/scaleway/builder.go
@@ -23,14 +23,14 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
 
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/scaleway/builder_test.go b/builder/scaleway/builder_test.go
index a1a0fccc3..cfb192614 100644
--- a/builder/scaleway/builder_test.go
+++ b/builder/scaleway/builder_test.go
@@ -32,7 +32,7 @@ func TestBuilder_Prepare_BadType(t *testing.T) {
 		"api_token": []string{},
 	}
 
-	warnings, err := b.Prepare(c)
+	_, warnings, err := b.Prepare(c)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -46,7 +46,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 	config := testConfig()
 
 	config["i_should_not_be_valid"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -60,7 +60,7 @@ func TestBuilderPrepare_Region(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "region")
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -72,7 +72,7 @@ func TestBuilderPrepare_Region(t *testing.T) {
 
 	config["region"] = expected
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -90,7 +90,7 @@ func TestBuilderPrepare_CommercialType(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "commercial_type")
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -102,7 +102,7 @@ func TestBuilderPrepare_CommercialType(t *testing.T) {
 
 	config["commercial_type"] = expected
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -120,7 +120,7 @@ func TestBuilderPrepare_Image(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "image")
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -132,7 +132,7 @@ func TestBuilderPrepare_Image(t *testing.T) {
 
 	config["image"] = expected
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -149,7 +149,7 @@ func TestBuilderPrepare_SnapshotName(t *testing.T) {
 	var b Builder
 	config := testConfig()
 
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -163,7 +163,7 @@ func TestBuilderPrepare_SnapshotName(t *testing.T) {
 
 	config["snapshot_name"] = "foobarbaz"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -173,7 +173,7 @@ func TestBuilderPrepare_SnapshotName(t *testing.T) {
 
 	config["snapshot_name"] = "{{timestamp}}"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -192,7 +192,7 @@ func TestBuilderPrepare_ServerName(t *testing.T) {
 	var b Builder
 	config := testConfig()
 
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -206,7 +206,7 @@ func TestBuilderPrepare_ServerName(t *testing.T) {
 
 	config["server_name"] = "foobar"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -216,7 +216,7 @@ func TestBuilderPrepare_ServerName(t *testing.T) {
 
 	config["server_name"] = "foobar-{{timestamp}}"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -226,7 +226,7 @@ func TestBuilderPrepare_ServerName(t *testing.T) {
 
 	config["server_name"] = "foobar-{{"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/tencentcloud/cvm/builder.go b/builder/tencentcloud/cvm/builder.go
index 8934d0649..596f52534 100644
--- a/builder/tencentcloud/cvm/builder.go
+++ b/builder/tencentcloud/cvm/builder.go
@@ -31,7 +31,7 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &b.config.ctx,
@@ -43,7 +43,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}, raws...)
 	b.config.ctx.EnableEnv = true
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	// Accumulate any errors
@@ -52,13 +52,13 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	errs = packer.MultiErrorAppend(errs, b.config.TencentCloudImageConfig.Prepare(&b.config.ctx)...)
 	errs = packer.MultiErrorAppend(errs, b.config.TencentCloudRunConfig.Prepare(&b.config.ctx)...)
 	if errs != nil && len(errs.Errors) > 0 {
-		return nil, errs
+		return nil, nil, errs
 	}
 
 	packer.LogSecretFilter.Set(b.config.SecretId, b.config.SecretKey)
 	log.Printf("[DEBUG]packer config: %v", b.config)
 
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/triton/builder.go b/builder/triton/builder.go
index ba6e2baa8..7a4cd54f1 100644
--- a/builder/triton/builder.go
+++ b/builder/triton/builder.go
@@ -20,7 +20,7 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	errs := &multierror.Error{}
 
 	err := config.Decode(&b.config, &config.DecodeOpts{
@@ -42,7 +42,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		b.config.Comm.SSHAgentAuth = true
 	}
 
-	return nil, errs.ErrorOrNil()
+	return nil, nil, errs.ErrorOrNil()
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/ucloud/uhost/builder.go b/builder/ucloud/uhost/builder.go
index a9903bf4a..6b2e25d4e 100644
--- a/builder/ucloud/uhost/builder.go
+++ b/builder/ucloud/uhost/builder.go
@@ -33,7 +33,7 @@ type Builder struct {
 	runner multistep.Runner
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &b.config.ctx,
@@ -45,7 +45,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}, raws...)
 	b.config.ctx.EnableEnv = true
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	// Accumulate any errors
@@ -55,11 +55,11 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	errs = packer.MultiErrorAppend(errs, b.config.RunConfig.Prepare(&b.config.ctx)...)
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return nil, errs
+		return nil, nil, errs
 	}
 
 	packer.LogSecretFilter.Set(b.config.PublicKey, b.config.PrivateKey)
-	return nil, nil
+	return nil, nil, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/ucloud/uhost/builder_test.go b/builder/ucloud/uhost/builder_test.go
index dca944bc5..70da65fa5 100644
--- a/builder/ucloud/uhost/builder_test.go
+++ b/builder/ucloud/uhost/builder_test.go
@@ -35,7 +35,7 @@ func TestBuilder_Prepare_BadType(t *testing.T) {
 		"public_key": []string{},
 	}
 
-	warnings, err := b.Prepare(c)
+	_, warnings, err := b.Prepare(c)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -50,7 +50,7 @@ func TestBuilderPrepare_ImageName(t *testing.T) {
 
 	// Test good
 	config["image_name"] = "foo"
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -61,7 +61,7 @@ func TestBuilderPrepare_ImageName(t *testing.T) {
 	// Test bad
 	config["image_name"] = "foo {{"
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -72,7 +72,7 @@ func TestBuilderPrepare_ImageName(t *testing.T) {
 	// Test bad
 	delete(config, "image_name")
 	b = Builder{}
-	warnings, err = b.Prepare(config)
+	_, warnings, err = b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -87,7 +87,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
@@ -113,7 +113,7 @@ func TestBuilderPrepare_ImageDestinations(t *testing.T) {
 			"description": "bar",
 		},
 	}
-	warnings, err := b.Prepare(config)
+	_, warnings, err := b.Prepare(config)
 	if len(warnings) > 0 {
 		t.Fatalf("bad: %#v", warnings)
 	}
diff --git a/builder/vagrant/builder.go b/builder/vagrant/builder.go
index 5fb189e13..d6cd83d0a 100644
--- a/builder/vagrant/builder.go
+++ b/builder/vagrant/builder.go
@@ -132,7 +132,7 @@ type Config struct {
 }
 
 // Prepare processes the build configuration parameters.
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	b.config = new(Config)
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
@@ -144,7 +144,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	// Accumulate any errors and warnings
@@ -215,10 +215,10 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 // Run executes a Packer build and returns a packer.Artifact representing
diff --git a/builder/vagrant/builder_test.go b/builder/vagrant/builder_test.go
index 65c2e5e9f..ee15b86e3 100644
--- a/builder/vagrant/builder_test.go
+++ b/builder/vagrant/builder_test.go
@@ -83,7 +83,7 @@ func TestBuilder_Prepare_ValidateSource(t *testing.T) {
 	}
 
 	for _, tc := range cases {
-		_, err := b.Prepare(tc.config)
+		_, _, err := b.Prepare(tc.config)
 		if (err != nil) != tc.errExpected {
 			t.Fatalf("Unexpected behavior from test case %#v; %s.", tc.config, tc.reason)
 		}
diff --git a/builder/virtualbox/iso/builder.go b/builder/virtualbox/iso/builder.go
index 02c7aeeac..ecdf1f470 100644
--- a/builder/virtualbox/iso/builder.go
+++ b/builder/virtualbox/iso/builder.go
@@ -130,7 +130,7 @@ type Config struct {
 	ctx interpolate.Context
 }
 
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	err := config.Decode(&b.config, &config.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &b.config.ctx,
@@ -145,7 +145,7 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		},
 	}, raws...)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	// Accumulate any errors and warnings
@@ -267,10 +267,10 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	if errs != nil && len(errs.Errors) > 0 {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/virtualbox/iso/builder_test.go b/builder/virtualbox/iso/builder_test.go
index 370ebe675..7f996efdf 100644
--- a/builder/virtualbox/iso/builder_test.go
+++ b/builder/virtualbox/iso/builder_test.go
@@ -32,7 +32,7 @@ func TestBuilder_ImplementsBuilder(t *testing.T) {
 func TestBuilderPrepare_Defaults(t *testing.T) {
 	var b Builder
 	config := testConfig()
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -62,7 +62,7 @@ func TestBuilderPrepare_DiskSize(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "disk_size")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -76,7 +76,7 @@ func TestBuilderPrepare_DiskSize(t *testing.T) {
 
 	config["disk_size"] = 60000
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -94,7 +94,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "floppy_files")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -109,7 +109,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	floppies_path := "../../../common/test-fixtures/floppies"
 	config["floppy_files"] = []string{fmt.Sprintf("%s/bar.bat", floppies_path), fmt.Sprintf("%s/foo.ps1", floppies_path)}
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -128,7 +128,7 @@ func TestBuilderPrepare_InvalidFloppies(t *testing.T) {
 	config := testConfig()
 	config["floppy_files"] = []string{"nonexistent.bat", "nonexistent.ps1"}
 	b = Builder{}
-	_, errs := b.Prepare(config)
+	_, _, errs := b.Prepare(config)
 	if errs == nil {
 		t.Fatalf("Nonexistent floppies should trigger multierror")
 	}
@@ -144,7 +144,7 @@ func TestBuilderPrepare_GuestAdditionsMode(t *testing.T) {
 
 	// test default mode
 	delete(config, "guest_additions_mode")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -155,7 +155,7 @@ func TestBuilderPrepare_GuestAdditionsMode(t *testing.T) {
 	// Test another mode
 	config["guest_additions_mode"] = "attach"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -170,7 +170,7 @@ func TestBuilderPrepare_GuestAdditionsMode(t *testing.T) {
 	// Test bad mode
 	config["guest_additions_mode"] = "teleport"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -184,7 +184,7 @@ func TestBuilderPrepare_GuestAdditionsPath(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "guest_additions_path")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -198,7 +198,7 @@ func TestBuilderPrepare_GuestAdditionsPath(t *testing.T) {
 
 	config["guest_additions_path"] = "foo"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -216,7 +216,7 @@ func TestBuilderPrepare_GuestAdditionsSHA256(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "guest_additions_sha256")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -230,7 +230,7 @@ func TestBuilderPrepare_GuestAdditionsSHA256(t *testing.T) {
 
 	config["guest_additions_sha256"] = "FOO"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -248,7 +248,7 @@ func TestBuilderPrepare_GuestAdditionsURL(t *testing.T) {
 	config := testConfig()
 
 	config["guest_additions_url"] = ""
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -262,7 +262,7 @@ func TestBuilderPrepare_GuestAdditionsURL(t *testing.T) {
 
 	config["guest_additions_url"] = "http://www.packer.io"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -277,7 +277,7 @@ func TestBuilderPrepare_HardDriveInterface(t *testing.T) {
 
 	// Test a default boot_wait
 	delete(config, "hard_drive_interface")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -292,7 +292,7 @@ func TestBuilderPrepare_HardDriveInterface(t *testing.T) {
 	// Test with a bad
 	config["hard_drive_interface"] = "fake"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -303,7 +303,7 @@ func TestBuilderPrepare_HardDriveInterface(t *testing.T) {
 	// Test with a good
 	config["hard_drive_interface"] = "sata"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -318,7 +318,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -333,7 +333,7 @@ func TestBuilderPrepare_ISOInterface(t *testing.T) {
 
 	// Test a default boot_wait
 	delete(config, "iso_interface")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -348,7 +348,7 @@ func TestBuilderPrepare_ISOInterface(t *testing.T) {
 	// Test with a bad
 	config["iso_interface"] = "fake"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -359,7 +359,7 @@ func TestBuilderPrepare_ISOInterface(t *testing.T) {
 	// Test with a good
 	config["iso_interface"] = "sata"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
diff --git a/builder/virtualbox/ovf/builder.go b/builder/virtualbox/ovf/builder.go
index b20b5b663..da63b6951 100644
--- a/builder/virtualbox/ovf/builder.go
+++ b/builder/virtualbox/ovf/builder.go
@@ -20,14 +20,14 @@ type Builder struct {
 }
 
 // Prepare processes the build configuration parameters.
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 // Run executes a Packer build and returns a packer.Artifact representing
diff --git a/builder/virtualbox/vm/builder.go b/builder/virtualbox/vm/builder.go
index 2336e58d5..ba76bacf1 100644
--- a/builder/virtualbox/vm/builder.go
+++ b/builder/virtualbox/vm/builder.go
@@ -20,14 +20,14 @@ type Builder struct {
 }
 
 // Prepare processes the build configuration parameters.
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 // Run executes a Packer build and returns a packer.Artifact representing
diff --git a/builder/vmware/iso/builder.go b/builder/vmware/iso/builder.go
index 7e94d9663..9de2d9284 100644
--- a/builder/vmware/iso/builder.go
+++ b/builder/vmware/iso/builder.go
@@ -19,15 +19,15 @@ type Builder struct {
 }
 
 // Prepare processes the build configuration parameters.
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 
 	b.config = *c
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 func (b *Builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
diff --git a/builder/vmware/iso/builder_test.go b/builder/vmware/iso/builder_test.go
index 195f8e381..38441d633 100644
--- a/builder/vmware/iso/builder_test.go
+++ b/builder/vmware/iso/builder_test.go
@@ -33,7 +33,7 @@ func TestBuilder_ImplementsBuilder(t *testing.T) {
 func TestBuilderPrepare_Defaults(t *testing.T) {
 	var b Builder
 	config := testConfig()
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -63,7 +63,7 @@ func TestBuilderPrepare_DiskSize(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "disk_size")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -77,7 +77,7 @@ func TestBuilderPrepare_DiskSize(t *testing.T) {
 
 	config["disk_size"] = 60000
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -95,7 +95,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	config := testConfig()
 
 	delete(config, "floppy_files")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -110,7 +110,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	floppies_path := "../../../common/test-fixtures/floppies"
 	config["floppy_files"] = []string{fmt.Sprintf("%s/bar.bat", floppies_path), fmt.Sprintf("%s/foo.ps1", floppies_path)}
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -129,7 +129,7 @@ func TestBuilderPrepare_InvalidFloppies(t *testing.T) {
 	config := testConfig()
 	config["floppy_files"] = []string{"nonexistent.bat", "nonexistent.ps1"}
 	b = Builder{}
-	_, errs := b.Prepare(config)
+	_, _, errs := b.Prepare(config)
 	if errs == nil {
 		t.Fatalf("Nonexistent floppies should trigger multierror")
 	}
@@ -149,7 +149,7 @@ func TestBuilderPrepare_RemoteType(t *testing.T) {
 	config["skip_validate_credentials"] = true
 	// Bad
 	config["remote_type"] = "foobar"
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -161,7 +161,7 @@ func TestBuilderPrepare_RemoteType(t *testing.T) {
 	// Bad
 	config["remote_host"] = ""
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -176,7 +176,7 @@ func TestBuilderPrepare_RemoteType(t *testing.T) {
 	config["remote_password"] = ""
 	config["remote_private_key_file"] = ""
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -189,7 +189,7 @@ func TestBuilderPrepare_RemoteType(t *testing.T) {
 	config["remote_host"] = "foobar.example.com"
 	config["remote_password"] = "supersecret"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -207,7 +207,7 @@ func TestBuilderPrepare_RemoteExport(t *testing.T) {
 	config["skip_validate_credentials"] = true
 	// Bad
 	config["remote_password"] = ""
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) != 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -218,7 +218,7 @@ func TestBuilderPrepare_RemoteExport(t *testing.T) {
 	// Good
 	config["remote_password"] = "supersecret"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) != 0 {
 		t.Fatalf("err: %s", err)
 	}
@@ -233,7 +233,7 @@ func TestBuilderPrepare_Format(t *testing.T) {
 
 	// Bad
 	config["format"] = "foobar"
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -252,7 +252,7 @@ func TestBuilderPrepare_Format(t *testing.T) {
 		config["skip_validate_credentials"] = true
 
 		b = Builder{}
-		warns, err = b.Prepare(config)
+		_, warns, err = b.Prepare(config)
 		if len(warns) > 0 {
 			t.Fatalf("bad: %#v", warns)
 		}
@@ -268,7 +268,7 @@ func TestBuilderPrepare_InvalidKey(t *testing.T) {
 
 	// Add a random key
 	config["i_should_not_be_valid"] = true
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -290,7 +290,7 @@ func TestBuilderPrepare_OutputDir(t *testing.T) {
 
 	config["output_directory"] = dir
 	b = Builder{}
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -301,7 +301,7 @@ func TestBuilderPrepare_OutputDir(t *testing.T) {
 	// Test with a good one
 	config["output_directory"] = "i-hope-i-dont-exist"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -316,7 +316,7 @@ func TestBuilderPrepare_ToolsUploadPath(t *testing.T) {
 
 	// Test a default
 	delete(config, "tools_upload_path")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -331,7 +331,7 @@ func TestBuilderPrepare_ToolsUploadPath(t *testing.T) {
 	// Test with a bad value
 	config["tools_upload_path"] = "{{{nope}"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -342,7 +342,7 @@ func TestBuilderPrepare_ToolsUploadPath(t *testing.T) {
 	// Test with a good one
 	config["tools_upload_path"] = "hey"
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -357,7 +357,7 @@ func TestBuilderPrepare_VMXTemplatePath(t *testing.T) {
 
 	// Test bad
 	config["vmx_template_path"] = "/i/dont/exist/forreal"
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -379,7 +379,7 @@ func TestBuilderPrepare_VMXTemplatePath(t *testing.T) {
 
 	config["vmx_template_path"] = tf.Name()
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -401,7 +401,7 @@ func TestBuilderPrepare_VMXTemplatePath(t *testing.T) {
 
 	config["vmx_template_path"] = tf2.Name()
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -417,7 +417,7 @@ func TestBuilderPrepare_VNCPort(t *testing.T) {
 	// Bad
 	config["vnc_port_min"] = 1000
 	config["vnc_port_max"] = 500
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -428,7 +428,7 @@ func TestBuilderPrepare_VNCPort(t *testing.T) {
 	// Bad
 	config["vnc_port_min"] = -500
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -440,7 +440,7 @@ func TestBuilderPrepare_VNCPort(t *testing.T) {
 	config["vnc_port_min"] = 500
 	config["vnc_port_max"] = 1000
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -458,7 +458,7 @@ func TestBuilderCheckCollisions(t *testing.T) {
 	}
 	{
 		var b Builder
-		warns, _ := b.Prepare(config)
+		_, warns, _ := b.Prepare(config)
 		if len(warns) != 1 {
 			t.Fatalf("Should have warning about two collisions.")
 		}
@@ -466,7 +466,7 @@ func TestBuilderCheckCollisions(t *testing.T) {
 	{
 		config["vmx_template_path"] = "some/path.vmx"
 		var b Builder
-		warns, _ := b.Prepare(config)
+		_, warns, _ := b.Prepare(config)
 		if len(warns) != 0 {
 			t.Fatalf("Should not check for collisions with custom template.")
 		}
@@ -484,7 +484,7 @@ func TestBuilderPrepare_CommConfig(t *testing.T) {
 		config["winrm_host"] = "1.2.3.4"
 
 		var b Builder
-		warns, err := b.Prepare(config)
+		_, warns, err := b.Prepare(config)
 		if len(warns) > 0 {
 			t.Fatalf("bad: %#v", warns)
 		}
@@ -512,7 +512,7 @@ func TestBuilderPrepare_CommConfig(t *testing.T) {
 		config["ssh_host"] = "1.2.3.4"
 
 		var b Builder
-		warns, err := b.Prepare(config)
+		_, warns, err := b.Prepare(config)
 		if len(warns) > 0 {
 			t.Fatalf("bad: %#v", warns)
 		}
diff --git a/builder/vmware/vmx/builder.go b/builder/vmware/vmx/builder.go
index e1e33dd7c..f33f14ffa 100644
--- a/builder/vmware/vmx/builder.go
+++ b/builder/vmware/vmx/builder.go
@@ -22,14 +22,14 @@ type Builder struct {
 }
 
 // Prepare processes the build configuration parameters.
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
 
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 // Run executes a Packer build and returns a packer.Artifact representing
diff --git a/builder/vmware/vmx/builder_test.go b/builder/vmware/vmx/builder_test.go
index 841d4f718..4e2a165a7 100644
--- a/builder/vmware/vmx/builder_test.go
+++ b/builder/vmware/vmx/builder_test.go
@@ -24,7 +24,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	config["source_path"] = tf.Name()
 
 	delete(config, "floppy_files")
-	warns, err := b.Prepare(config)
+	_, warns, err := b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -39,7 +39,7 @@ func TestBuilderPrepare_FloppyFiles(t *testing.T) {
 	floppies_path := "../../../common/test-fixtures/floppies"
 	config["floppy_files"] = []string{fmt.Sprintf("%s/bar.bat", floppies_path), fmt.Sprintf("%s/foo.ps1", floppies_path)}
 	b = Builder{}
-	warns, err = b.Prepare(config)
+	_, warns, err = b.Prepare(config)
 	if len(warns) > 0 {
 		t.Fatalf("bad: %#v", warns)
 	}
@@ -58,7 +58,7 @@ func TestBuilderPrepare_InvalidFloppies(t *testing.T) {
 	config := testConfig(t)
 	config["floppy_files"] = []string{"nonexistent.bat", "nonexistent.ps1"}
 	b = Builder{}
-	_, errs := b.Prepare(config)
+	_, _, errs := b.Prepare(config)
 	if errs == nil {
 		t.Fatalf("Nonexistent floppies should trigger multierror")
 	}
diff --git a/builder/yandex/builder.go b/builder/yandex/builder.go
index 69bb9469f..e59ae9b25 100644
--- a/builder/yandex/builder.go
+++ b/builder/yandex/builder.go
@@ -24,13 +24,13 @@ type Builder struct {
 }
 
 // Prepare processes the build configuration parameters.
-func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+func (b *Builder) Prepare(raws ...interface{}) ([]string, []string, error) {
 	c, warnings, errs := NewConfig(raws...)
 	if errs != nil {
-		return warnings, errs
+		return nil, warnings, errs
 	}
 	b.config = c
-	return warnings, nil
+	return nil, warnings, nil
 }
 
 // Run executes a yandex Packer build and returns a packer.Artifact
diff --git a/command/build_parallel_test.go b/command/build_parallel_test.go
index a2dc240ae..a6228be32 100644
--- a/command/build_parallel_test.go
+++ b/command/build_parallel_test.go
@@ -28,7 +28,9 @@ type ParallelTestBuilder struct {
 	wg sync.WaitGroup
 }
 
-func (b *ParallelTestBuilder) Prepare(raws ...interface{}) ([]string, error) { return nil, nil }
+func (b *ParallelTestBuilder) Prepare(raws ...interface{}) ([]string, []string, error) {
+	return nil, nil, nil
+}
 
 func (b *ParallelTestBuilder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
 	ui.Say("building")
@@ -39,7 +41,7 @@ func (b *ParallelTestBuilder) Run(ctx context.Context, ui packer.Ui, hook packer
 // LockedBuilder wont run until unlock is called
 type LockedBuilder struct{ unlock chan interface{} }
 
-func (b *LockedBuilder) Prepare(raws ...interface{}) ([]string, error) { return nil, nil }
+func (b *LockedBuilder) Prepare(raws ...interface{}) ([]string, []string, error) { return nil, nil, nil }
 
 func (b *LockedBuilder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
 	ui.Say("locking build")
diff --git a/common/shell-local/config.go b/common/shell-local/config.go
index 46f2f8088..500cf0132 100644
--- a/common/shell-local/config.go
+++ b/common/shell-local/config.go
@@ -5,6 +5,7 @@ package shell_local
 import (
 	"errors"
 	"fmt"
+	// "log"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -49,10 +50,6 @@ type Config struct {
 }
 
 func Decode(config *Config, raws ...interface{}) error {
-	// Create passthrough for build-generated data so we can fill it in once we know
-	// it
-	config.ctx.Data = packer.BasicPlaceholderData()
-
 	err := configHelper.Decode(&config, &configHelper.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &config.ctx,
@@ -63,7 +60,8 @@ func Decode(config *Config, raws ...interface{}) error {
 		},
 	}, raws...)
 	if err != nil {
-		return fmt.Errorf("Error decoding config: %s, config is %#v, and raws is %#v", err, config, raws)
+		return fmt.Errorf("Error decoding config: %s", err)
+		// return fmt.Errorf("Error decoding config: %s, config is %#v, and raws is %#v", err, config, raws)
 	}
 
 	return nil
diff --git a/helper/config/decode.go b/helper/config/decode.go
index 43673e1dc..5aefadcb4 100644
--- a/helper/config/decode.go
+++ b/helper/config/decode.go
@@ -41,9 +41,11 @@ func Decode(target interface{}, config *DecodeOpts, raws ...interface{}) error {
 		config = &DecodeOpts{Interpolate: true}
 	}
 
+	// Detect user variables from the raws and merge them into our context
+	ctxData, raws := DetectContextData(raws...)
+
 	// Interpolate first
 	if config.Interpolate {
-		// Detect user variables from the raws and merge them into our context
 		ctx, err := DetectContext(raws...)
 		if err != nil {
 			return err
@@ -55,6 +57,9 @@ func Decode(target interface{}, config *DecodeOpts, raws ...interface{}) error {
 			config.InterpolateContext.BuildType = ctx.BuildType
 			config.InterpolateContext.TemplatePath = ctx.TemplatePath
 			config.InterpolateContext.UserVariables = ctx.UserVariables
+			if config.InterpolateContext.Data == nil {
+				config.InterpolateContext.Data = ctxData
+			}
 		}
 		ctx = config.InterpolateContext
 
@@ -103,7 +108,7 @@ func Decode(target interface{}, config *DecodeOpts, raws ...interface{}) error {
 		for _, unused := range md.Unused {
 			if unused != "type" && !strings.HasPrefix(unused, "packer_") {
 				err = multierror.Append(err, fmt.Errorf(
-					"unknown configuration key: %q", unused))
+					"unknown configuration key: %q; raws is %#v \n\n and ctx data is %#v", unused, raws, ctxData))
 			}
 		}
 		if err != nil {
@@ -114,6 +119,39 @@ func Decode(target interface{}, config *DecodeOpts, raws ...interface{}) error {
 	return nil
 }
 
+func DetectContextData(raws ...interface{}) (map[interface{}]interface{}, []interface{}) {
+	// In provisioners, the last value pulled from raws is the placeholder data
+	// for build-specific variables. Pull these out to add to interpolation
+	// context.
+
+	// Internally, our tests may cause this to be read as a map[string]string
+	placeholderData := raws[len(raws)-1]
+	if pd, ok := placeholderData.(map[string]string); ok {
+		if uuid, ok := pd["PackerRunUUID"]; ok {
+			if strings.Contains(uuid, "Generated_PackerRunUUID.") {
+				cast := make(map[interface{}]interface{})
+				for k, v := range pd {
+					cast[k] = v
+				}
+				raws = raws[:len(raws)-1]
+				return cast, raws
+			}
+		}
+	}
+
+	// but with normal interface conversion across the rpc, it'll look like a
+	// map[interface]interface, not a map[string]string
+	if pd, ok := placeholderData.(map[interface{}]interface{}); ok {
+		if uuid, ok := pd["PackerRunUUID"]; ok {
+			if strings.Contains(uuid.(string), "Generated_PackerRunUUID.") {
+				raws = raws[:len(raws)-1]
+				return pd, raws
+			}
+		}
+	}
+	return nil, raws
+}
+
 // DetectContext builds a base interpolate.Context, automatically
 // detecting things like user variables from the raw configuration params.
 func DetectContext(raws ...interface{}) (*interpolate.Context, error) {
diff --git a/packer/build.go b/packer/build.go
index badd67c1f..9c83c8992 100644
--- a/packer/build.go
+++ b/packer/build.go
@@ -5,6 +5,8 @@ import (
 	"fmt"
 	"log"
 	"sync"
+
+	"github.com/hashicorp/packer/helper/common"
 )
 
 const (
@@ -148,17 +150,30 @@ func (b *coreBuild) Prepare() (warn []string, err error) {
 	}
 
 	// Prepare the builder
-	warn, err = b.builder.Prepare(b.builderConfig, packerConfig)
+	generatedVars, warn, err := b.builder.Prepare(b.builderConfig, packerConfig)
 	if err != nil {
 		log.Printf("Build '%s' prepare failure: %s\n", b.name, err)
 		return
 	}
 
+	// If the builder has provided a list of to-be-generated variables that
+	// should be made accessible to provisioners, pass that list into
+	// the provisioner prepare() so that the provisioner can appropriately
+	// validate user input against what will become available.
+	generatedPlaceholderMap := BasicPlaceholderData()
+	if generatedVars != nil {
+		for _, k := range generatedVars {
+			generatedPlaceholderMap[k] = fmt.Sprintf("Generated_%s. "+
+				common.PlaceholderMsg, k)
+		}
+	}
+
 	// Prepare the provisioners
 	for _, coreProv := range b.provisioners {
 		configs := make([]interface{}, len(coreProv.config), len(coreProv.config)+1)
 		copy(configs, coreProv.config)
 		configs = append(configs, packerConfig)
+		configs = append(configs, generatedPlaceholderMap)
 
 		if err = coreProv.provisioner.Prepare(configs...); err != nil {
 			return
@@ -170,6 +185,7 @@ func (b *coreBuild) Prepare() (warn []string, err error) {
 		configs := make([]interface{}, len(b.cleanupProvisioner.config), len(b.cleanupProvisioner.config)+1)
 		copy(configs, b.cleanupProvisioner.config)
 		configs = append(configs, packerConfig)
+		configs = append(configs, generatedPlaceholderMap)
 		err = b.cleanupProvisioner.provisioner.Prepare(configs...)
 		if err != nil {
 			return
diff --git a/packer/build_test.go b/packer/build_test.go
index 0c58b405d..8102c4e67 100644
--- a/packer/build_test.go
+++ b/packer/build_test.go
@@ -69,7 +69,7 @@ func TestBuild_Prepare(t *testing.T) {
 	if !prov.PrepCalled {
 		t.Fatal("prep should be called")
 	}
-	if !reflect.DeepEqual(prov.PrepConfigs, []interface{}{42, packerConfig}) {
+	if !reflect.DeepEqual(prov.PrepConfigs, []interface{}{42, packerConfig, BasicPlaceholderData()}) {
 		t.Fatalf("bad: %#v", prov.PrepConfigs)
 	}
 
@@ -144,7 +144,7 @@ func TestBuild_Prepare_Debug(t *testing.T) {
 	if !prov.PrepCalled {
 		t.Fatal("prepare should be called")
 	}
-	if !reflect.DeepEqual(prov.PrepConfigs, []interface{}{42, packerConfig}) {
+	if !reflect.DeepEqual(prov.PrepConfigs, []interface{}{42, packerConfig, BasicPlaceholderData()}) {
 		t.Fatalf("bad: %#v", prov.PrepConfigs)
 	}
 }
diff --git a/packer/builder.go b/packer/builder.go
index e24073128..349f94ba8 100644
--- a/packer/builder.go
+++ b/packer/builder.go
@@ -25,9 +25,10 @@ type Builder interface {
 	// Each of the configuration values should merge into the final
 	// configuration.
 	//
-	// Prepare should return a list of warnings along with any errors
-	// that occurred while preparing.
-	Prepare(...interface{}) ([]string, error)
+	// Prepare should return a list of variables that will be made accessible to
+	// users during the provison methods, a list of warnings along with any
+	// errors that occurred while preparing.
+	Prepare(...interface{}) ([]string, []string, error)
 
 	// Run is where the actual build should take place. It takes a Build and a Ui.
 	Run(context.Context, Ui, Hook) (Artifact, error)
diff --git a/packer/builder_mock.go b/packer/builder_mock.go
index 5ebd2d993..47dc5aec0 100644
--- a/packer/builder_mock.go
+++ b/packer/builder_mock.go
@@ -23,10 +23,10 @@ type MockBuilder struct {
 	RunFn         func(ctx context.Context)
 }
 
-func (tb *MockBuilder) Prepare(config ...interface{}) ([]string, error) {
+func (tb *MockBuilder) Prepare(config ...interface{}) ([]string, []string, error) {
 	tb.PrepareCalled = true
 	tb.PrepareConfig = config
-	return tb.PrepareWarnings, nil
+	return nil, tb.PrepareWarnings, nil
 }
 
 func (tb *MockBuilder) Run(ctx context.Context, ui Ui, h Hook) (Artifact, error) {
diff --git a/packer/plugin/builder.go b/packer/plugin/builder.go
index 44af7dd67..35665dc7b 100644
--- a/packer/plugin/builder.go
+++ b/packer/plugin/builder.go
@@ -12,7 +12,7 @@ type cmdBuilder struct {
 	client  *Client
 }
 
-func (b *cmdBuilder) Prepare(config ...interface{}) ([]string, error) {
+func (b *cmdBuilder) Prepare(config ...interface{}) ([]string, []string, error) {
 	defer func() {
 		r := recover()
 		b.checkExit(r, nil)
diff --git a/packer/provisioner.go b/packer/provisioner.go
index d20e4d360..b7f729bc3 100644
--- a/packer/provisioner.go
+++ b/packer/provisioner.go
@@ -64,7 +64,7 @@ func BasicPlaceholderData() map[string]string {
 	placeholderData["User"] = fmt.Sprintf(msg, "User")
 	placeholderData["Password"] = fmt.Sprintf(msg, "Password")
 	placeholderData["ConnType"] = fmt.Sprintf(msg, "Type")
-	placeholderData["PACKER_RUN_UUID"] = fmt.Sprintf(msg, "PACKER_RUN_UUID")
+	placeholderData["PackerRunUUID"] = fmt.Sprintf(msg, "PackerRunUUID")
 	placeholderData["SSHPublicKey"] = fmt.Sprintf(msg, "SSHPublicKey")
 	placeholderData["SSHPrivateKey"] = fmt.Sprintf(msg, "SSHPrivateKey")
 
@@ -75,6 +75,28 @@ func BasicPlaceholderData() map[string]string {
 	return placeholderData
 }
 
+func CastDataToMap(data interface{}) map[string]interface{} {
+	// Provisioners expect a map[string]interface{} in their data field, but
+	// it gets converted into a map[interface]interface on the way over the
+	// RPC. Check that data can be cast into such a form, and cast it.
+	cast := make(map[string]interface{})
+	interMap, ok := data.(map[interface{}]interface{})
+	if !ok {
+		log.Printf("Unable to read map[string]interface out of data."+
+			"Using empty interface: %#v", data)
+	} else {
+		for key, val := range interMap {
+			keyString, ok := key.(string)
+			if ok {
+				cast[keyString] = val
+			} else {
+				log.Printf("Error casting generated data key to a string.")
+			}
+		}
+	}
+	return cast
+}
+
 // Runs the provisioners in order.
 func (h *ProvisionHook) Run(ctx context.Context, name string, ui Ui, comm Communicator, data interface{}) error {
 	// Shortcut
@@ -91,25 +113,7 @@ func (h *ProvisionHook) Run(ctx context.Context, name string, ui Ui, comm Commun
 	for _, p := range h.Provisioners {
 		ts := CheckpointReporter.AddSpan(p.TypeName, "provisioner", p.Config)
 
-		// Provisioners expect a map[string]interface{} in their data field, but
-		// it gets converted into a map[interface]interface on the way over the
-		// RPC. Check that data can be cast into such a form, and cast it.
-		cast := make(map[string]interface{})
-		interMap, ok := data.(map[interface{}]interface{})
-		if !ok {
-			log.Printf("Unable to read map[string]interface out of data." +
-				"Using empty interface.")
-		} else {
-			for key, val := range interMap {
-				keyString, ok := key.(string)
-				if ok {
-					cast[keyString] = val
-				} else {
-					log.Printf("Error casting generated data key to a string.")
-				}
-			}
-		}
-
+		cast := CastDataToMap(data)
 		err := p.Provisioner.Provision(ctx, ui, comm, cast)
 
 		ts.End(err)
diff --git a/packer/rpc/builder.go b/packer/rpc/builder.go
index 4b95bea3e..5be4cc2bb 100644
--- a/packer/rpc/builder.go
+++ b/packer/rpc/builder.go
@@ -30,22 +30,23 @@ type BuilderPrepareArgs struct {
 }
 
 type BuilderPrepareResponse struct {
-	Warnings []string
-	Error    *BasicError
+	GeneratedVars []string
+	Warnings      []string
+	Error         *BasicError
 }
 
-func (b *builder) Prepare(config ...interface{}) ([]string, error) {
+func (b *builder) Prepare(config ...interface{}) ([]string, []string, error) {
 	var resp BuilderPrepareResponse
 	cerr := b.client.Call("Builder.Prepare", &BuilderPrepareArgs{config}, &resp)
 	if cerr != nil {
-		return nil, cerr
+		return nil, nil, cerr
 	}
 	var err error = nil
 	if resp.Error != nil {
 		err = resp.Error
 	}
 
-	return resp.Warnings, err
+	return resp.GeneratedVars, resp.Warnings, err
 }
 
 func (b *builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (packer.Artifact, error) {
@@ -87,10 +88,11 @@ func (b *builder) Run(ctx context.Context, ui packer.Ui, hook packer.Hook) (pack
 }
 
 func (b *BuilderServer) Prepare(args *BuilderPrepareArgs, reply *BuilderPrepareResponse) error {
-	warnings, err := b.builder.Prepare(args.Configs...)
+	generated, warnings, err := b.builder.Prepare(args.Configs...)
 	*reply = BuilderPrepareResponse{
-		Warnings: warnings,
-		Error:    NewBasicError(err),
+		GeneratedVars: generated,
+		Warnings:      warnings,
+		Error:         NewBasicError(err),
 	}
 	return nil
 }
diff --git a/packer/rpc/builder_test.go b/packer/rpc/builder_test.go
index 9d89239b0..75d442865 100644
--- a/packer/rpc/builder_test.go
+++ b/packer/rpc/builder_test.go
@@ -20,7 +20,7 @@ func TestBuilderPrepare(t *testing.T) {
 
 	// Test Prepare
 	config := 42
-	warnings, err := bClient.Prepare(config)
+	_, warnings, err := bClient.Prepare(config)
 	if err != nil {
 		t.Fatalf("bad: %s", err)
 	}
@@ -50,7 +50,7 @@ func TestBuilderPrepare_Warnings(t *testing.T) {
 	b.PrepareWarnings = expected
 
 	// Test Prepare
-	warnings, err := bClient.Prepare(nil)
+	_, warnings, err := bClient.Prepare(nil)
 	if err != nil {
 		t.Fatalf("bad: %s", err)
 	}
diff --git a/post-processor/checksum/post-processor_test.go b/post-processor/checksum/post-processor_test.go
index fce5fd1cf..e52974ec4 100644
--- a/post-processor/checksum/post-processor_test.go
+++ b/post-processor/checksum/post-processor_test.go
@@ -57,7 +57,7 @@ func setup(t *testing.T) (packer.Ui, packer.Artifact, error) {
 
 	// Prepare the file builder
 	builder := file.Builder{}
-	warnings, err := builder.Prepare(tpl.Builders["file"].Config)
+	_, warnings, err := builder.Prepare(tpl.Builders["file"].Config)
 	if len(warnings) > 0 {
 		for _, warn := range warnings {
 			return nil, nil, fmt.Errorf("Configuration warning: %s", warn)
diff --git a/post-processor/compress/post-processor_test.go b/post-processor/compress/post-processor_test.go
index 0f5e1a19e..5b4cfcd9f 100644
--- a/post-processor/compress/post-processor_test.go
+++ b/post-processor/compress/post-processor_test.go
@@ -197,7 +197,7 @@ func setup(t *testing.T) (packer.Ui, packer.Artifact, error) {
 
 	// Prepare the file builder
 	builder := file.Builder{}
-	warnings, err := builder.Prepare(tpl.Builders["file"].Config)
+	_, warnings, err := builder.Prepare(tpl.Builders["file"].Config)
 	if len(warnings) > 0 {
 		for _, warn := range warnings {
 			return nil, nil, fmt.Errorf("Configuration warning: %s", warn)
diff --git a/provisioner/ansible-local/provisioner_test.go b/provisioner/ansible-local/provisioner_test.go
index 180bd0dc9..1b8e5e34a 100644
--- a/provisioner/ansible-local/provisioner_test.go
+++ b/provisioner/ansible-local/provisioner_test.go
@@ -343,7 +343,7 @@ func testProvisionerProvisionDockerWithPlaybookFiles(t *testing.T, templateStrin
 
 	// Setup the builder
 	builder := &docker.Builder{}
-	warnings, err := builder.Prepare(tpl.Builders["docker"].Config)
+	_, warnings, err := builder.Prepare(tpl.Builders["docker"].Config)
 	if err != nil {
 		t.Fatalf("Error preparing configuration %s", err)
 	}
diff --git a/provisioner/ansible/provisioner.go b/provisioner/ansible/provisioner.go
index 201bfd086..4ae101f6a 100644
--- a/provisioner/ansible/provisioner.go
+++ b/provisioner/ansible/provisioner.go
@@ -80,9 +80,6 @@ type Provisioner struct {
 func (p *Provisioner) Prepare(raws ...interface{}) error {
 	p.done = make(chan struct{})
 
-	// Create passthrough for build-generated data
-	p.config.ctx.Data = packer.BasicPlaceholderData()
-
 	err := config.Decode(&p.config, &config.DecodeOpts{
 		Interpolate:        true,
 		InterpolateContext: &p.config.ctx,
diff --git a/template/interpolate/funcs.go b/template/interpolate/funcs.go
index b47ceb2dd..dfbaa9b50 100644
--- a/template/interpolate/funcs.go
+++ b/template/interpolate/funcs.go
@@ -166,21 +166,22 @@ func funcGenTemplateDir(ctx *Context) interface{} {
 
 func funcGenGenerated(ctx *Context) interface{} {
 	return func(s string) (string, error) {
-		if data, ok := ctx.Data.(map[string]string); ok {
+		if data, ok := ctx.Data.(map[interface{}]interface{}); ok {
 			// PlaceholderData has been passed into generator, so if the given
 			// key already exists in data, then we know it's an "allowed" key
 			if heldPlace, ok := data[s]; ok {
-				// If we're in the first interpolation pass, the goal is to
-				// make sure that we pass the value through.
-				// TODO match against an actual string constant
-				if strings.Contains(heldPlace, common.PlaceholderMsg) {
-					return fmt.Sprintf("{{.%s}}", s), nil
-				} else {
-					return heldPlace, nil
+				if hp, ok := heldPlace.(string); ok {
+					// If we're in the first interpolation pass, the goal is to
+					// make sure that we pass the value through.
+					// TODO match against an actual string constant
+					if strings.Contains(hp, common.PlaceholderMsg) {
+						return fmt.Sprintf("{{.%s}}", s), nil
+					} else {
+						return hp, nil
+					}
 				}
-			} else {
-				return "", fmt.Errorf("loaded data, but couldnt find %s in it.", s)
 			}
+			return "", fmt.Errorf("loaded data, but couldnt find %s in it.", s)
 		}
 
 		return "", fmt.Errorf("Error validating computed variable: the given "+