packer: Builders can take multiple configs

packer/build_test.go

@@ -40,7 +40,7 @@ func TestBuild_Prepare(t *testing.T) {
 
 	build.Prepare()
 	assert.True(builder.prepareCalled, "prepare should be called")
-	assert.Equal(builder.prepareConfig, 42, "prepare config should be 42")
+	assert.Equal(builder.prepareConfig, []interface{}{42}, "prepare config should be 42")
 
 	// Verify provisioners were prepared
 	coreProv := coreB.provisioners[0]

packer/builder.go

@@ -3,11 +3,18 @@ package packer
 // Implementers of Builder are responsible for actually building images
 // on some platform given some configuration.
 type Builder interface {
-	// Prepare is responsible for reading in some configuration, in the raw form
-	// of map[string]interface{}, and storing that state for use later. Any setup
-	// should be done in this method. Note that NO side effects should really take
-	// place in prepare. It is meant as a state setup step only.
-	Prepare(config interface{}) error
+	// Prepare is responsible for configuring the builder and validating
+	// that configuration. Any setup should be done in this method. Note that
+	// NO side effects should take place in prepare, it is meant as a state
+	// setup only. Calling Prepare is not necessarilly followed by a Run.
+	//
+	// The parameters to Prepare are a set of interface{} values of the
+	// configuration. These are almost always `map[string]interface{}`
+	// parsed from a template, but no guarantee is made.
+	//
+	// Each of the configuration values should merge into the final
+	// configuration.
+	Prepare(...interface{}) error
 
 	// Run is where the actual build should take place. It takes a Build and a Ui.
 	Run(ui Ui, hook Hook, cache Cache) (Artifact, error)

packer/builder_test.go

@@ -2,7 +2,7 @@ package packer
 
 type TestBuilder struct {
 	prepareCalled bool
-	prepareConfig interface{}
+	prepareConfig []interface{}
 	runCalled     bool
 	runCache      Cache
 	runHook       Hook
@@ -10,7 +10,7 @@ type TestBuilder struct {
 	cancelCalled  bool
 }
 
-func (tb *TestBuilder) Prepare(config interface{}) error {
+func (tb *TestBuilder) Prepare(config ...interface{}) error {
 	tb.prepareCalled = true
 	tb.prepareConfig = config
 	return nil

packer/plugin/builder.go

@@ -10,13 +10,13 @@ type cmdBuilder struct {
 	client  *Client
 }
 
-func (b *cmdBuilder) Prepare(config interface{}) error {
+func (b *cmdBuilder) Prepare(config ...interface{}) error {
 	defer func() {
 		r := recover()
 		b.checkExit(r, nil)
 	}()
 
-	return b.builder.Prepare(config)
+	return b.builder.Prepare(config...)
 }
 
 func (b *cmdBuilder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packer.Artifact, error) {

packer/plugin/builder_test.go

@@ -8,7 +8,7 @@ import (
 
 type helperBuilder byte
 
-func (helperBuilder) Prepare(interface{}) error {
+func (helperBuilder) Prepare(...interface{}) error {
 	return nil
 }
 

packer/rpc/builder.go

@@ -21,7 +21,7 @@ type BuilderServer struct {
 }
 
 type BuilderPrepareArgs struct {
-	Config interface{}
+	Configs []interface{}
 }
 
 type BuilderRunArgs struct {
@@ -38,7 +38,7 @@ func Builder(client *rpc.Client) *builder {
 	return &builder{client}
 }
 
-func (b *builder) Prepare(config interface{}) (err error) {
+func (b *builder) Prepare(config ...interface{}) (err error) {
 	cerr := b.client.Call("Builder.Prepare", &BuilderPrepareArgs{config}, &err)
 	if cerr != nil {
 		err = cerr
@@ -109,7 +109,7 @@ func (b *builder) Cancel() {
 }
 
 func (b *BuilderServer) Prepare(args *BuilderPrepareArgs, reply *error) error {
-	err := b.builder.Prepare(args.Config)
+	err := b.builder.Prepare(args.Configs...)
 	if err != nil {
 		*reply = NewBasicError(err)
 	}

packer/rpc/builder_test.go

@@ -12,7 +12,7 @@ var testBuilderArtifact = &testArtifact{}
 
 type testBuilder struct {
 	prepareCalled bool
-	prepareConfig interface{}
+	prepareConfig []interface{}
 	runCalled     bool
 	runCache      packer.Cache
 	runHook       packer.Hook
@@ -23,7 +23,7 @@ type testBuilder struct {
 	nilRunResult bool
 }
 
-func (b *testBuilder) Prepare(config interface{}) error {
+func (b *testBuilder) Prepare(config ...interface{}) error {
 	b.prepareCalled = true
 	b.prepareConfig = config
 	return nil
@@ -68,7 +68,7 @@ func TestBuilderRPC(t *testing.T) {
 	bClient := Builder(client)
 	bClient.Prepare(config)
 	assert.True(b.prepareCalled, "prepare should be called")
-	assert.Equal(b.prepareConfig, 42, "prepare should be called with right arg")
+	assert.Equal(b.prepareConfig, []interface{}{42}, "prepare should be called with right arg")
 
 	// Test Run
 	cache := new(testCache)