builder/amazon-chroot: nullable copy_files

Allow users to specify that they don't want any
files copied into the chroot by setting an empty
copy_files list

builder/amazon/chroot/builder.go

@@ -87,10 +87,6 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		b.config.ChrootMounts = make([][]string, 0)
 	}
 
-	if b.config.CopyFiles == nil {
-		b.config.CopyFiles = make([]string, 0)
-	}
-
 	if len(b.config.ChrootMounts) == 0 {
 		b.config.ChrootMounts = [][]string{
 			{"proc", "proc", "/proc"},
@@ -101,9 +97,13 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		}
 	}
 
-	if len(b.config.CopyFiles) == 0 && !b.config.FromScratch {
+	// set default copy file if we're not giving our own
+	if b.config.CopyFiles == nil {
+		b.config.CopyFiles = make([]string, 0)
+		if !b.config.FromScratch {
 			b.config.CopyFiles = []string{"/etc/resolv.conf"}
 		}
+	}
 
 	if b.config.CommandWrapper == "" {
 		b.config.CommandWrapper = "{{.Command}}"

builder/amazon/chroot/builder_test.go

@@ -1,8 +1,9 @@
 package chroot
 
 import (
-	"github.com/mitchellh/packer/packer"
 	"testing"
+
+	"github.com/mitchellh/packer/packer"
 )
 
 func testConfig() map[string]interface{} {
@@ -117,3 +118,33 @@ func TestBuilderPrepare_CommandWrapper(t *testing.T) {
 		t.Errorf("err: %s", err)
 	}
 }
+
+func TestBuilderPrepare_CopyFiles(t *testing.T) {
+	b := &Builder{}
+	config := testConfig()
+
+	warnings, err := b.Prepare(config)
+	if len(warnings) > 0 {
+		t.Fatalf("bad: %#v", warnings)
+	}
+	if err != nil {
+		t.Errorf("err: %s", err)
+	}
+
+	if len(b.config.CopyFiles) != 1 && b.config.CopyFiles[0] != "/etc/resolv.conf" {
+		t.Errorf("Was expecting default value for copy_files.")
+	}
+
+	config["copy_files"] = []string{}
+	warnings, err = b.Prepare(config)
+	if len(warnings) > 0 {
+		t.Fatalf("bad: %#v", warnings)
+	}
+	if err != nil {
+		t.Errorf("err: %s", err)
+	}
+
+	if len(b.config.CopyFiles) > 0 {
+		t.Errorf("Was expecting no default value for copy_files.")
+	}
+}