packer: Builder.Prepare can return an error

builder/amazonebs/builder.go

@@ -15,6 +15,8 @@ type Builder struct {
 	config config
 }
 
-func (*Builder) Prepare(interface{}) {}
+func (*Builder) Prepare(interface{}) error {
+	return nil
+}
 
 func (*Builder) Run(packer.Build, packer.Ui) {}

packer/build.go

@@ -3,7 +3,7 @@ package packer
 // A Build represents a single job within Packer that is responsible for
 // building some machine image artifact. Builds are meant to be parallelized.
 type Build interface {
-	Prepare()
+	Prepare() error
 	Run(ui Ui)
 }
 
@@ -29,15 +29,15 @@ type coreBuild struct {
 //
 // Run is where the actual build should take place. It takes a Build and a Ui.
 type Builder interface {
-	Prepare(config interface{})
+	Prepare(config interface{}) error
 	Run(build Build, ui Ui)
 }
 
 // Prepare prepares the build by doing some initialization for the builder
 // and any hooks. This _must_ be called prior to Run.
-func (b *coreBuild) Prepare() {
+func (b *coreBuild) Prepare() error {
 	b.prepareCalled = true
-	b.builder.Prepare(b.rawConfig)
+	return b.builder.Prepare(b.rawConfig)
 }
 
 // Runs the actual build. Prepare must be called prior to running this.

packer/build_test.go

@@ -13,9 +13,10 @@ type TestBuilder struct {
 	runUi Ui
 }
 
-func (tb *TestBuilder) Prepare(config interface{}) {
+func (tb *TestBuilder) Prepare(config interface{}) error {
 	tb.prepareCalled = true
 	tb.prepareConfig = config
+	return nil
 }
 
 func (tb *TestBuilder) Run(b Build, ui Ui) {

packer/plugin/builder.go

@@ -13,13 +13,13 @@ type cmdBuilder struct {
 	client *client
 }
 
-func (b *cmdBuilder) Prepare(config interface{}) {
+func (b *cmdBuilder) Prepare(config interface{}) error {
 	defer func() {
 		r := recover()
 		b.checkExit(r, nil)
 	}()
 
-	b.builder.Prepare(config)
+	return b.builder.Prepare(config)
 }
 
 func (b *cmdBuilder) Run(build packer.Build, ui packer.Ui) {

packer/plugin/builder_test.go

@@ -9,7 +9,9 @@ import (
 
 type helperBuilder byte
 
-func (helperBuilder) Prepare(interface{}) {}
+func (helperBuilder) Prepare(interface{}) error {
+	return nil
+}
 
 func (helperBuilder) Run(packer.Build, packer.Ui) {}
 

packer/rpc/build.go

@@ -23,8 +23,9 @@ type BuildRunArgs struct {
 	UiRPCAddress string
 }
 
-func (b *Build) Prepare() {
-	b.client.Call("Build.Prepare", new(interface{}), new(interface{}))
+func (b *Build) Prepare() (err error) {
+	b.client.Call("Build.Prepare", new(interface{}), &err)
+	return
 }
 
 func (b *Build) Run(ui packer.Ui) {
@@ -36,10 +37,8 @@ func (b *Build) Run(ui packer.Ui) {
 	b.client.Call("Build.Run", args, new(interface{}))
 }
 
-func (b *BuildServer) Prepare(args *BuildPrepareArgs, reply *interface{}) error {
-	b.build.Prepare()
-
-	*reply = nil
+func (b *BuildServer) Prepare(args *BuildPrepareArgs, reply *error) error {
+	*reply = b.build.Prepare()
 	return nil
 }
 

packer/rpc/build_test.go

@@ -13,8 +13,9 @@ type testBuild struct {
 	runUi packer.Ui
 }
 
-func (b *testBuild) Prepare() {
+func (b *testBuild) Prepare() error {
 	b.prepareCalled = true
+	return nil
 }
 
 func (b *testBuild) Run(ui packer.Ui) {

packer/rpc/builder.go

@@ -29,8 +29,9 @@ func Builder(client *rpc.Client) *builder {
 	return &builder{client}
 }
 
-func (b *builder) Prepare(config interface{}) {
-	b.client.Call("Builder.Prepare", &BuilderPrepareArgs{config}, new(interface{}))
+func (b *builder) Prepare(config interface{}) (err error) {
+	b.client.Call("Builder.Prepare", &BuilderPrepareArgs{config}, &err)
+	return
 }
 
 func (b *builder) Run(build packer.Build, ui packer.Ui) {
@@ -44,9 +45,8 @@ func (b *builder) Run(build packer.Build, ui packer.Ui) {
 	b.client.Call("Builder.Run", args, new(interface{}))
 }
 
-func (b *BuilderServer) Prepare(args *BuilderPrepareArgs, reply *interface{}) error {
-	b.builder.Prepare(args.Config)
-	*reply = nil
+func (b *BuilderServer) Prepare(args *BuilderPrepareArgs, reply *error) error {
+	*reply = b.builder.Prepare(args.Config)
 	return nil
 }
 

packer/rpc/builder_test.go

@@ -15,9 +15,10 @@ type testBuilder struct {
 	runUi packer.Ui
 }
 
-func (b *testBuilder) Prepare(config interface{}) {
+func (b *testBuilder) Prepare(config interface{}) error {
 	b.prepareCalled = true
 	b.prepareConfig = config
+	return nil
 }
 
 func (b *testBuilder) Run(build packer.Build, ui packer.Ui) {