packer/rpc: fix interface returns for msgpack [GH-1076]

packer/rpc/build.go

@@ -21,7 +21,7 @@ type BuildServer struct {
 
 type BuildPrepareResponse struct {
 	Warnings []string
-	Error    error
+	Error    *BasicError
 }
 
 func (b *build) Name() (result string) {
@@ -34,8 +34,12 @@ func (b *build) Prepare() ([]string, error) {
 	if cerr := b.client.Call("Build.Prepare", new(interface{}), &resp); cerr != nil {
 		return nil, cerr
 	}
+	var err error = nil
+	if resp.Error != nil {
+		err = resp.Error
+	}
 
-	return resp.Warnings, resp.Error
+	return resp.Warnings, err
 }
 
 func (b *build) Run(ui packer.Ui, cache packer.Cache) ([]packer.Artifact, error) {
@@ -90,7 +94,7 @@ func (b *BuildServer) Prepare(args *interface{}, resp *BuildPrepareResponse) err
 	warnings, err := b.build.Prepare()
 	*resp = BuildPrepareResponse{
 		Warnings: warnings,
-		Error:    err,
+		Error:    NewBasicError(err),
 	}
 	return nil
 }

packer/rpc/post_processor.go

@@ -25,14 +25,14 @@ type PostProcessorConfigureArgs struct {
 }
 
 type PostProcessorProcessResponse struct {
-	Err      error
+	Err      *BasicError
 	Keep     bool
 	StreamId uint32
 }
 
 func (p *postProcessor) Configure(raw ...interface{}) (err error) {
 	args := &PostProcessorConfigureArgs{Configs: raw}
-	if cerr := p.client.Call("PostProcessor.Configure", args, &err); cerr != nil {
+	if cerr := p.client.Call("PostProcessor.Configure", args, new(interface{})); cerr != nil {
 		err = cerr
 	}
 
@@ -67,13 +67,9 @@ func (p *postProcessor) PostProcess(ui packer.Ui, a packer.Artifact) (packer.Art
 	return client.Artifact(), response.Keep, nil
 }
 
-func (p *PostProcessorServer) Configure(args *PostProcessorConfigureArgs, reply *error) error {
-	*reply = p.p.Configure(args.Configs...)
-	if *reply != nil {
-		*reply = NewBasicError(*reply)
-	}
-
-	return nil
+func (p *PostProcessorServer) Configure(args *PostProcessorConfigureArgs, reply *interface{}) error {
+	err := p.p.Configure(args.Configs...)
+	return err
 }
 
 func (p *PostProcessorServer) PostProcess(streamId uint32, reply *PostProcessorProcessResponse) error {
@@ -92,12 +88,8 @@ func (p *PostProcessorServer) PostProcess(streamId uint32, reply *PostProcessorP
 		go server.Serve()
 	}
 
-	if err != nil {
-		err = NewBasicError(err)
-	}
-
 	*reply = PostProcessorProcessResponse{
-		Err:      err,
+		Err:      NewBasicError(err),
 		Keep:     keep,
 		StreamId: streamId,
 	}