packer/rpc: Return proper nil artifact if nil is returned

packer/rpc/builder.go

@@ -84,7 +84,12 @@ func (b *builder) Run(ui packer.Ui, hook packer.Hook) packer.Artifact {
 		panic(err)
 	}
 
-	client, err := rpc.Dial("tcp", <-artifactAddress)
+	address := <-artifactAddress
+	if address == "" {
+		return nil
+	}
+
+	client, err := rpc.Dial("tcp", address)
 	if err != nil {
 		panic(err)
 	}
@@ -127,12 +132,16 @@ func (b *BuilderServer) Run(args *BuilderRunArgs, reply *interface{}) error {
 		hook := Hook(client)
 		ui := &Ui{client}
 		artifact := b.builder.Run(ui, hook)
+		responseAddress := ""
 
+		if artifact != nil {
 			// Wrap the artifact
 			server := rpc.NewServer()
 			RegisterArtifact(server, artifact)
+			responseAddress = serveSingleConn(server)
+		}
 
-		responseWriter.Encode(&BuilderRunResponse{serveSingleConn(server)})
+		responseWriter.Encode(&BuilderRunResponse{responseAddress})
 	}()
 
 	return nil

packer/rpc/builder_test.go

@@ -16,6 +16,8 @@ type testBuilder struct {
 	runHook       packer.Hook
 	runUi         packer.Ui
 	cancelCalled  bool
+
+	nilRunResult bool
 }
 
 func (b *testBuilder) Prepare(config interface{}) error {
@@ -28,7 +30,12 @@ func (b *testBuilder) Run(ui packer.Ui, hook packer.Hook) packer.Artifact {
 	b.runCalled = true
 	b.runHook = hook
 	b.runUi = ui
+
+	if !b.nilRunResult {
 		return testBuilderArtifact
+	} else {
+		return nil
+	}
 }
 
 func (b *testBuilder) Cancel() {
@@ -74,6 +81,11 @@ func TestBuilderRPC(t *testing.T) {
 		assert.Equal(artifact.Id(), testBuilderArtifact.Id(), "should have artifact Id")
 	}
 
+	// Test run with nil result
+	b.nilRunResult = true
+	artifact = bClient.Run(ui, hook)
+	assert.Nil(artifact, "should be nil")
+
 	// Test Cancel
 	bClient.Cancel()
 	assert.True(b.cancelCalled, "cancel should be called")