while I'm at it, kill this race condition in uploads, too

packer/rpc/communicator.go

@@ -132,7 +132,12 @@ func (c *communicator) Start(cmd *packer.RemoteCmd) (err error) {
 func (c *communicator) Upload(path string, r io.Reader, fi *os.FileInfo) (err error) {
 	// Pipe the reader through to the connection
 	streamId := c.mux.NextId()
-	go serveSingleCopy("uploadData", c.mux, streamId, nil, r)
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		serveSingleCopy("uploadData", c.mux, streamId, nil, r)
+	}()
 
 	args := CommunicatorUploadArgs{
 		Path:           path,
@@ -144,6 +149,7 @@ func (c *communicator) Upload(path string, r io.Reader, fi *os.FileInfo) (err er
 	}
 
 	err = c.client.Call("Communicator.Upload", &args, new(interface{}))
+	wg.Wait()
 	return
 }