packer/plugin: fix data race reading stdout in Client

packer/plugin/client.go

@@ -2,7 +2,6 @@ package plugin
 
 import (
 	"bufio"
-	"bytes"
 	"errors"
 	"fmt"
 	"github.com/mitchellh/packer/packer"
@@ -222,7 +221,7 @@ func (c *Client) Start() (address string, err error) {
 		fmt.Sprintf("PACKER_PLUGIN_MAX_PORT=%d", c.config.MaxPort),
 	}
 
-	stdout := new(bytes.Buffer)
+	stdout_r, stdout_w := io.Pipe()
 	stderr_r, stderr_w := io.Pipe()
 
 	cmd := c.config.Cmd
@@ -230,7 +229,7 @@ func (c *Client) Start() (address string, err error) {
 	cmd.Env = append(cmd.Env, env...)
 	cmd.Stdin = os.Stdin
 	cmd.Stderr = stderr_w
-	cmd.Stdout = stdout
+	cmd.Stdout = stdout_w
 
 	log.Printf("Starting plugin: %s %#v", cmd.Path, cmd.Args)
 	err = cmd.Start()
@@ -253,9 +252,10 @@ func (c *Client) Start() (address string, err error) {
 
 	// Start goroutine to wait for process to exit
 	go func() {
-		// Make sure we close the write end of our stderr listener so
-		// that the log goroutine ends properly.
+		// Make sure we close the write end of our stderr/stdout so
+		// that the readers send EOF properly.
 		defer stderr_w.Close()
+		defer stdout_w.Close()
 
 		// Wait for the command to end.
 		cmd.Wait()
@@ -271,6 +271,34 @@ func (c *Client) Start() (address string, err error) {
 	// Start goroutine that logs the stderr
 	go c.logStderr(stderr_r)
 
+	// Start a goroutine that is going to be reading the lines
+	// out of stdout
+	linesCh := make(chan []byte)
+	go func() {
+		defer close(linesCh)
+
+		buf := bufio.NewReader(stdout_r)
+		for {
+			line, err := buf.ReadBytes('\n')
+			if line != nil {
+				linesCh <- line
+			}
+
+			if err == io.EOF {
+				return
+			}
+		}
+	}()
+
+	// Make sure after we exit we read the lines from stdout forever
+	// so they dont' block since it is an io.Pipe
+	defer func() {
+		go func() {
+			for _ = range linesCh {
+			}
+		}()
+	}()
+
 	// Some channels for the next step
 	timeout := time.After(c.config.StartTimeout)
 
@@ -281,6 +309,12 @@ func (c *Client) Start() (address string, err error) {
 		case <-timeout:
 			err = errors.New("timeout while waiting for plugin to start")
 			done = true
+		case line := <-linesCh:
+			// Trim the address and reset the err since we were able
+			// to read some sort of address.
+			c.address = strings.TrimSpace(string(line))
+			address = c.address
+			return
 		default:
 		}
 
@@ -289,15 +323,6 @@ func (c *Client) Start() (address string, err error) {
 			done = true
 		}
 
-		if line, lerr := stdout.ReadBytes('\n'); lerr == nil {
-			// Trim the address and reset the err since we were able
-			// to read some sort of address.
-			c.address = strings.TrimSpace(string(line))
-			address = c.address
-			err = nil
-			break
-		}
-
 		// If error is nil from previously, return now
 		if err != nil {
 			return