packer/plugin: more stable stderr logging

packer/plugin/client.go

 package plugin
 
 import (
+	"bufio"
 	"bytes"
 	"errors"
 	"fmt"
@@ -53,6 +54,10 @@ type ClientConfig struct {
 	// StartTimeout is the timeout to wait for the plugin to say it
 	// has started successfully.
 	StartTimeout time.Duration
+
+	// If non-nil, then the stderr of the client will be written to here
+	// (as well as the log).
+	Stderr io.Writer
 }
 
 // This makes sure all the managed subprocesses are killed and properly
@@ -211,13 +216,13 @@ func (c *Client) Start() (address string, err error) {
 	}
 
 	stdout := new(bytes.Buffer)
-	stderr := new(bytes.Buffer)
+	stderr_r, stderr_w := io.Pipe()
 
 	cmd := c.config.Cmd
 	cmd.Env = append(cmd.Env, os.Environ()...)
 	cmd.Env = append(cmd.Env, env...)
 	cmd.Stdin = os.Stdin
-	cmd.Stderr = stderr
+	cmd.Stderr = stderr_w
 	cmd.Stdout = stdout
 
 	log.Printf("Starting plugin: %s %#v", cmd.Path, cmd.Args)
@@ -241,13 +246,14 @@ func (c *Client) Start() (address string, err error) {
 
 	// Start goroutine to wait for process to exit
 	go func() {
+		defer stderr_w.Close()
 		cmd.Wait()
 		log.Printf("%s: plugin process exited\n", cmd.Path)
 		c.exited = true
 	}()
 
 	// Start goroutine that logs the stderr
-	go c.logStderr(stderr)
+	go c.logStderr(stderr_r)
 
 	// Some channels for the next step
 	timeout := time.After(c.config.StartTimeout)
@@ -288,22 +294,21 @@ func (c *Client) Start() (address string, err error) {
 	return
 }
 
-func (c *Client) logStderr(buf *bytes.Buffer) {
-	for done := false; !done; {
-		if c.Exited() {
-			done = true
-		}
-
-		var err error
-		for err != io.EOF {
-			var line string
-			line, err = buf.ReadString('\n')
+func (c *Client) logStderr(r io.Reader) {
+	bufR := bufio.NewReader(r)
+	for {
+		line, err := bufR.ReadString('\n')
 		if line != "" {
 			log.Printf("%s: %s", c.config.Cmd.Path, line)
+
+			if c.config.Stderr != nil {
+				c.config.Stderr.Write([]byte(line))
 			}
 		}
 
-		time.Sleep(10 * time.Millisecond)
+		if err == io.EOF {
+			break
+		}
 	}
 
 	// Flag that we've completed logging for others

packer/plugin/client_test.go

 package plugin
 
 import (
+	"bytes"
 	"io/ioutil"
 	"os"
+	"strings"
 	"testing"
 	"time"
 )
@@ -50,6 +52,32 @@ func TestClient_Start_Timeout(t *testing.T) {
 	}
 }
 
+func TestClient_Stderr(t *testing.T) {
+	stderr := new(bytes.Buffer)
+	process := helperProcess("stderr")
+	c := NewClient(&ClientConfig{
+		Cmd:    process,
+		Stderr: stderr,
+	})
+	defer c.Kill()
+
+	if _, err := c.Start(); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	for !c.Exited() {
+		time.Sleep(10 * time.Millisecond)
+	}
+
+	if !strings.Contains(stderr.String(), "HELLO\n") {
+		t.Fatalf("bad log data: '%s'", stderr.String())
+	}
+
+	if !strings.Contains(stderr.String(), "WORLD\n") {
+		t.Fatalf("bad log data: '%s'", stderr.String())
+	}
+}
+
 func TestClient_Stdin(t *testing.T) {
 	// Overwrite stdin for this test with a temporary file
 	tf, err := ioutil.TempFile("", "packer")

packer/plugin/plugin_test.go

@@ -67,6 +67,10 @@ func TestHelperProcess(*testing.T) {
 	case "start-timeout":
 		time.Sleep(1 * time.Minute)
 		os.Exit(1)
+	case "stderr":
+		fmt.Println(":1234")
+		log.Println("HELLO")
+		log.Println("WORLD")
 	case "stdin":
 		fmt.Println(":1234")
 		data := make([]byte, 5)