os/exec: return idempotent Closer from StdinPipe

Before this fix, it was always an error to use the Close method on the
io.WriteCloser obtained from Cmd.StdinPipe, as it would race with the
Close performed by Cmd.Wait.

Fixes #6270.

R=golang-dev, r, remyoudompheng, bradfitz, dsymonds
CC=golang-dev
https://golang.org/cl/13329043

2 files changed, 59 insertions, 2 deletions

diff --git a/src/pkg/os/exec/exec.go b/src/pkg/os/exec/exec.go
index a3bbcf3005..582930f2c4 100644
--- a/src/pkg/os/exec/exec.go
+++ b/src/pkg/os/exec/exec.go
@@ -13,6 +13,7 @@ import (
 	"io"
 	"os"
 	"strconv"
+	"sync"
 	"syscall"
 )
 
@@ -357,6 +358,8 @@ func (c *Cmd) CombinedOutput() ([]byte, error) {
 
 // StdinPipe returns a pipe that will be connected to the command's
 // standard input when the command starts.
+// If the returned WriteCloser is not closed before Wait is called,
+// Wait will close it.
 func (c *Cmd) StdinPipe() (io.WriteCloser, error) {
 	if c.Stdin != nil {
 		return nil, errors.New("exec: Stdin already set")
@@ -370,8 +373,23 @@ func (c *Cmd) StdinPipe() (io.WriteCloser, error) {
 	}
 	c.Stdin = pr
 	c.closeAfterStart = append(c.closeAfterStart, pr)
-	c.closeAfterWait = append(c.closeAfterWait, pw)
-	return pw, nil
+	wc := &closeOnce{File: pw}
+	c.closeAfterWait = append(c.closeAfterWait, wc)
+	return wc, nil
+}
+
+type closeOnce struct {
+	*os.File
+
+	close    sync.Once
+	closeErr error
+}
+
+func (c *closeOnce) Close() error {
+	c.close.Do(func() {
+		c.closeErr = c.File.Close()
+	})
+	return c.closeErr
 }
 
 // StdoutPipe returns a pipe that will be connected to the command's
diff --git a/src/pkg/os/exec/exec_test.go b/src/pkg/os/exec/exec_test.go
index d7e8573a08..c380d6506c 100644
--- a/src/pkg/os/exec/exec_test.go
+++ b/src/pkg/os/exec/exec_test.go
@@ -152,6 +152,34 @@ func TestPipes(t *testing.T) {
 	check("Wait", err)
 }
 
+const stdinCloseTestString = "Some test string."
+
+// Issue 6270.
+func TestStdinClose(t *testing.T) {
+	check := func(what string, err error) {
+		if err != nil {
+			t.Fatalf("%s: %v", what, err)
+		}
+	}
+	cmd := helperCommand("stdinClose")
+	stdin, err := cmd.StdinPipe()
+	check("StdinPipe", err)
+	// Check that we can access methods of the underlying os.File.`
+	if _, ok := stdin.(interface {
+		Fd() uintptr
+	}); !ok {
+		t.Error("can't access methods of underlying *os.File")
+	}
+	check("Start", cmd.Start())
+	go func() {
+		_, err := io.Copy(stdin, strings.NewReader(stdinCloseTestString))
+		check("Copy", err)
+		// Before the fix, this next line would race with cmd.Wait.
+		check("Close", stdin.Close())
+	}()
+	check("Wait", cmd.Wait())
+}
+
 // Issue 5071
 func TestPipeLookPathLeak(t *testing.T) {
 	fd0 := numOpenFDS(t)
@@ -507,6 +535,17 @@ func TestHelperProcess(*testing.T) {
 				os.Exit(1)
 			}
 		}
+	case "stdinClose":
+		b, err := ioutil.ReadAll(os.Stdin)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+		if s := string(b); s != stdinCloseTestString {
+			fmt.Fprintf(os.Stderr, "Error: Read %q, want %q", s, stdinCloseTestString)
+			os.Exit(1)
+		}
+		os.Exit(0)
 	case "read3": // read fd 3
 		fd3 := os.NewFile(3, "fd3")
 		bs, err := ioutil.ReadAll(fd3)