From 10e2ffdf2ca657567fc1708f6387fef69a8445b6 Mon Sep 17 00:00:00 2001
From: Andrew Gerrand <adg@golang.org>
Date: Thu, 29 Aug 2013 14:41:44 +1000
Subject: [PATCH] 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
---
 src/pkg/os/exec/exec.go      | 22 ++++++++++++++++++--
 src/pkg/os/exec/exec_test.go | 39 ++++++++++++++++++++++++++++++++++++
 2 files changed, 59 insertions(+), 2 deletions(-)

diff --git a/src/pkg/os/exec/exec.go b/src/pkg/os/exec/exec.go
index a3bbcf3005a..582930f2c42 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 d7e8573a084..c380d6506c4 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)