1 files changed, 34 insertions, 4 deletions

diff --git a/src/pkg/os/exec/exec.go b/src/pkg/os/exec/exec.go
index a3bbcf300..491cc242b 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 +358,10 @@ func (c *Cmd) CombinedOutput() ([]byte, error) {
 
 // StdinPipe returns a pipe that will be connected to the command's
 // standard input when the command starts.
+// The pipe will be closed automatically after Wait sees the command exit.
+// A caller need only call Close to force the pipe to close sooner.
+// For example, if the command being run will not exit until standard input
+// is closed, the caller must close the pipe.
 func (c *Cmd) StdinPipe() (io.WriteCloser, error) {
 	if c.Stdin != nil {
 		return nil, errors.New("exec: Stdin already set")
@@ -370,13 +375,33 @@ 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
 // standard output when the command starts.
-// The pipe will be closed automatically after Wait sees the command exit.
+//
+// Wait will close the pipe after seeing the command exit, so most callers
+// need not close the pipe themselves; however, an implication is that
+// it is incorrect to call Wait before all reads from the pipe have completed.
+// For the same reason, it is incorrect to call Run when using StdoutPipe.
+// See the example for idiomatic usage.
 func (c *Cmd) StdoutPipe() (io.ReadCloser, error) {
 	if c.Stdout != nil {
 		return nil, errors.New("exec: Stdout already set")
@@ -396,7 +421,12 @@ func (c *Cmd) StdoutPipe() (io.ReadCloser, error) {
 
 // StderrPipe returns a pipe that will be connected to the command's
 // standard error when the command starts.
-// The pipe will be closed automatically after Wait sees the command exit.
+//
+// Wait will close the pipe after seeing the command exit, so most callers
+// need not close the pipe themselves; however, an implication is that
+// it is incorrect to call Wait before all reads from the pipe have completed.
+// For the same reason, it is incorrect to use Run when using StderrPipe.
+// See the StdoutPipe example for idiomatic usage.
 func (c *Cmd) StderrPipe() (io.ReadCloser, error) {
 	if c.Stderr != nil {
 		return nil, errors.New("exec: Stderr already set")