os/exec: fix plan9 build

Fixes build from https://golang.org/cl/12152

Plan 9 lacks syscall.EPIPE. I was misled by api/go1.txt and also
forgot to use the trybots. :(

Change-Id: I4982fe969ad4a8724090cb03009bfb21780d8aa7
Reviewed-on: https://go-review.googlesource.com/12153
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/exec/exec.go

@@ -157,6 +157,11 @@ func (c *Cmd) argv() []string {
 	return []string{c.Path}
 }
 
+// skipStdinCopyError optionally specifies a function which reports
+// whether the provided the stdin copy error should be ignored.
+// It is non-nil everywhere but Plan 9, which lacks EPIPE. See exec_posix.go.
+var skipStdinCopyError func(error) bool
+
 func (c *Cmd) stdin() (f *os.File, err error) {
 	if c.Stdin == nil {
 		f, err = os.Open(os.DevNull)
@@ -180,16 +185,9 @@ func (c *Cmd) stdin() (f *os.File, err error) {
 	c.closeAfterWait = append(c.closeAfterWait, pw)
 	c.goroutine = append(c.goroutine, func() error {
 		_, err := io.Copy(pw, c.Stdin)
-
-		// Ignore EPIPE errors copying to stdin if the program
-		// completed successfully otherwise.
-		// See Issue 9173.
-		if pe, ok := err.(*os.PathError); ok &&
-			pe.Op == "write" && pe.Path == "|1" &&
-			pe.Err == syscall.EPIPE {
+		if skip := skipStdinCopyError; skip != nil && skip(err) {
 			err = nil
 		}
-
 		if err1 := pw.Close(); err == nil {
 			err = err1
 		}

src/os/exec/exec_posix.go

@@ -0,0 +1,24 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !plan9
+
+package exec
+
+import (
+	"os"
+	"syscall"
+)
+
+func init() {
+	skipStdinCopyError = func(err error) bool {
+		// Ignore EPIPE errors copying to stdin if the program
+		// completed successfully otherwise.
+		// See Issue 9173.
+		pe, ok := err.(*os.PathError)
+		return ok &&
+			pe.Op == "write" && pe.Path == "|1" &&
+			pe.Err == syscall.EPIPE
+	}
+}