os/exec: ignore pipe write errors when command completes successfully

Fixes #9173

Change-Id: I83530533db84b07cb88dbf6ec690be48a06a9d7d
Reviewed-on: https://go-review.googlesource.com/12152Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/exec/exec.go

@@ -180,6 +180,16 @@ 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 {
+			err = nil
+		}
+
 		if err1 := pw.Close(); err == nil {
 			err = err1
 		}

src/os/exec/exec_test.go

@@ -765,3 +765,24 @@ func TestHelperProcess(*testing.T) {
 		os.Exit(2)
 	}
 }
+
+// Issue 9173: ignore stdin pipe writes if the program completes successfully.
+func TestIgnorePipeErrorOnSuccess(t *testing.T) {
+	testenv.MustHaveExec(t)
+
+	// We really only care about testing this on Unixy things.
+	if runtime.GOOS == "windows" || runtime.GOOS == "plan9" {
+		t.Skipf("skipping test on %q", runtime.GOOS)
+	}
+
+	cmd := helperCommand(t, "echo", "foo")
+	var out bytes.Buffer
+	cmd.Stdin = strings.NewReader(strings.Repeat("x", 10<<20))
+	cmd.Stdout = &out
+	if err := cmd.Run(); err != nil {
+		t.Fatal(err)
+	}
+	if got, want := out.String(), "foo\n"; got != want {
+		t.Errorf("output = %q; want %q", got, want)
+	}
+}