os/exec: ignore hungup error while copying stdin on Plan 9

Fixes #35753

Change-Id: I38674c59c601785eb25b778dc25efdb92231dd9b
Reviewed-on: https://go-review.googlesource.com/c/go/+/208223
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/exec/exec.go

@@ -238,7 +238,6 @@ func (c *Cmd) argv() []string {
 
 // skipStdinCopyError optionally specifies a function which reports
 // whether the provided 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) {

src/os/exec/exec_plan9.go

@@ -0,0 +1,19 @@
+// Copyright 2019 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.
+
+package exec
+
+import "os"
+
+func init() {
+	skipStdinCopyError = func(err error) bool {
+		// Ignore hungup errors copying to stdin if the program
+		// completed successfully otherwise.
+		// See Issue 35753.
+		pe, ok := err.(*os.PathError)
+		return ok &&
+			pe.Op == "write" && pe.Path == "|1" &&
+			pe.Err.Error() == "i/o on hungup channel"
+	}
+}

src/os/exec/exec_test.go

@@ -974,11 +974,6 @@ func (delayedInfiniteReader) Read(b []byte) (int, error) {
 func TestIgnorePipeErrorOnSuccess(t *testing.T) {
 	testenv.MustHaveExec(t)
 
-	// We really only care about testing this on Unixy and Windowsy things.
-	if runtime.GOOS == "plan9" {
-		t.Skipf("skipping test on %q", runtime.GOOS)
-	}
-
 	testWith := func(r io.Reader) func(*testing.T) {
 		return func(t *testing.T) {
 			cmd := helperCommand(t, "echo", "foo")