os: don't use waitid on Darwin

According to issue #19314 waitid on Darwin returns if the process is
stopped, even though we specify WEXITED.

Fixes #19314.

Change-Id: I95faf196c11e43b7741efff79351bab45c811bc2
Reviewed-on: https://go-review.googlesource.com/37610
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/exec/exec_posix_test.go

@@ -11,6 +11,7 @@ import (
 	"strconv"
 	"syscall"
 	"testing"
+	"time"
 )
 
 func TestCredentialNoSetGroups(t *testing.T) {
@@ -43,3 +44,40 @@ func TestCredentialNoSetGroups(t *testing.T) {
 		t.Errorf("Failed to run command: %v", err)
 	}
 }
+
+// For issue #19314: make sure that SIGSTOP does not cause the process
+// to appear done.
+func TestWaitid(t *testing.T) {
+	t.Parallel()
+
+	cmd := helperCommand(t, "sleep")
+	if err := cmd.Start(); err != nil {
+		t.Fatal(err)
+	}
+
+	// The sleeps here are unnecessary in the sense that the test
+	// should still pass, but they are useful to make it more
+	// likely that we are testing the expected state of the child.
+	time.Sleep(100 * time.Millisecond)
+
+	if err := cmd.Process.Signal(syscall.SIGSTOP); err != nil {
+		cmd.Process.Kill()
+		t.Fatal(err)
+	}
+
+	ch := make(chan error)
+	go func() {
+		ch <- cmd.Wait()
+	}()
+
+	time.Sleep(100 * time.Millisecond)
+
+	if err := cmd.Process.Signal(syscall.SIGCONT); err != nil {
+		t.Error(err)
+		syscall.Kill(cmd.Process.Pid, syscall.SIGCONT)
+	}
+
+	cmd.Process.Kill()
+
+	<-ch
+}

src/os/exec/exec_test.go

@@ -868,6 +868,9 @@ func TestHelperProcess(*testing.T) {
 	case "stderrfail":
 		fmt.Fprintf(os.Stderr, "some stderr text\n")
 		os.Exit(1)
+	case "sleep":
+		time.Sleep(3 * time.Second)
+		os.Exit(0)
 	default:
 		fmt.Fprintf(os.Stderr, "Unknown command %q\n", cmd)
 		os.Exit(2)

src/os/wait_unimp.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build dragonfly nacl netbsd openbsd solaris
+// +build darwin dragonfly nacl netbsd openbsd solaris
 
 package os
 

src/os/wait_waitid.go

@@ -2,7 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin linux
+// We used to used this code for Darwin, but according to issue #19314
+// waitid returns if the process is stopped, even when using WEXITED.
+
+// +build linux
 
 package os