os/exec: add closeOnce.WriteString method

Add an explicit WriteString method to closeOnce that acquires the
writers lock.  This overrides the one promoted from the
embedded *os.File field.  The promoted one naturally does not acquire
the lock, and can therefore race with the Close method.

Fixes #17647.

Change-Id: I3460f2a0d503449481cfb2fd4628b4855ab0ecdf
Reviewed-on: https://go-review.googlesource.com/33298
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/dist/test.go

@@ -1055,7 +1055,7 @@ func (t *tester) runFlag(rx string) string {
 func (t *tester) raceTest(dt *distTest) error {
 	t.addCmd(dt, "src", "go", "test", "-race", "-i", "runtime/race", "flag", "os/exec")
 	t.addCmd(dt, "src", "go", "test", "-race", t.runFlag("Output"), "runtime/race")
-	t.addCmd(dt, "src", "go", "test", "-race", "-short", t.runFlag("TestParse|TestEcho"), "flag", "os/exec")
+	t.addCmd(dt, "src", "go", "test", "-race", "-short", t.runFlag("TestParse|TestEcho|TestStdinCloseRace"), "flag", "os/exec")
 	// We don't want the following line, because it
 	// slows down all.bash (by 10 seconds on my laptop).
 	// The race builder should catch any error here, but doesn't.
@@ -1068,7 +1068,7 @@ func (t *tester) raceTest(dt *distTest) error {
 	}
 	if t.extLink() {
 		// Test with external linking; see issue 9133.
-		t.addCmd(dt, "src", "go", "test", "-race", "-short", "-ldflags=-linkmode=external", t.runFlag("TestParse|TestEcho"), "flag", "os/exec")
+		t.addCmd(dt, "src", "go", "test", "-race", "-short", "-ldflags=-linkmode=external", t.runFlag("TestParse|TestEcho|TestStdinCloseRace"), "flag", "os/exec")
 	}
 	return nil
 }

src/os/exec/exec.go

@@ -579,6 +579,13 @@ func (c *closeOnce) Write(b []byte) (int, error) {
 	return n, err
 }
 
+func (c *closeOnce) WriteString(s string) (int, error) {
+	c.writers.RLock()
+	n, err := c.File.WriteString(s)
+	c.writers.RUnlock()
+	return n, err
+}
+
 // StdoutPipe returns a pipe that will be connected to the command's
 // standard output when the command starts.
 //

src/os/exec/exec_test.go

@@ -246,6 +246,32 @@ func TestStdinClose(t *testing.T) {
 	check("Wait", cmd.Wait())
 }
 
+// Issue 17647.
+func TestStdinCloseRace(t *testing.T) {
+	cmd := helperCommand(t, "stdinClose")
+	stdin, err := cmd.StdinPipe()
+	if err != nil {
+		t.Fatalf("StdinPipe: %v", err)
+	}
+	if err := cmd.Start(); err != nil {
+		t.Fatalf("Start: %v", err)
+	}
+	go func() {
+		if err := cmd.Process.Kill(); err != nil {
+			t.Errorf("Kill: %v", err)
+		}
+	}()
+	go func() {
+		io.Copy(stdin, strings.NewReader(stdinCloseTestString))
+		if err := stdin.Close(); err != nil {
+			t.Errorf("stdin.Close: %v", err)
+		}
+	}()
+	if err := cmd.Wait(); err == nil {
+		t.Fatalf("Wait: succeeded unexpectedly")
+	}
+}
+
 // Issue 5071
 func TestPipeLookPathLeak(t *testing.T) {
 	fd0, lsof0 := numOpenFDS(t)