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>

2 files changed, 33 insertions, 0 deletions

diff --git a/src/os/exec/exec.go b/src/os/exec/exec.go
index a3a0f20ebc..c4c5168b98 100644
--- a/src/os/exec/exec.go
+++ b/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.
 //
diff --git a/src/os/exec/exec_test.go b/src/os/exec/exec_test.go
index 67fe14faae..d3ac7ab4b9 100644
--- a/src/os/exec/exec_test.go
+++ b/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)