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>

1 files changed, 7 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.
 //