transport/file: avoid race with Command.Wait, fixes #463

Pipe returned by Command.StderrPipe() has a race with Read and
Command.Wait(). We use a io.Pipe() instead and ensure it is closed
after Wait().

Author: smola

plumbing/transport/file/client.go

@@ -46,16 +46,22 @@ func (r *runner) Command(cmd string, ep transport.Endpoint, auth transport.AuthM
 }
 
 type command struct {
-	cmd    *exec.Cmd
-	closed bool
+	cmd          *exec.Cmd
+	stderrCloser io.Closer
+	closed       bool
 }
 
 func (c *command) Start() error {
 	return c.cmd.Start()
 }
 
 func (c *command) StderrPipe() (io.Reader, error) {
-	return c.cmd.StderrPipe()
+	// Pipe returned by Command.StderrPipe has a race with Read + Command.Wait.
+	// We use an io.Pipe and close it after the command finishes.
+	r, w := io.Pipe()
+	c.cmd.Stderr = w
+	c.stderrCloser = r
+	return r, nil
 }
 
 func (c *command) StdinPipe() (io.WriteCloser, error) {
@@ -72,7 +78,11 @@ func (c *command) Close() error {
 		return nil
 	}
 
-	defer func() { c.closed = true }()
+	defer func() {
+		c.closed = true
+		_ = c.stderrCloser.Close()
+	}()
+
 	err := c.cmd.Wait()
 	if _, ok := err.(*os.PathError); ok {
 		return nil