os/exec: make TestPipeLookPathLeak more verbose when it fails

Trying to understand the linux-386-387 failures:
http://build.golang.org/log/78a91da173c11e986b4e623527c2d0b746f4e814

Also modernize the closeOnce code with a method value, while I
was looking.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews, iant
https://golang.org/cl/87950044

1 files changed, 8 insertions, 6 deletions

diff --git a/src/pkg/os/exec/exec.go b/src/pkg/os/exec/exec.go
index 44b9cc08ce..a70ed0d20c 100644
--- a/src/pkg/os/exec/exec.go
+++ b/src/pkg/os/exec/exec.go
@@ -429,15 +429,17 @@ func (c *Cmd) StdinPipe() (io.WriteCloser, error) {
 type closeOnce struct {
 	*os.File
 
-	close    sync.Once
-	closeErr error
+	once sync.Once
+	err  error
 }
 
 func (c *closeOnce) Close() error {
-	c.close.Do(func() {
-		c.closeErr = c.File.Close()
-	})
-	return c.closeErr
+	c.once.Do(c.close)
+	return c.err
+}
+
+func (c *closeOnce) close() {
+	c.err = c.File.Close()
 }
 
 // StdoutPipe returns a pipe that will be connected to the command's