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

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

src/pkg/os/exec/exec_test.go

@@ -214,7 +214,7 @@ func TestStdinClose(t *testing.T) {
 
 // Issue 5071
 func TestPipeLookPathLeak(t *testing.T) {
-	fd0 := numOpenFDS(t)
+	fd0, lsof0 := numOpenFDS(t)
 	for i := 0; i < 4; i++ {
 		cmd := exec.Command("something-that-does-not-exist-binary")
 		cmd.StdoutPipe()
@@ -224,19 +224,19 @@ func TestPipeLookPathLeak(t *testing.T) {
 			t.Fatal("unexpected success")
 		}
 	}
-	fdGrowth := numOpenFDS(t) - fd0
+	open, lsof := numOpenFDS(t)
+	fdGrowth := open - fd0
 	if fdGrowth > 2 {
-		t.Errorf("leaked %d fds; want ~0", fdGrowth)
+		t.Errorf("leaked %d fds; want ~0; have:\n%s\noriginally:\n%s", fdGrowth, lsof, lsof0)
 	}
 }
 
-func numOpenFDS(t *testing.T) int {
+func numOpenFDS(t *testing.T) (n int, lsof []byte) {
 	lsof, err := exec.Command("lsof", "-n", "-p", strconv.Itoa(os.Getpid())).Output()
 	if err != nil {
 		t.Skip("skipping test; error finding or running lsof")
-		return 0
 	}
-	return bytes.Count(lsof, []byte("\n"))
+	return bytes.Count(lsof, []byte("\n")), lsof
 }
 
 var testedAlreadyLeaked = false