net/http: deflake new TestInterruptWithPanic_h2

TestInterruptWithPanic_h2 was added yesterday in
https://golang.org/cl/33099 and https://golang.org/cl/33103

Deflake it. The http2 server sends an error before logging.

Rather than reorder the http2 code to log before writing the RSTStream
frame, just loop for a bit waiting for the condition we're
expecting.

This goes from 2 in 500 flakes for me to unreproducible.

Change-Id: I062866a5977f50c820965aaf83882ddd7bf98f91
Reviewed-on: https://go-review.googlesource.com/33140
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/net/http/clientserver_test.go

@@ -1161,7 +1161,6 @@ func testInterruptWithPanic(t *testing.T, h2 bool, panicValue interface{}) {
 	defer afterTest(t)
 
 	var errorLog lockedBytesBuffer
-
 	cst := newClientServerTest(t, h2, HandlerFunc(func(w ResponseWriter, r *Request) {
 		io.WriteString(w, msg)
 		w.(Flusher).Flush()
@@ -1182,20 +1181,30 @@ func testInterruptWithPanic(t *testing.T, h2 bool, panicValue interface{}) {
 	if err == nil {
 		t.Errorf("client read all successfully; want some error")
 	}
-	wantStackLogged := panicValue != nil && panicValue != ErrAbortHandler
+	logOutput := func() string {
 		errorLog.Lock()
-	gotLog := errorLog.String()
+		defer errorLog.Unlock()
+		return errorLog.String()
+	}
+	wantStackLogged := panicValue != nil && panicValue != ErrAbortHandler
+
+	if err := waitErrCondition(5*time.Second, 10*time.Millisecond, func() error {
+		gotLog := logOutput()
 		if !wantStackLogged {
 			if gotLog == "" {
-			return
+				return nil
 			}
-		t.Fatalf("want no log output; got: %s", gotLog)
+			return fmt.Errorf("want no log output; got: %s", gotLog)
 		}
 		if gotLog == "" {
-		t.Fatalf("wanted a stack trace logged; got nothing")
+			return fmt.Errorf("wanted a stack trace logged; got nothing")
 		}
 		if !strings.Contains(gotLog, "created by ") && strings.Count(gotLog, "\n") < 6 {
-		t.Errorf("output doesn't look like a panic stack trace. Got: %s", gotLog)
+			return fmt.Errorf("output doesn't look like a panic stack trace. Got: %s", gotLog)
+		}
+		return nil
+	}); err != nil {
+		t.Fatal(err)
 	}
 }
 

src/net/http/main_test.go

@@ -134,3 +134,16 @@ func waitCondition(waitFor, checkEvery time.Duration, fn func() bool) bool {
 	}
 	return false
 }
+
+// waitErrCondition is like waitCondition but with errors instead of bools.
+func waitErrCondition(waitFor, checkEvery time.Duration, fn func() error) error {
+	deadline := time.Now().Add(waitFor)
+	var err error
+	for time.Now().Before(deadline) {
+		if err = fn(); err == nil {
+			return nil
+		}
+		time.Sleep(checkEvery)
+	}
+	return err
+}