net/http: unflake TestTimeoutHandlerSuperfluousLogs

Uses 2 channels to synchronize that test, because
relying on sleeps creates flaky behavior, thus:

a) 1 buffered channel to send back the last spurious line
without having to reason about "happens before" behavior
a) 1 buffered channel at the end of the handler; it'll
be controlled by whether we expect to timeout or not,
but will always be closed when the test ends

Fixes #35051

Change-Id: Iff735aa8d1ed9de8d92b792374ec161cc0a72798
Reviewed-on: https://go-review.googlesource.com/c/go/+/208477
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/net/http/serve_test.go

@@ -6306,6 +6306,10 @@ func testContentEncodingNoSniffing(t *testing.T, h2 bool) {
 // Issue 30803: ensure that TimeoutHandler logs spurious
 // WriteHeader calls, for consistency with other Handlers.
 func TestTimeoutHandlerSuperfluousLogs(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping in short mode")
+	}
+
 	setParallel(t)
 	defer afterTest(t)
 
@@ -6314,29 +6318,30 @@ func TestTimeoutHandlerSuperfluousLogs(t *testing.T) {
 	testFuncName := runtime.FuncForPC(pc).Name()
 
 	timeoutMsg := "timed out here!"
-	maxTimeout := 200 * time.Millisecond
 
 	tests := []struct {
 		name        string
-		sleepTime time.Duration
+		mustTimeout bool
 		wantResp    string
 	}{
 		{
 			name:     "return before timeout",
-			sleepTime: 0,
 			wantResp: "HTTP/1.1 404 Not Found\r\nContent-Length: 0\r\n\r\n",
 		},
 		{
 			name:        "return after timeout",
-			sleepTime: maxTimeout * 2,
+			mustTimeout: true,
 			wantResp: fmt.Sprintf("HTTP/1.1 503 Service Unavailable\r\nContent-Length: %d\r\n\r\n%s",
 				len(timeoutMsg), timeoutMsg),
 		},
 	}
 
 	for _, tt := range tests {
+		tt := tt
 		t.Run(tt.name, func(t *testing.T) {
-			var lastSpuriousLine int32
+			exitHandler := make(chan bool, 1)
+			defer close(exitHandler)
+			lastLine := make(chan int, 1)
 
 			sh := HandlerFunc(func(w ResponseWriter, r *Request) {
 				w.WriteHeader(404)
@@ -6344,14 +6349,23 @@ func TestTimeoutHandlerSuperfluousLogs(t *testing.T) {
 				w.WriteHeader(404)
 				w.WriteHeader(404)
 				_, _, line, _ := runtime.Caller(0)
-				atomic.StoreInt32(&lastSpuriousLine, int32(line))
-
-				<-time.After(tt.sleepTime)
+				lastLine <- line
+				<-exitHandler
 			})
 
+			if !tt.mustTimeout {
+				exitHandler <- true
+			}
+
 			logBuf := new(bytes.Buffer)
 			srvLog := log.New(logBuf, "", 0)
-			th := TimeoutHandler(sh, maxTimeout, timeoutMsg)
+			// When expecting to timeout, we'll keep the duration short.
+			dur := 20 * time.Millisecond
+			if !tt.mustTimeout {
+				// Otherwise, make it arbitrarily long to reduce the risk of flakes.
+				dur = 10 * time.Second
+			}
+			th := TimeoutHandler(sh, dur, timeoutMsg)
 			cst := newClientServerTest(t, h1Mode /* the test is protocol-agnostic */, th, optWithServerLog(srvLog))
 			defer cst.close()
 
@@ -6379,10 +6393,12 @@ func TestTimeoutHandlerSuperfluousLogs(t *testing.T) {
 				t.Fatalf("Server logs count mismatch\ngot %d, want %d\n\nGot\n%s\n", g, w, blob)
 			}
 
+			lastSpuriousLine := <-lastLine
+			firstSpuriousLine := lastSpuriousLine - 3
 			// Now ensure that the regexes match exactly.
 			//      "http: superfluous response.WriteHeader call from <fn>.func\d.\d (<curFile>:lastSpuriousLine-[1, 3]"
 			for i, logEntry := range logEntries {
-				wantLine := atomic.LoadInt32(&lastSpuriousLine) - 3 + int32(i)
+				wantLine := firstSpuriousLine + i
 				pat := fmt.Sprintf("^http: superfluous response.WriteHeader call from %s.func\\d+.\\d+ \\(%s:%d\\)$",
 					testFuncName, curFileBaseName, wantLine)
 				re := regexp.MustCompile(pat)