Make the singlethreaded code simpler by using boolean guard (knative#…

…10624)

* Make the singlethreaded code simpler by using boolean guard

No need an int for that. Just checking whther there's something
holding the semaphore is enough.

Change-Id: I455ff55a661b680290f58eb7e4a0f0d5ef5085a9

* return

Change-Id: I661bd1b95fe6f621100ba901c2f5ca6bd9b503ac

test/e2e/subroutes_test.go

@@ -53,7 +53,7 @@ func TestSubrouteLocalSTS(t *testing.T) { // We can't use a longer more descript
 	t.Log("Creating a Service for the helloworld test app.")
 	names := test.ResourceNames{
 		Service: test.ObjectNameForTest(t),
-		Image:   "helloworld",
+		Image:   test.HelloWorld,
 	}
 
 	test.EnsureTearDown(t, clients, &names)

test/test_images/singlethreaded/main.go

@@ -27,18 +27,17 @@ import (
 	"knative.dev/serving/test"
 )
 
-var lockedFlag = atomic.NewInt32(-1)
+var isLocked = atomic.NewBool(false)
 
 func handler(w http.ResponseWriter, r *http.Request) {
-	v := lockedFlag.Inc() // Returns the new value.
-	defer lockedFlag.Dec()
-	if v > 0 {
-		// Return HTTP 500 if more than 1 request at a time gets in
-		w.WriteHeader(http.StatusInternalServerError)
+	if isLocked.CAS(false /*was unlocked*/, true /*lock*/) {
+		defer isLocked.Store(false)
+		time.Sleep(500 * time.Millisecond)
+		fmt.Fprintf(w, "One at a time")
 		return
 	}
-	time.Sleep(500 * time.Millisecond)
-	fmt.Fprintf(w, "One at a time")
+	// Return HTTP 500 if more than 1 request at a time gets in
+	w.WriteHeader(http.StatusInternalServerError)
 }
 
 func main() {