Finally nail the race which made H2 tests randomly "just not work":

One cannot use a counter as boolean when you have both waiters and
read ahead,  if one frame heas been read ahead and one stream is
waiting for another frame, the counter will be zero indicating
no frames should be read.

bin/varnishtest/vtc_http2.c

@@ -103,6 +103,7 @@ struct stream {
 	pthread_t		tp;
 	struct http		*hp;
 	int64_t			ws;
+	int			wf;
 
 	VTAILQ_HEAD(, frame)   fq;
 
@@ -720,6 +721,7 @@ receive_frame(void *priv)
 	AZ(pthread_mutex_lock(&hp->mtx));
 	while (hp->h2) {
 		/*no wanted frames? */
+		assert(hp->wf >= 0);
 		if (hp->wf == 0) {
 			AZ(pthread_cond_wait(&hp->cond, &hp->mtx));
 			continue;
@@ -824,8 +826,12 @@ receive_frame(void *priv)
 
 		AZ(pthread_mutex_lock(&hp->mtx));
 		VTAILQ_INSERT_HEAD(&s->fq, f, list);
-		hp->wf--;
-		AZ(pthread_cond_signal(&s->cond));
+		if (s->wf) {
+			assert(hp->wf > 0);
+			hp->wf--;
+			s->wf = 0;
+			AZ(pthread_cond_signal(&s->cond));
+		}
 		continue;
 	}
 	AZ(pthread_mutex_unlock(&hp->mtx));
@@ -2060,8 +2066,10 @@ rxstuff(struct stream *s)
 	CHECK_OBJ_NOTNULL(s, STREAM_MAGIC);
 
 	AZ(pthread_mutex_lock(&s->hp->mtx));
-	s->hp->wf++;
 	if (VTAILQ_EMPTY(&s->fq)) {
+		assert(s->hp->wf >= 0);
+		s->hp->wf++;
+		s->wf = 1;
 		AZ(pthread_cond_signal(&s->hp->cond));
 		AZ(pthread_cond_wait(&s->cond, &s->hp->mtx));
 	}