[master] 5337a9a Finally nail the race which made H2 tests randomly "just not work":

[Poul-Henning Kamp]

commit 5337a9a98f2c5ad1d37330f738c1aa0bd4859660
Author: Poul-Henning Kamp <phk at FreeBSD.org>
Date:   Sat Feb 11 12:29:14 2017 +0000

    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.

diff --git a/bin/varnishtest/vtc_http2.c b/bin/varnishtest/vtc_http2.c
index f7f0e34..542f2db 100644
--- a/bin/varnishtest/vtc_http2.c
+++ b/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));
 	}