Use a separate condvar for connection-level flow control updates

The current flow control code's use of h2->cond is racy.

h2->cond is already used for handing over a DATA frame to a stream
thread. In the event that we have both streams waiting on this condvar
for window updates and at the same time the rxthread gets signaled for a
DATA frame, we could end up waking up the wrong thread and the rxthread
gets stuck forever.

This commit addresses this by using a separate condvar for window
updates.

An alternative would be to always issue a broadcast on h2->cond instead
of signal, but I found this approach much cleaner.

Probably fixes: #2623

bin/varnishd/http2/cache_http2.h

@@ -145,6 +145,7 @@ struct h2_sess {
 	pthread_t			rxthr;
 	struct h2_req			*mailcall;
 	pthread_cond_t			*cond;
+	pthread_cond_t			winupd_cond[1];
 
 	struct sess			*sess;
 	int				refcnt;

bin/varnishd/http2/cache_http2_proto.c

@@ -369,7 +369,7 @@ h2_rx_window_update(struct worker *wrk, struct h2_sess *h2, struct h2_req *r2)
 	Lck_Lock(&h2->sess->mtx);
 	r2->t_window += wu;
 	if (r2 == h2->req0)
-		AZ(pthread_cond_broadcast(h2->cond));
+		AZ(pthread_cond_broadcast(h2->winupd_cond));
 	else if (r2->cond != NULL)
 		AZ(pthread_cond_signal(r2->cond));
 	Lck_Unlock(&h2->sess->mtx);

bin/varnishd/http2/cache_http2_send.c

@@ -213,7 +213,7 @@ h2_do_window(struct worker *wrk, struct h2_req *r2,
 			r2->cond = NULL;
 		}
 		while (h2->req0->t_window <= 0 && h2_errcheck(r2, h2) == 0) {
-			AZ(Lck_CondWait(h2->cond, &h2->sess->mtx, 0));
+			AZ(Lck_CondWait(h2->winupd_cond, &h2->sess->mtx, 0));
 		}
 
 		if (h2_errcheck(r2, h2) == 0) {

bin/varnishd/http2/cache_http2_session.c

@@ -123,6 +123,7 @@ h2_init_sess(const struct worker *wrk, struct sess *sp,
 		h2->htc->rfd = &sp->fd;
 		h2->sess = sp;
 		h2->rxthr = pthread_self();
+		AZ(pthread_cond_init(h2->winupd_cond, NULL));
 		VTAILQ_INIT(&h2->streams);
 		VTAILQ_INIT(&h2->txqueue);
 		h2_local_settings(&h2->local_settings);
@@ -150,6 +151,7 @@ h2_del_sess(struct worker *wrk, struct h2_sess *h2, enum sess_close reason)
 	assert(VTAILQ_EMPTY(&h2->streams));
 
 	VHT_Fini(h2->dectbl);
+	AZ(pthread_cond_destroy(h2->winupd_cond));
 	req = h2->srq;
 	AZ(req->ws->r);
 	sp = h2->sess;
@@ -404,7 +406,7 @@ h2_new_session(struct worker *wrk, void *arg)
 		if (r2->cond != NULL)
 			AZ(pthread_cond_signal(r2->cond));
 	}
-	AZ(pthread_cond_broadcast(h2->cond));
+	AZ(pthread_cond_broadcast(h2->winupd_cond));
 	Lck_Unlock(&h2->sess->mtx);
 	while (1) {
 		again = 0;