Merge r5102: Retry backend fetches

Retry backend fetches one time if we got a recycled connection and the
transfer failed before we received anything.

Fixes: #749


git-svn-id: http://www.varnish-cache.org/svn/branches/2.1@5313 d4fa192b-c00b-0410-8231-f00ffab90ce4

bin/varnishd/cache_center.c

@@ -442,6 +442,15 @@ cnt_fetch(struct sess *sp)
 	http_Setup(sp->wrk->beresp, sp->wrk->ws);
 
 	i = FetchHdr(sp);
+	/*
+	 * If we recycle a backend connection, there is a finite chance
+	 * that the backend closed it before we get a request to it.
+	 * Do a single retry in that case.
+	 */
+	if (i == 1) {
+		VSL_stats->backend_retry++;
+		i = FetchHdr(sp);
+	}
 
 	/*
 	 * Save a copy before it might get mangled in VCL.  When it comes to

include/stat_field.h

@@ -152,3 +152,5 @@ MAC_STAT(esi_errors,		uint64_t, 0, 'a', "ESI parse errors (unlock)")
 MAC_STAT(accept_fail,		uint64_t, 0, 'a', "Accept failures")
 MAC_STAT(client_drop_late,	uint64_t, 0, 'a', "Connection dropped late")
 MAC_STAT(uptime,		uint64_t, 0, 'a', "Client uptime")
+
+MAC_STAT(backend_retry,		uint64_t, 0, 'a', "Backend conn. retry")