[master] acd2198 Fix race in the VSL segment counter initialization

[Martin Blix Grydeland]

commit acd219844d551883507829984964ad880db27fef
Author: Martin Blix Grydeland <martin at varnish-software.com>
Date:   Tue Mar 7 17:13:26 2017 +0100

    Fix race in the VSL segment counter initialization
    
    Fix a stupid misstake where the segment counter in the VSL segment was
    set just before the entire struct was cleared during
    initialization. This didn't cause problems in the common case because
    it would be set correctly on the very first log line produced. But it
    did open a race where an unfortunate early log client would assume it
    was lagging indefinetely behind varnishd on the log reading and report
    overflow error. This was most commonly experienced with varnishtest.
    
    Fixes: #2241

diff --git a/bin/varnishd/cache/cache_shmlog.c b/bin/varnishd/cache/cache_shmlog.c
index d8e3c7d..66fac59 100644
--- a/bin/varnishd/cache/cache_shmlog.c
+++ b/bin/varnishd/cache/cache_shmlog.c
@@ -507,13 +507,13 @@ VSM_Init(void)
 	   problems with regard to readers on that event visible */
 	vsl_segment_n = UINT_MAX - (VSL_SEGMENTS - 1);
 	AZ(vsl_segment_n % VSL_SEGMENTS);
-	vsl_head->segment_n = vsl_segment_n;
 	vsl_ptr = vsl_head->log;
 	*vsl_ptr = VSL_ENDMARKER;
 
 	memset(vsl_head, 0, sizeof *vsl_head);
 	vsl_head->segsize = vsl_segsize;
 	vsl_head->offset[0] = 0;
+	vsl_head->segment_n = vsl_segment_n;
 	for (i = 1; i < VSL_SEGMENTS; i++)
 		vsl_head->offset[i] = -1;
 	VWMB();