[PATCH 1/2] Remove negative Age assertion and truncate to zero instead.

[Martin Blix Grydeland]

The Age reported on response objects is calculated from the last
request timestamp taken. For a cache hit that hasn't been on a
waitinglist, that will be the Start timestamp. This opens a race where
the requests' last timestamp could be before the objects t_origin.

Truncate the Age to zero rather than assert in that case.

Fixes: #1486
---
 bin/varnishd/cache/cache_req_fsm.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/bin/varnishd/cache/cache_req_fsm.c b/bin/varnishd/cache/cache_req_fsm.c
index 2518422..af5d76b 100644
--- a/bin/varnishd/cache/cache_req_fsm.c
+++ b/bin/varnishd/cache/cache_req_fsm.c
@@ -117,10 +117,15 @@ cnt_deliver(struct worker *wrk, struct req *req)
 	/* We base Age calculation upon the last timestamp taken during
 	   client request processing. This gives some inaccuracy, but
 	   since Age is only full second resolution that shouldn't
-	   matter. */
-	assert(req->t_prev > req->obj->exp.t_origin);
-	http_PrintfHeader(req->resp, "Age: %.0f",
-	    req->t_prev - req->obj->exp.t_origin);
+	   matter. (Last request timestamp could be a Start timestamp
+	   taken before the object entered into cache leading to negative
+	   age. Truncate to zero in that case).
+	*/
+	if (req->t_prev > req->obj->exp.t_origin)
+		http_PrintfHeader(req->resp, "Age: %.0f",
+		    req->t_prev - req->obj->exp.t_origin);
+	else
+		http_PrintfHeader(req->resp, "Age: %.0f", 0.);
 
 	http_SetHeader(req->resp, "Via: 1.1 varnish (v4)");
 
-- 
1.9.2