Add cache_hit_grace counter

The counter cache_hit_grace counts the number of grace hits. To be
precise, it counts the number of times lookup returns an expired
object, but vcl_hit is called and decides to return(deliver).

Every time cache_hit_grace is incremented, cache_hit is also
incremented (so this commit does not change the cache_hit counter).

bin/varnishd/VSC_main.vsc

@@ -56,6 +56,13 @@
 	Count of cache hits.  A cache hit indicates that an object has been
 	delivered to a client without fetching it from a backend server.
 
+.. varnish_vsc:: cache_hit_grace
+	:oneliner:	Cache grace hits
+
+	Count of cache hits with grace. A cache hit with grace is a cache
+	hit where the object is expired. Note that such hits are also
+	included in the cache_hit counter.
+
 .. varnish_vsc:: cache_hitpass
 	:oneliner:	Cache hits for pass.
 

bin/varnishd/cache/cache_req_fsm.c

@@ -523,6 +523,8 @@ cnt_lookup(struct worker *wrk, struct req *req)
 		}
 		wrk->stats->cache_hit++;
 		req->is_hit = 1;
+		if (lr == HSH_EXP || lr == HSH_EXPBUSY)
+			wrk->stats->cache_hit_grace++;
 		req->req_step = R_STP_DELIVER;
 		return (REQ_FSM_MORE);
 	case VCL_RET_MISS:

bin/varnishtest/tests/b00052.vtc

+varnishtest "The cache_hit_grace counter"
+
+server s1 {
+	# normal fetch
+	rxreq
+	expect req.url == "/1"
+	txresp -hdr "Age: 1" -hdr "Cache-Control: max-age=2" -body "1"
+
+	# background fetch:
+	rxreq
+	expect req.url == "/1"
+	txresp -body "2"
+
+	# normal fetch
+	rxreq
+	expect req.url == "/2"
+	txresp
+} -start
+
+varnish v1 -vcl+backend { } -start
+
+client c1 {
+	txreq -url "/1"
+	rxresp
+	expect resp.body == "1"
+} -run
+
+delay 2
+
+# Get a grace hit, will trigger a background fetch
+
+client c2 {
+	txreq -url "/1"
+	rxresp
+	expect resp.body == "1"
+} -run
+
+delay 2
+
+client c3 {
+	txreq -url "/2"
+	rxresp
+	txreq -url "/1"
+	rxresp
+	expect resp.body == "2"
+} -run
+
+# Check that counters are correct:
+
+varnish v1 -expect cache_hit == 2
+varnish v1 -expect cache_hit_grace == 1
+varnish v1 -expect cache_miss == 2