Add ws_backend_overflow counter

bin/varnishd/VSC_main.vsc

@@ -594,6 +594,13 @@
 	Number of times we failed a response due to running out of
 	workspace memory during delivery.
 
+.. varnish_vsc:: ws_backend_overflow
+	:level: diag
+	:group: wrk
+	:oneliner: workspace_backend overflows
+
+	Number of times we ran out of space in workspace_backend.
+
 .. varnish_vsc:: shm_records
 	:level:	diag
 	:oneliner:	SHM records

bin/varnishd/cache/cache_busyobj.c

@@ -167,6 +167,9 @@ VBO_ReleaseBusyObj(struct worker *wrk, struct busyobj **pbo)
 
 	AZ(bo->htc);
 
+	if (WS_Overflowed(bo->ws))
+		wrk->stats->ws_backend_overflow++;
+
 	if (bo->fetch_objcore != NULL) {
 		AN(wrk);
 		(void)HSH_DerefObjCore(wrk, &bo->fetch_objcore,

bin/varnishtest/tests/r01739.vtc

@@ -27,3 +27,4 @@ client c1 {
 } -run
 
 logexpect l1 -wait
+varnish v1 -expect ws_backend_overflow == 1

bin/varnishtest/tests/r01990.vtc

@@ -40,3 +40,4 @@ client c1 {
 } -run
 
 logexpect l1 -wait
+varnish v1 -expect ws_backend_overflow == 1