Push pool stats to global stats on delayed VCL release

Often in test cases the siutation comes where multiple threads go idle at
the same time, and no new activity happens after that. This would
potentially create the situation where the last thread to go idle sees
that some thread is currently pushing stats to global, so it leaves its
stats contribution on the pool stats to avoid contention. But without new
activity in the future, the pool stats may stay in the pool accumulator
indefinitely.

This patch adds a check to see if there are unaccumulated statistics
lingering in the pool accumulator when a thread wakes after idling to
release its VCL reference. If there are, the thread takes on the task of
pushing the pool stats to the global stats. This should help with test
case stability, and potentially help with underflowed stats reported by
some users.

bin/varnishd/cache/cache_wrk.c

@@ -398,10 +398,32 @@ Pool_Work_Thread(struct pool *pp, struct worker *wrk)
 				i = Lck_CondWait(&wrk->cond, &pp->mtx, tmo);
 				if (i == ETIMEDOUT)
 					VCL_Rel(&wrk->vcl);
-			} while (wrk->task->func == NULL);
-			tpx = *wrk->task;
-			tp = &tpx;
-			wrk->stats->summs++;
+				if (wrk->task->func != NULL) {
+					/* We have been handed a new task */
+					tpx = *wrk->task;
+					tp = &tpx;
+					wrk->stats->summs++;
+				} else if (pp->b_stat != NULL &&
+				    pp->a_stat->summs) {
+					/* Woken up to release the VCL,
+					 * and noticing that there are
+					 * pool stats not pushed to the
+					 * global stats and no active
+					 * thread currently doing
+					 * it. Remove ourself from the
+					 * idle queue and take on the
+					 * task. */
+					assert(pp->nidle > 0);
+					VTAILQ_REMOVE(&pp->idle_queue,
+					    wrk->task, list);
+					pp->nidle--;
+					tps.func = pool_stat_summ;
+					tps.priv = pp->a_stat;
+					pp->a_stat = pp->b_stat;
+					pp->b_stat = NULL;
+					tp = &tps;
+				}
+			} while (tp == NULL);
 		}
 		Lck_Unlock(&pp->mtx);