Optimize watchcache by not starting a gorotuine for all Get/List requests setting RV=0

staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher_whitebox_test.go

@@ -210,7 +210,7 @@ TestCase:
 			break TestCase
 		default:
 		}
-		w.Stop()
+		w.stopThreadUnsafe()
 	}
 }
 
@@ -551,7 +551,7 @@ func TestCacheWatcherStoppedInAnotherGoroutine(t *testing.T) {
 		case <-time.After(time.Second):
 			t.Fatal("expected received a event on ResultChan")
 		}
-		w.Stop()
+		w.stopThreadUnsafe()
 	}
 }
 

staging/src/k8s.io/apiserver/pkg/storage/cacher/watch_cache.go

@@ -420,6 +420,15 @@ func (w *watchCache) List() []interface{} {
 // You HAVE TO explicitly call w.RUnlock() after this function.
 func (w *watchCache) waitUntilFreshAndBlock(resourceVersion uint64, trace *utiltrace.Trace) error {
 	startTime := w.clock.Now()
+
+	// In case resourceVersion is 0, we accept arbitrarily stale result.
+	// As a result, the condition in the below for loop will never be
+	// satisfied (w.resourceVersion is never negative), this call will
+	// never hit the w.cond.Wait().
+	// As a result - we can optimize the code by not firing the wakeup
+	// function (and avoid starting a gorotuine), especially given that
+	// resourceVersion=0 is the most common case.
+	if resourceVersion > 0 {
 		go func() {
 			// Wake us up when the time limit has expired.  The docs
 			// promise that time.After (well, NewTimer, which it calls)
@@ -431,6 +440,7 @@ func (w *watchCache) waitUntilFreshAndBlock(resourceVersion uint64, trace *utilt
 			<-w.clock.After(blockTimeout)
 			w.cond.Broadcast()
 		}()
+	}
 
 	w.RLock()
 	if trace != nil {