simplify periodical executor background routine (#339)

core/executors/periodicalexecutor.go

@@ -93,15 +93,12 @@ func (pe *PeriodicalExecutor) Wait() {
 func (pe *PeriodicalExecutor) addAndCheck(task interface{}) (interface{}, bool) {
 	pe.lock.Lock()
 	defer func() {
-		var start bool
 		if !pe.guarded {
 			pe.guarded = true
-			start = true
+			// defer to unlock quickly
+			defer pe.backgroundFlush()
 		}
 		pe.lock.Unlock()
-		if start {
-			pe.backgroundFlush()
-		}
 	}()
 
 	if pe.container.AddTask(task) {
@@ -148,14 +145,12 @@ func (pe *PeriodicalExecutor) cleanup() (stop bool) {
 	pe.guarded = false
 	if atomic.LoadInt32(&pe.inflight) == 0 {
 		stop = true
+		// defer to unlock quickly
+		// flush again to avoid missing tasks
+		defer pe.Flush()
 	}
 	pe.lock.Unlock()
 
-	if stop {
-		// flush again to avoid missing tasks
-		pe.Flush()
-	}
-
 	return
 }