fix issue #317 (#331)

core/executors/periodicalexecutor.go

@@ -134,8 +134,7 @@ func (pe *PeriodicalExecutor) backgroundFlush() {
 					pe.guarded = false
 					pe.lock.Unlock()
 
-					// flush again to avoid missing tasks
-					pe.Flush()
+					pe.cleanup()
 					return
 				}
 			}
@@ -143,6 +142,22 @@ func (pe *PeriodicalExecutor) backgroundFlush() {
 	})
 }
 
+func (pe *PeriodicalExecutor) cleanup() {
+	// avoid deadlock in Add()
+	for {
+		select {
+		case vals := <-pe.commander:
+			pe.enterExecution()
+			pe.confirmChan <- lang.Placeholder
+			pe.executeTasks(vals)
+		default:
+			// flush again to avoid missing tasks
+			pe.Flush()
+			return
+		}
+	}
+}
+
 func (pe *PeriodicalExecutor) doneExecution() {
 	pe.waitGroup.Done()
 }