fix mapreduce problem when reducer doesn't write

core/mr/mapreduce.go

@@ -16,7 +16,10 @@ const (
 	minWorkers     = 1
 )
 
-var ErrCancelWithNil = errors.New("mapreduce cancelled with nil")
+var (
+	ErrCancelWithNil  = errors.New("mapreduce cancelled with nil")
+	ErrReduceNoOutput = errors.New("reduce not writing value")
+)
 
 type (
 	GenerateFunc    func(source chan<- interface{})
@@ -93,7 +96,14 @@ func MapReduceWithSource(source <-chan interface{}, mapper MapperFunc, reducer R
 	collector := make(chan interface{}, options.workers)
 	done := syncx.NewDoneChan()
 	writer := newGuardedWriter(output, done.Done())
+	var closeOnce sync.Once
 	var retErr errorx.AtomicError
+	finish := func() {
+		closeOnce.Do(func() {
+			done.Close()
+			close(output)
+		})
+	}
 	cancel := once(func(err error) {
 		if err != nil {
 			retErr.Set(err)
@@ -102,14 +112,15 @@ func MapReduceWithSource(source <-chan interface{}, mapper MapperFunc, reducer R
 		}
 
 		drain(source)
-		done.Close()
-		close(output)
+		finish()
 	})
 
 	go func() {
 		defer func() {
 			if r := recover(); r != nil {
 				cancel(fmt.Errorf("%v", r))
+			} else {
+				finish()
 			}
 		}()
 		reducer(collector, writer, cancel)
@@ -122,7 +133,7 @@ func MapReduceWithSource(source <-chan interface{}, mapper MapperFunc, reducer R
 	} else if ok {
 		return value, nil
 	} else {
-		return nil, nil
+		return nil, ErrReduceNoOutput
 	}
 }