[#72] Add ListenWithError method in Listener interface

Listen and ListenWithError methods check if subscriber channel
has been closed. If so, ListenWithError passes error message
into provided channel.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/morph/event/listener.go

@@ -17,10 +17,16 @@ type Listener interface {
 	// Must start the event listener.
 	//
 	// Must listen to events with the parser installed.
-	//
-	// Must return an error if event listening could not be started.
 	Listen(context.Context)
 
+	// Must start the event listener.
+	//
+	// Must listen to events with the parser installed.
+	//
+	// Must send error to channel if subscriber channel has been closed or
+	// it could not be started.
+	ListenWithError(context.Context, chan<- error)
+
 	// Must set the parser of particular contract event.
 	//
 	// Parser of each event must be set once. All parsers must be set before Listen call.
@@ -78,7 +84,7 @@ var (
 // Returns an error if listener was already started.
 func (s listener) Listen(ctx context.Context) {
 	s.once.Do(func() {
-		if err := s.listen(ctx); err != nil {
+		if err := s.listen(ctx, nil); err != nil {
 			s.log.Error("could not start listen to events",
 				zap.String("error", err.Error()),
 			)
@@ -86,7 +92,24 @@ func (s listener) Listen(ctx context.Context) {
 	})
 }
 
-func (s listener) listen(ctx context.Context) error {
+// ListenWithError starts the listening for events with registered handlers and
+// passing error message to intError channel if subscriber channel has been closed.
+//
+// Executes once, all subsequent calls do nothing.
+//
+// Returns an error if listener was already started.
+func (s listener) ListenWithError(ctx context.Context, intError chan<- error) {
+	s.once.Do(func() {
+		if err := s.listen(ctx, intError); err != nil {
+			s.log.Error("could not start listen to events",
+				zap.String("error", err.Error()),
+			)
+			intError <- err
+		}
+	})
+}
+
+func (s listener) listen(ctx context.Context, intError chan<- error) error {
 	// create the list of listening contract hashes
 	hashes := make([]util.Uint160, 0)
 
@@ -115,12 +138,12 @@ func (s listener) listen(ctx context.Context) error {
 		return err
 	}
 
-	s.listenLoop(ctx, chEvent)
+	s.listenLoop(ctx, chEvent, intError)
 
 	return nil
 }
 
-func (s listener) listenLoop(ctx context.Context, chEvent <-chan *result.NotificationEvent) {
+func (s listener) listenLoop(ctx context.Context, chEvent <-chan *result.NotificationEvent, intErr chan<- error) {
 loop:
 	for {
 		select {
@@ -132,6 +155,10 @@ loop:
 		case notifyEvent, ok := <-chEvent:
 			if !ok {
 				s.log.Warn("stop event listener by channel")
+				if intErr != nil {
+					intErr <- errors.New("event subscriber connection has been terminated")
+				}
+
 				break loop
 			} else if notifyEvent == nil {
 				s.log.Warn("nil notification event was caught")