fix: break out of correct loop when slow subscribers

datanode/candlesv2/candle_updates.go

@@ -104,7 +104,6 @@ func (s *CandleUpdates) run(ctx context.Context) {
 			if len(candles) > 0 {
 				lastCandle = &candles[len(candles)-1]
 			}
-
 			subscriptions = s.sendCandlesToSubscribers(candles, subscriptions)
 		}
 	}
@@ -224,12 +223,13 @@ func (s *CandleUpdates) getCandleUpdates(ctx context.Context, lastCandle *entiti
 func (s *CandleUpdates) sendCandlesToSubscribers(candles []entities.Candle, subscriptions map[string]chan entities.Candle) map[string]chan entities.Candle {
 	ret := subscriptions
 	for subscriptionID, outCh := range subscriptions {
+	loop:
 		for _, candle := range candles {
 			select {
 			case outCh <- candle:
 			default:
-				ret = removeSubscription(subscriptions, subscriptionID)
-				break
+				ret = removeSubscription(ret, subscriptionID)
+				break loop
 			}
 		}
 	}

datanode/candlesv2/candle_updates_test.go

@@ -339,6 +339,37 @@ func TestSubscribeAndUnSubscribeWithNonReturningSource(t *testing.T) {
 	updates.Unsubscribe(subID2)
 }
 
+func TestMultipleSlowConsumers(t *testing.T) {
+	nSends := 100
+	testCandleSource := &testCandleSource{candles: make(chan []entities.Candle, nSends), errorCh: make(chan error)}
+	// ensure the sub channels are buffered
+	updates := candlesv2.NewCandleUpdates(context.Background(), logging.NewTestLogger(), "testCandles",
+		testCandleSource, newTestCandleConfig(5).CandleUpdates)
+	startTime := time.Now()
+
+	updated := startTime
+	firstCandle := createCandle(startTime, updated, 1, 1, 1, 1, 10, 200)
+	lastCandle := firstCandle // just for the sake of types
+
+	wg := sync.WaitGroup{}
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		testCandleSource.candles <- []entities.Candle{firstCandle}
+
+		// keep updating the most recent candle
+		for i := 0; i < nSends; i++ {
+			updated = updated.Add(time.Second * time.Duration(i))
+			lastCandle = createCandle(startTime, updated, 1, 1, 1, 1, 10, 200)
+			testCandleSource.candles <- []entities.Candle{lastCandle, lastCandle, lastCandle}
+		}
+	}()
+	// ensure the first candle is sent
+	_, _, _ = updates.Subscribe()
+	_, _, _ = updates.Subscribe()
+	wg.Wait()
+}
+
 func newTestCandleConfig(subscribeBufferSize int) candlesv2.Config {
 	conf := candlesv2.NewDefaultConfig()
 	conf.CandleUpdates = candlesv2.CandleUpdatesConfig{