Added an external go routine to ensure stale batches get flushed

batchconsumer/batchermanager.go

@@ -137,25 +137,30 @@ func (b *batcherManager) startMessageHandler(
 	batchMsg <-chan tagMsgPair, lastIgnored, lastProcessed <-chan kcl.SequencePair,
 	shutdown <-chan chan<- struct{},
 ) {
+	flushStaleBatches := make(chan struct{})
+
+	go func() {
+		for { // Flush batches that haven't been updated recently
+			<-time.NewTimer(time.Second).C
+			flushStaleBatches <- struct{}{}
+		}
+	}()
+
 	go func() {
 		var lastProcessedPair kcl.SequencePair
 		var lastIgnoredPair kcl.SequencePair
 		batchers := map[string]*batcher{}
 
 		for {
-			for tag, batcher := range batchers { // Flush batcher that hasn't been updated recently
+			select {
+			case <-flushStaleBatches:
+				for tag, batcher := range batchers {
 					if b.batchInterval <= time.Now().Sub(batcher.LastUpdated) {
 						b.sendBatch(batcher, tag)
 						b.sendCheckpoint(tag, lastIgnoredPair, batchers)
 						batcher.Clear()
 					}
 				}
-
-			select {
-			case <-time.NewTimer(time.Second).C:
-				// Timer is janky way to ensure the code above is ran at regular intervals
-				// Can't put above code under this case because it's very possible that this
-				// timer is always pre-empted by other channels
 			case tmp := <-batchMsg:
 				batcher, ok := batchers[tmp.tag]
 				if !ok {