Rehydrate more opportunistically

federationsender/queue/destinationqueue.go

@@ -173,12 +173,11 @@ func (oq *destinationQueue) backgroundSend() {
 		// e.g. in response to EDUs.
 		transactionID := gomatrixserverlib.TransactionID("")
 
-		// If we haven't reached the PDU limit yet then rehydrate the
-		// PDU queue from the database. We'll retrieve the events in the
-		// next transaction. Note that this does *not* necessarily mean
-		// we will fill the buffer - the important thing here is that we
-		// still continue to send transactions in order.
-		if len(oq.pendingPDUs) < maxPDUsPerTransaction {
+		// Retrieve the events in the next transaction. Note that this
+		// does *not* necessarily mean we will fill the buffer - the
+		// important thing here is that we still continue to send
+		// transactions in order.
+		hydrate := func() {
 			txid, pdus, err := oq.db.GetNextTransactionPDUs(
 				context.TODO(), // context
 				oq.destination, // server name
@@ -186,18 +185,25 @@ func (oq *destinationQueue) backgroundSend() {
 			)
 			if err != nil {
 				log.WithError(err).Errorf("failed to get next transaction PDUs for server %q", oq.destination)
-				continue
+				return
 			}
 			transactionID = txid
 			oq.pendingPDUs = append(oq.pendingPDUs, pdus...)
 		}
 
+		// If we haven't reached the PDU limit yet then rehydrate the
+		// PDU queue from the database.
+		if len(oq.pendingPDUs) < maxPDUsPerTransaction {
+			hydrate()
+		}
+
 		// Wait either for incoming events, or until we hit an
 		// idle timeout.
 		if len(oq.pendingPDUs) == 0 {
 			select {
 			case <-oq.incomingPDUs:
 				// There are new PDUs waiting in the database.
+				hydrate()
 			case edu := <-oq.incomingEDUs:
 				// Likewise for EDUs, although we should probably not try
 				// too hard with some EDUs (like typing notifications) after