Make the fix for #2718 the same as dendrite main (#31)

Co-authored-by: Tak Wai Wong <tak@hntlabs.com>

appservice/consumers/roomserver.go

@@ -22,6 +22,7 @@ import (
 	"math"
 	"net/http"
 	"net/url"
+	"strconv"
 	"time"
 
 	"github.com/matrix-org/gomatrixserverlib"
@@ -151,15 +152,25 @@ func (s *OutputRoomEventConsumer) onMessage(
 		return true
 	}
 
+	txnID := ""
+	// Try to get the message metadata, if we're able to, use the timestamp as the txnID
+	if len(msgs) > 0 {
+		metadata, err := msgs[0].Metadata()
+		if err == nil {
+			txnID = strconv.Itoa(int(metadata.Timestamp.UnixNano()))
+		}
+	}
+
 	// Send event to any relevant application services. If we hit
 	// an error here, return false, so that we negatively ack.
 	log.WithField("appservice", state.ID).Debugf("Appservice worker sending %d events(s) from roomserver", len(events))
-	return s.sendEvents(ctx, state, events) == nil
+	return s.sendEvents(txnID, ctx, state, events) == nil
 }
 
 // sendEvents passes events to the appservice by using the transactions
 // endpoint. It will block for the backoff period if necessary.
 func (s *OutputRoomEventConsumer) sendEvents(
+	txnID string,
 	ctx context.Context, state *appserviceState,
 	events []*gomatrixserverlib.HeaderedEvent,
 ) error {
@@ -173,10 +184,10 @@ func (s *OutputRoomEventConsumer) sendEvents(
 		return err
 	}
 
-	// If the number of items in the array is different,
-	// then this should be treated as a different transaction. Incorporate the length
-	// of events into the transaction ID.
-	txnID := fmt.Sprintf("%d_%d", events[0].Event.OriginServerTS(), len(transaction))
+	// If txnID is not defined, generate one from the events.
+	if txnID == "" {
+		txnID = fmt.Sprintf("%d_%d", events[0].Event.OriginServerTS(), len(transaction))
+	}
 
 	// Send the transaction to the appservice.
 	// https://matrix.org/docs/spec/application_service/r0.1.2#put-matrix-app-v1-transactions-txnid