Bug fix #2718 appservice txnid should be different for each batch of events (#2719)

See issue: [#2718](https://github.com/matrix-org/dendrite/issues/2718)
for more details.

The fix assumes that if the number of transaction items are different,
then the txnid should be different.

txnid := OriginalServerTS()_len(transactions) 

The case that it doesn't address is if the txnid generated this way is
the same for 2 different batches of events which have the same
OriginalServerTS and the same array length.

Another option:

txnid := OriginalServerTS()_hash(transactions)

Would love to hear other ideas and ways to fix this.

### Pull Request Checklist

* [x ] I have added added tests for PR _or_ I have justified why this PR
doesn't need tests.
* [x ] Pull request includes a [sign
off](https://github.com/matrix-org/dendrite/blob/main/docs/CONTRIBUTING.md#sign-off)

Signed-off-by: `Tak Wai Wong <tak@hntlabs.com>`

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"
@@ -151,10 +152,17 @@ func (s *OutputRoomEventConsumer) onMessage(
 		return true
 	}
 
+	txnID := ""
+	// Try to get the message metadata, if we're able to, use the timestamp as the txnID
+	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(ctx, state, events, txnID) == nil
 }
 
 // sendEvents passes events to the appservice by using the transactions
@@ -162,6 +170,7 @@ func (s *OutputRoomEventConsumer) onMessage(
 func (s *OutputRoomEventConsumer) sendEvents(
 	ctx context.Context, state *appserviceState,
 	events []*gomatrixserverlib.HeaderedEvent,
+	txnID string,
 ) error {
 	// Create the transaction body.
 	transaction, err := json.Marshal(
@@ -173,13 +182,14 @@ func (s *OutputRoomEventConsumer) sendEvents(
 		return err
 	}
 
-	// TODO: We should probably be more intelligent and pick something not
+	// If txnID is not defined, generate one from the events.
-	// in the control of the event. A NATS timestamp header or something maybe.
+	if txnID == "" {
-	txnID := events[0].Event.OriginServerTS()
+		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
-	address := fmt.Sprintf("%s/transactions/%d?access_token=%s", state.URL, txnID, url.QueryEscape(state.HSToken))
+	address := fmt.Sprintf("%s/transactions/%s?access_token=%s", state.URL, txnID, url.QueryEscape(state.HSToken))
 	req, err := http.NewRequestWithContext(ctx, "PUT", address, bytes.NewBuffer(transaction))
 	if err != nil {
 		return err