fix: Don't re-play m.dummy to_device events

If both ends had m.dummy events queued as last messages an an olm
session corrupted, then the clients landed in an infinite game of
ping-pong. It was so stable, that the clients could have won the
ping-pong world championships!

lib/encryption/olm_manager.dart

@@ -667,11 +667,16 @@ class OlmManager {
         return;
       }
       final lastSentMessage = json.decode(lastSentMessageRes.first);
+      // We do *not* want to re-play m.dummy events, as they hold no value except of saying
+      // what olm session is the most recent one. In fact, if we *do* replay them, then
+      // we can easily land in an infinite ping-pong trap!
+      if (lastSentMessage['type'] != EventTypes.Dummy) {
         // okay, time to send the message!
         await client.sendToDeviceEncrypted(
             [device], lastSentMessage['type'], lastSentMessage['content']);
       }
     }
+  }
 
   void dispose() {
     for (final sessions in olmSessions.values) {

test/encryption/olm_manager_test.dart

@@ -234,6 +234,30 @@ void main() {
           FakeMatrixApi.calledEndpoints.keys.any(
               (k) => k.startsWith('/client/r0/sendToDevice/m.room.encrypted')),
           false);
+
+      // don't replay if the last event is m.dummy itself
+      FakeMatrixApi.calledEndpoints.clear();
+      await client.database.setLastSentMessageUserDeviceKey(
+          json.encode({
+            'type': 'm.dummy',
+            'content': {},
+          }),
+          client.id,
+          userId,
+          deviceId);
+      event = ToDeviceEvent(
+        sender: userId,
+        type: 'm.dummy',
+        content: {},
+        encryptedContent: {
+          'sender_key': senderKey,
+        },
+      );
+      await client.encryption.olmManager.handleToDeviceEvent(event);
+      expect(
+          FakeMatrixApi.calledEndpoints.keys.any(
+              (k) => k.startsWith('/client/r0/sendToDevice/m.room.encrypted')),
+          false);
     });
 
     test('dispose client', () async {