From b41c7b1bc627ab8ff2eb7fbe0a75df9bd00f27c2 Mon Sep 17 00:00:00 2001
From: Sorunome <mail@sorunome.de>
Date: Tue, 21 Sep 2021 09:58:11 +0200
Subject: [PATCH] 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       | 11 ++++++++---
 test/encryption/olm_manager_test.dart | 24 ++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/lib/encryption/olm_manager.dart b/lib/encryption/olm_manager.dart
index 62290380..1e73559f 100644
--- a/lib/encryption/olm_manager.dart
+++ b/lib/encryption/olm_manager.dart
@@ -667,9 +667,14 @@ class OlmManager {
         return;
       }
       final lastSentMessage = json.decode(lastSentMessageRes.first);
-      // okay, time to send the message!
-      await client.sendToDeviceEncrypted(
-          [device], lastSentMessage['type'], lastSentMessage['content']);
+      // 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']);
+      }
     }
   }
 
diff --git a/test/encryption/olm_manager_test.dart b/test/encryption/olm_manager_test.dart
index 858f5914..3b063bd8 100644
--- a/test/encryption/olm_manager_test.dart
+++ b/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 {