Fix a bug introduced in Synapse v1.0.0 whereby device list updates would not be sent to remote homeservers if there were too many to send at once. (#11729)

Co-authored-by: Brendan Abolivier <babolivier@matrix.org>

tests/storage/test_devices.py

@@ -94,7 +94,7 @@ class DeviceStoreTestCase(HomeserverTestCase):
     def test_get_device_updates_by_remote(self):
         device_ids = ["device_id1", "device_id2"]
 
-        # Add two device updates with a single stream_id
+        # Add two device updates with sequential `stream_id`s
         self.get_success(
             self.store.add_device_change_to_streams("user_id", device_ids, ["somehost"])
         )
@@ -107,6 +107,54 @@ class DeviceStoreTestCase(HomeserverTestCase):
         # Check original device_ids are contained within these updates
         self._check_devices_in_updates(device_ids, device_updates)
 
+    def test_get_device_updates_by_remote_can_limit_properly(self):
+        """
+        Tests that `get_device_updates_by_remote` returns an appropriate
+        stream_id to resume fetching from (without skipping any results).
+        """
+
+        # Add some device updates with sequential `stream_id`s
+        device_ids = [
+            "device_id1",
+            "device_id2",
+            "device_id3",
+            "device_id4",
+            "device_id5",
+        ]
+        self.get_success(
+            self.store.add_device_change_to_streams("user_id", device_ids, ["somehost"])
+        )
+
+        # Get all device updates ever meant for this remote
+        next_stream_id, device_updates = self.get_success(
+            self.store.get_device_updates_by_remote("somehost", -1, limit=3)
+        )
+
+        # Check the first three original device_ids are contained within these updates
+        self._check_devices_in_updates(device_ids[:3], device_updates)
+
+        # Get the next batch of device updates
+        next_stream_id, device_updates = self.get_success(
+            self.store.get_device_updates_by_remote("somehost", next_stream_id, limit=3)
+        )
+
+        # Check the last two original device_ids are contained within these updates
+        self._check_devices_in_updates(device_ids[3:], device_updates)
+
+        # Add some more device updates to ensure it still resumes properly
+        device_ids = ["device_id6", "device_id7"]
+        self.get_success(
+            self.store.add_device_change_to_streams("user_id", device_ids, ["somehost"])
+        )
+
+        # Get the next batch of device updates
+        next_stream_id, device_updates = self.get_success(
+            self.store.get_device_updates_by_remote("somehost", next_stream_id, limit=3)
+        )
+
+        # Check the newly-added device_ids are contained within these updates
+        self._check_devices_in_updates(device_ids, device_updates)
+
     def _check_devices_in_updates(self, expected_device_ids, device_updates):
         """Check that an specific device ids exist in a list of device update EDUs"""
         self.assertEqual(len(device_updates), len(expected_device_ids))