Add a looping call to retry resyncs

synapse/handlers/device.py

@@ -535,6 +535,10 @@ class DeviceListUpdater(object):
             iterable=True,
         )
 
+        # Attempt to resync out of sync device lists every 30s.
+        self._resync_retry_in_progress = False
+        self.clock.looping_call(self._maybe_retry_device_resync, 30 * 1000)
+
     @trace
     @defer.inlineCallbacks
     def incoming_device_list_update(self, origin, edu_content):
@@ -678,6 +682,40 @@ class DeviceListUpdater(object):
 
         return False
 
+    async def _maybe_retry_device_resync(self):
+        """Retry to resync device lists that are out of sync, except if another retry is
+        in progress.
+        """
+        if self._resync_retry_in_progress:
+            return
+
+        try:
+            # Prevent another call of this function to retry resyncing device lists so
+            # we don't send too many requests.
+            self._resync_retry_in_progress = True
+            # Get all of the users that need resyncing.
+            need_resync = await self.store.get_user_ids_requiring_device_list_resync()
+            # Iterate over the set of user IDs.
+            for user_id in need_resync:
+                # Try to resync the current user's devices list. Exception handling
+                # isn't necessary here, since user_device_resync catches all instances
+                # of "Exception" that might be raised from the federation request. This
+                # means that if an exception is raised by this function, it must be
+                # because of a database issue, which means _maybe_retry_device_resync
+                # probably won't be able to go much further anyway.
+                result = await self.user_device_resync(user_id=user_id)
+                # user_device_resync only returns a result if it managed to successfully
+                # resync and update the database. Updating the table of users requiring
+                # resync isn't necessary here as user_device_resync already does it
+                # (through self.store.update_remote_device_list_cache).
+                if result:
+                    logger.debug(
+                        "Successfully resynced the device list for %s" % user_id,
+                    )
+        finally:
+            # Allow future calls to retry resyncinc out of sync device lists.
+            self._resync_retry_in_progress = False
+
     @defer.inlineCallbacks
     def user_device_resync(self, user_id):
         """Fetches all devices for a user and updates the device cache with them.