1 files changed, 15 insertions, 6 deletions
diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index 9e52af5f13..9356ae998e 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -1030,7 +1030,7 @@ class DeviceListWorkerUpdater:
 
     async def multi_user_device_resync(
         self, user_ids: List[str], mark_failed_as_stale: bool = True
-    ) -> Dict[str, Optional[JsonDict]]:
+    ) -> Dict[str, Optional[JsonMapping]]:
         """
         Like `user_device_resync` but operates on multiple users **from the same origin**
         at once.
@@ -1059,6 +1059,7 @@ class DeviceListUpdater(DeviceListWorkerUpdater):
         self._notifier = hs.get_notifier()
 
         self._remote_edu_linearizer = Linearizer(name="remote_device_list")
+        self._resync_linearizer = Linearizer(name="remote_device_resync")
 
         # user_id -> list of updates waiting to be handled.
         self._pending_updates: Dict[
@@ -1301,7 +1302,7 @@ class DeviceListUpdater(DeviceListWorkerUpdater):
 
     async def multi_user_device_resync(
         self, user_ids: List[str], mark_failed_as_stale: bool = True
-    ) -> Dict[str, Optional[JsonDict]]:
+    ) -> Dict[str, Optional[JsonMapping]]:
         """
         Like `user_device_resync` but operates on multiple users **from the same origin**
         at once.
@@ -1321,9 +1322,11 @@ class DeviceListUpdater(DeviceListWorkerUpdater):
         failed = set()
         # TODO(Perf): Actually batch these up
         for user_id in user_ids:
-            user_result, user_failed = await self._user_device_resync_returning_failed(
-                user_id
-            )
+            async with self._resync_linearizer.queue(user_id):
+                (
+                    user_result,
+                    user_failed,
+                ) = await self._user_device_resync_returning_failed(user_id)
             result[user_id] = user_result
             if user_failed:
                 failed.add(user_id)
@@ -1335,7 +1338,7 @@ class DeviceListUpdater(DeviceListWorkerUpdater):
 
     async def _user_device_resync_returning_failed(
         self, user_id: str
-    ) -> Tuple[Optional[JsonDict], bool]:
+    ) -> Tuple[Optional[JsonMapping], bool]:
         """Fetches all devices for a user and updates the device cache with them.
 
         Args:
@@ -1348,6 +1351,12 @@ class DeviceListUpdater(DeviceListWorkerUpdater):
               e.g. due to a connection problem.
             - True iff the resync failed and the device list should be marked as stale.
         """
+        # Check that we haven't gone and fetched the devices since we last
+        # checked if we needed to resync these device lists.
+        if await self.store.get_users_whose_devices_are_cached([user_id]):
+            cached = await self.store.get_cached_devices_for_user(user_id)
+            return cached, False
+
         logger.debug("Attempting to resync the device list for %s", user_id)
         log_kv({"message": "Doing resync to update device list."})
         # Fetch all devices for the user.