2 files changed, 6 insertions, 8 deletions
diff --git a/changelog.d/18391.bugfix b/changelog.d/18391.bugfix
new file mode 100644
index 0000000000..bbcb7b7a28
--- /dev/null
+++ b/changelog.d/18391.bugfix
@@ -0,0 +1 @@
+Prevent race-condition in `_maybe_retry_device_resync` entrance.
diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index 1efd039f22..f8b547bbed 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -20,6 +20,7 @@
 #
 #
 import logging
+from threading import Lock
 from typing import (
     TYPE_CHECKING,
     AbstractSet,
@@ -1237,7 +1238,7 @@ class DeviceListUpdater(DeviceListWorkerUpdater):
         )
 
         # Attempt to resync out of sync device lists every 30s.
-        self._resync_retry_in_progress = False
+        self._resync_retry_lock = Lock()
         self.clock.looping_call(
             run_as_background_process,
             30 * 1000,
@@ -1419,13 +1420,10 @@ class DeviceListUpdater(DeviceListWorkerUpdater):
         """Retry to resync device lists that are out of sync, except if another retry is
         in progress.
         """
-        if self._resync_retry_in_progress:
+        # If the lock can not be acquired we want to always return immediately instead of blocking here
+        if not self._resync_retry_lock.acquire(blocking=False):
             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()
 
@@ -1466,8 +1464,7 @@ class DeviceListUpdater(DeviceListWorkerUpdater):
                         e,
                     )
         finally:
-            # Allow future calls to retry resyncinc out of sync device lists.
-            self._resync_retry_in_progress = False
+            self._resync_retry_lock.release()
 
     async def multi_user_device_resync(
         self, user_ids: List[str], mark_failed_as_stale: bool = True