fix(device-handler): make _maybe_retry_device_resync thread-safe

Author: 3nprob

changelog.d/18391.fix

@@ -0,0 +1 @@
+Prevent race-condition in `_maybe_retry_device_resync` entrance.

synapse/handlers/device.py

@@ -20,6 +20,7 @@
 #
 #
 import logging
+from threading import Lock
 from typing import (
     TYPE_CHECKING,
     AbstractSet,
@@ -1237,7 +1238,7 @@ def __init__(self, hs: "HomeServer", device_handler: DeviceHandler):
         )
 
         # 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 +1420,9 @@ async def _maybe_retry_device_resync(self) -> None:
         """Retry to resync device lists that are out of sync, except if another retry is
         in progress.
         """
-        if self._resync_retry_in_progress:
+        if self._resync_retry_lock.locked():
             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
+        with self._resync_retry_lock:
             # Get all of the users that need resyncing.
             need_resync = await self.store.get_user_ids_requiring_device_list_resync()
 
@@ -1465,9 +1462,6 @@ async def _maybe_retry_device_resync(self) -> None:
                         user_id,
                         e,
                     )
-        finally:
-            # Allow future calls to retry resyncinc out of sync device lists.
-            self._resync_retry_in_progress = False
 
     async def multi_user_device_resync(
         self, user_ids: List[str], mark_failed_as_stale: bool = True