fix(device-handler): make _maybe_retry_device_resync thread-safe (#18391)

A race-condition may render concurrent retry loops.

Use an actual `Lock` for guarding single access of device resyncing
retrying.

### Pull Request Checklist

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
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correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

changelog.d/18391.bugfix

@@ -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 @@ 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