diff --git a/packages/overlays/matrix-synapse/patches/0028-fix-device-handler-make-_maybe_retry_device_resync-t.patch b/packages/overlays/matrix-synapse/patches/0028-fix-device-handler-make-_maybe_retry_device_resync-t.patch
new file mode 100644
index 0000000..87859bf
--- /dev/null
+++ b/packages/overlays/matrix-synapse/patches/0028-fix-device-handler-make-_maybe_retry_device_resync-t.patch
@@ -0,0 +1,94 @@
+From 33ba8860c43d4770ea119a09a4fcbbf366f3b32e Mon Sep 17 00:00:00 2001
+From: 3nprob <74199244+3nprob@users.noreply.github.com>
+Date: Mon, 26 May 2025 14:21:43 +0000
+Subject: [PATCH 28/34] 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
+style](https://element-hq.github.io/synapse/latest/code_style.html) is
+correct
+(run the
+[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))
+---
+ changelog.d/18391.bugfix | 1 +
+ synapse/handlers/device.py | 13 +++++--------
+ 2 files changed, 6 insertions(+), 8 deletions(-)
+ create mode 100644 changelog.d/18391.bugfix
+
+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
+--
+2.49.0
+
|
