From 1409c409b0e7fa2bf6f8622ea862734dc1090e2a Mon Sep 17 00:00:00 2001
From: Erik Johnston <erikj@element.io>
Date: Mon, 19 Oct 2026 08:18:45 +0000
Subject: [PATCH] Release a lock directly while the server shuts down

A lock released in the background ran its delete as a background
process. Once shutdown has begun, starting one raises. So a persist
that had committed failed as it released the purge lock, and the lock's
row stayed until the reaper removed it. The delete is now awaited once
the server is shutting down, as it was before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PVLsNh6akt4ifEizY4D1ta
---
 synapse/server.py                      |  5 +++++
 synapse/storage/databases/main/lock.py |  3 ++-
 tests/handlers/test_worker_lock.py     | 22 ++++++++++++++++++++++
 3 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/synapse/server.py b/synapse/server.py
index fffc144aed..a731bb3066 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -371,6 +371,11 @@ class HomeServer(metaclass=abc.ABCMeta):
         # For every server we spawn in the process, track it in the metrics
         synapse_server_name_info.labels(**{SERVER_NAME_LABEL: self.hostname}).set(1)
 
+    def is_shutting_down(self) -> bool:
+        """Whether `shutdown` has been called. Background processes can no
+        longer be started."""
+        return self._is_shutdown
+
     def run_as_background_process(
         self,
         desc: "LiteralString",
diff --git a/synapse/storage/databases/main/lock.py b/synapse/storage/databases/main/lock.py
index 3bbb2a5780..963ce9d1da 100644
--- a/synapse/storage/databases/main/lock.py
+++ b/synapse/storage/databases/main/lock.py
@@ -532,7 +532,8 @@ class Lock:
             },
             desc="drop_lock",
         )
-        if wait:
+        if wait or self._hs.is_shutting_down():
+            # No background process can start once shutdown has begun.
             await delete
         else:
             self._hs.run_as_background_process("Lock.release", lambda: delete)
diff --git a/tests/handlers/test_worker_lock.py b/tests/handlers/test_worker_lock.py
index 5563ec4683..fd209b6e25 100644
--- a/tests/handlers/test_worker_lock.py
+++ b/tests/handlers/test_worker_lock.py
@@ -59,6 +59,28 @@ class WorkerLockTestCase(unittest.HomeserverTestCase):
         self.get_success(d2)
         self.get_success(lock2.__aexit__(None, None, None))
 
+    def test_release_in_background_while_shutting_down(self) -> None:
+        """A lock released in the background once shutdown has begun is still
+        released, without failing the caller."""
+        lock = self.worker_lock_handler.acquire_read_write_lock(
+            "name", "key", write=False, release_in_background=True
+        )
+        self.get_success(lock.__aenter__())
+
+        # `shutdown` sets this first, then waits for the rest to stop.
+        self.hs._is_shutdown = True
+        try:
+            self.get_success(lock.__aexit__(None, None, None))
+        finally:
+            self.hs._is_shutdown = False
+
+        rows = self.get_success(
+            self.store.db_pool.simple_select_list(
+                "worker_read_write_locks", {"lock_name": "name"}, ["token"]
+            )
+        )
+        self.assertEqual(rows, [])
+
     def test_timeouts_for_lock_locally(self) -> None:
         """
         Test that we regularly retry to reacquire locks.