Fix dropping locks on shut down (#10433)

changelog.d/10433.bugfix

@@ -0,0 +1 @@
+Fix error while dropping locks on shutdown. Introduced in v1.38.0.

synapse/storage/databases/main/lock.py

@@ -78,7 +78,11 @@ async def _on_shutdown(self) -> None:
         """Called when the server is shutting down"""
         logger.info("Dropping held locks due to shutdown")
 
-        for (lock_name, lock_key), token in self._live_tokens.items():
+        # We need to take a copy of the tokens dict as dropping the locks will
+        # cause the dictionary to change.
+        tokens = dict(self._live_tokens)
+
+        for (lock_name, lock_key), token in tokens.items():
             await self._drop_lock(lock_name, lock_key, token)
 
         logger.info("Dropped locks due to shutdown")

tests/storage/databases/main/test_lock.py

@@ -98,3 +98,16 @@ def test_drop(self):
 
         lock2 = self.get_success(self.store.try_acquire_lock("name", "key"))
         self.assertIsNotNone(lock2)
+
+    def test_shutdown(self):
+        """Test that shutting down Synapse releases the locks"""
+        # Acquire two locks
+        lock = self.get_success(self.store.try_acquire_lock("name", "key1"))
+        self.assertIsNotNone(lock)
+        lock2 = self.get_success(self.store.try_acquire_lock("name", "key2"))
+        self.assertIsNotNone(lock2)
+
+        # Now call the shutdown code
+        self.get_success(self.store._on_shutdown())
+
+        self.assertEqual(self.store._live_tokens, {})