database/LockStore: use Linearizer for lock acquisition

This fixes the "test_acquire_contention" test case added in the previous
commit.

Signed-off-by: Sumner Evans <[email protected]>

synapse/storage/databases/main/lock.py

@@ -26,6 +26,7 @@
     LoggingTransaction,
 )
 from synapse.util import Clock
+from synapse.util.async_helpers import Linearizer
 from synapse.util.stringutils import random_string
 
 if TYPE_CHECKING:
@@ -84,6 +85,8 @@ def __init__(
             self._on_shutdown,
         )
 
+        self.limiter = Linearizer("database_lock_linearizer")
+
     @wrap_as_background_process("LockStore._on_shutdown")
     async def _on_shutdown(self) -> None:
         """Called when the server is shutting down"""
@@ -103,6 +106,16 @@ async def try_acquire_lock(self, lock_name: str, lock_key: str) -> Optional["Loc
         context manager if the lock is successfully acquired, which *must* be
         used (otherwise the lock will leak).
         """
+        async with self.limiter.queue((lock_name, lock_key)):
+            return await self._try_acquire_lock(lock_name, lock_key)
+
+    async def _try_acquire_lock(
+        self, lock_name: str, lock_key: str
+    ) -> Optional["Lock"]:
+        """Try to acquire a lock for the given name/key. Will return an async
+        context manager if the lock is successfully acquired, which *must* be
+        used (otherwise the lock will leak).
+        """
 
         # Check if this process has taken out a lock and if it's still valid.
         lock = self._live_tokens.get((lock_name, lock_key))