Initialize locks lazily

It seems that during class creation time, the event loop that is automatically
detected by asyncio.Lock is not the same event loop that ends up being used
by asyncio.run. Therefore, when two coroutines try to access the lock at the
same time and one of them needs to await it, a RuntimeError is raised because
of mismatched event loops.

server/asyncio_extensions.py

@@ -89,11 +89,14 @@ def _synchronize(
     lock: Optional[asyncio.Lock] = None
 ) -> AsyncFunc:
     """Wrap an async function with an async lock."""
-    if lock is None:
-        lock = asyncio.Lock()
-
     @wraps(function)
     async def wrapped(*args, **kwargs):
+        nonlocal lock
+
+        # During testing, functions are called from multiple loops
+        if lock is None or lock._loop != asyncio.get_event_loop():
+            lock = asyncio.Lock()
+
         async with lock:
             return await function(*args, **kwargs)
 

tests/unit_tests/test_matchmaker_queue.py

@@ -1,7 +1,9 @@
 import asyncio
 import functools
+import time
 from concurrent.futures import CancelledError, TimeoutError
 
+import mock
 import pytest
 from hypothesis import given
 from hypothesis import strategies as st
@@ -406,3 +408,33 @@ async def find_matches():
     matchmaker_queue.on_match_found.assert_called_once_with(
         s2, s3, matchmaker_queue
     )
+
+
+@pytest.mark.asyncio
+async def test_find_matches_synchronized(queue_factory):
+    is_matching = False
+
+    def make_matches(*args):
+        nonlocal is_matching
+
+        assert not is_matching, "Function call not synchronized"
+        is_matching = True
+
+        time.sleep(0.2)
+
+        is_matching = False
+        return []
+
+    with mock.patch(
+        "server.matchmaker.matchmaker_queue.make_matches",
+        make_matches
+    ):
+        queues = [queue_factory(f"Queue{i}") for i in range(5)]
+        # Ensure that find_matches does not short circuit
+        for queue in queues:
+            queue._queue = {mock.Mock(): 1, mock.Mock(): 2}
+            queue.find_teams = mock.Mock()
+
+        await asyncio.gather(*[
+            queue.find_matches() for queue in queues
+        ])