Add regression test

Author: NoahStapp

test/asynchronous/test_async_loop_unblocked.py

@@ -0,0 +1,57 @@
+# Copyright 2025-present MongoDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Test that the asynchronous API does not block the event loop."""
+from __future__ import annotations
+
+import asyncio
+import time
+from test.asynchronous import AsyncIntegrationTest
+
+from pymongo.errors import ServerSelectionTimeoutError
+
+
+class TestClientLoopUnblocked(AsyncIntegrationTest):
+    async def test_client_does_not_block_loop(self):
+        # Use an unreachable TEST-NET host to ensure that the client times out attempting to create a connection.
+        client = self.simple_client("192.0.2.1", serverSelectionTimeoutMS=500)
+        latencies = []
+
+        # If the loop is being blocked, at least one iteration will have a latency much more than 0.1 seconds
+        async def background_task():
+            last_run = None
+            try:
+                while True:
+                    if last_run:
+                        latencies.append(time.monotonic() - last_run)
+                    last_run = time.monotonic()
+                    await asyncio.sleep(0.1)
+            except asyncio.CancelledError:
+                latencies.append(time.monotonic() - last_run)
+                raise
+
+        t = asyncio.create_task(background_task())
+
+        with self.assertRaisesRegex(ServerSelectionTimeoutError, "No servers found yet"):
+            await client.admin.command("ping")
+
+        t.cancel()
+        with self.assertRaises(asyncio.CancelledError):
+            await t
+
+        self.assertLessEqual(
+            sorted(latencies, reverse=True)[0],
+            0.2,
+            "Task took longer than twice its sleep time to run again",
+        )

tools/synchro.py

@@ -186,6 +186,7 @@ def async_only_test(f: str) -> bool:
         "test_async_cancellation.py",
         "test_async_loop_safety.py",
         "test_async_contextvars_reset.py",
+        "test_async_loop_unblocked.py",
     ]