PYTHON 5212 - Use asyncio.loop.sock_connect in _async_create_connection (#2383)

Author: NoahStapp

doc/changelog.rst

@@ -8,6 +8,23 @@ PyMongo 4.14 brings a number of changes including:
 - Added :attr:`bson.codec_options.TypeRegistry.codecs` and :attr:`bson.codec_options.TypeRegistry.fallback_encoder` properties
   to allow users to directly access the type codecs and fallback encoder for a given :class:`bson.codec_options.TypeRegistry`.
 
+Changes in Version 4.13.2 (2025/06/17)
+--------------------------------------
+
+Version 4.13.2 is a bug fix release.
+
+- Fixed a bug where ``AsyncMongoClient`` would block the event loop while creating new connections,
+  potentially significantly increasing latency for ongoing operations.
+
+Issues Resolved
+...............
+
+See the `PyMongo 4.13.2 release notes in JIRA`_ for the list of resolved issues
+in this release.
+
+.. _PyMongo 4.13.2 release notes in JIRA: https://jira.mongodb.org/secure/ReleaseNote.jspa?projectId=10004&version=43937
+
+
 Changes in Version 4.13.1 (2025/06/10)
 --------------------------------------
 

pymongo/pool_shared.py

@@ -206,7 +206,8 @@ async def _async_create_connection(address: _Address, options: PoolOptions) -> s
         # SOCK_CLOEXEC not supported for Unix sockets.
         _set_non_inheritable_non_atomic(sock.fileno())
         try:
-            sock.connect(host)
+            sock.setblocking(False)
+            await asyncio.get_running_loop().sock_connect(sock, host)
             return sock
         except OSError:
             sock.close()
@@ -241,14 +242,22 @@ async def _async_create_connection(address: _Address, options: PoolOptions) -> s
                 timeout = options.connect_timeout
             elif timeout <= 0:
                 raise socket.timeout("timed out")
-            sock.settimeout(timeout)
             sock.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, True)
             _set_keepalive_times(sock)
-            sock.connect(sa)
+            # Socket needs to be non-blocking during connection to not block the event loop
+            sock.setblocking(False)
+            await asyncio.wait_for(
+                asyncio.get_running_loop().sock_connect(sock, sa), timeout=timeout
+            )
+            sock.settimeout(timeout)
             return sock
+        except asyncio.TimeoutError as e:
+            sock.close()
+            err = socket.timeout("timed out")
+            err.__cause__ = e
         except OSError as e:
-            err = e
             sock.close()
+            err = e  # type: ignore[assignment]
 
     if err is not None:
         raise err

test/asynchronous/test_async_loop_unblocked.py

@@ -0,0 +1,56 @@
+# 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():
+            start = time.monotonic()
+            try:
+                while True:
+                    start = time.monotonic()
+                    await asyncio.sleep(0.1)
+                    latencies.append(time.monotonic() - start)
+            except asyncio.CancelledError:
+                latencies.append(time.monotonic() - start)
+                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],
+            1.0,
+            "Background task was blocked from running",
+        )

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",
     ]