Detailed steps to reproduce the problem?

I found a race condition during the thread sanitizer investigation in PYTHON-6048. It can be reproduced by instrumenting the C extensions with -fsanitize=thread. Closing a client reports a data race between close() tearing down monitor sockets and a monitor thread still inside recv() on the same connection.

The tradeoff for fixing the race for synchronous client: close() blocks for up to ~1 s (typical ≤ 0.5 s) instead of returning instantly at 0 ms.

Overall current state:

• Sync clients: close() signals the monitor threads (heartbeat, RTT, and SRV polling for mongodb+srv:// clients) to stop but never joins them. The threads keep running after close() returns; that is the race the sanitizer caught.
• Async clients: close() joins monitor tasks, but with no timeout, so a monitor that never stops hangs close() forever.
• In both cases, joining can also happen while the topology lock is held, blocking unrelated server selection.

Desired state:

• close() waits for all of its monitor threads to stop before returning.
• Monitors are joined after the topology lock is released, within a single ~1 s cap, regardless of monitor count, so shutdown stays bounded no matter how many monitors the client has.
• Closing a mongodb+srv:// client costs no more than closing any other client: the SRV polling thread is covered by the same single deadline, with no SRV-specific fixed delay.
• Subinterpreter shutdown also joins monitors against a single deadline, not ~1 s per monitor.

Related tickets:

• PYTHON-2338 (unclosed _RttMonitor sockets when the executor stops before check-in)
• PYTHON-5053 (unbounded close joins)
• PYTHON-5418 makes monitor threads non-daemon in subinterpreters and joins them at teardown ~1 s per executor, so the single shared deadline should also cover subinterpreter shutdown.

The exact Python version used, with patch level:

3.14.2 free-threaded (CPython 3.14.2t); the behavior reproduces on any CPython version.

The exact version of PyMongo used, with patch level:

4.19.0.dev0 (PYTHON-6048 branch); has_c() = True. The race window dates back to PYTHON-2123 (2020).

Describe how MongoDB is set up. Local vs Hosted, version, topology, load balanced, etc.

Local mongod 8.0, standalone and replica set topologies; streaming server monitoring (serverMonitoringMode=stream).

The operating system and version (e.g. Windows 7, OSX 10.8, ...)

Linux (Ubuntu 22.04/26.04, aarch64).

Web framework or asynchronous network library used, if any, with version (e.g. Django 1.7, mod_wsgi 4.3.0, gevent 1.0.1, Tornado 4.0.2, ...)

None.

Security Vulnerabilities

None identified - this is a correctness/latency issue, not a security vulnerability.