[NoahStapp]

PYTHON-6182

Changes in this PR

Disable TLS session reuse on Python 3.11 due to a memory leak bug in CPython.

Test Plan

New tests.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[Copilot]

🟡 Changes recommended

The version guard leaves TLS session reuse enabled on affected Python 3.12.0–3.12.6 and 3.13.0 releases.

1 open issue

• Use fixed Python patch-version boundaries for the TLS leak fix · New

What changed in this PR

This PR disables TLS session reuse on Python 3.11 in PyMongo’s shared connection pool to avoid a CPython memory leak.

Changes:

• Prevents session-cache allocation on Python versions below 3.12.
• Adds cache-disabled assertions and skips reuse tests on affected runtimes.
• Documents the workaround and upstream fix versions.

File	Description
test/​test_ssl.py	Updates generated synchronous TLS cache tests.
test/​asynchronous/​test_ssl.py	Adds version-dependent TLS cache assertions and skips.
pymongo/​pool_shared.py	Gates session-cache allocation by Python version.
doc/​changelog.rst	Records the workaround and CPython fix versions.

_{Review effort: Balanced}

[aclark4life]

Agree

[NoahStapp]

We specifically said in our meeting that we do not distinguish between patch versions of CPython. If users want to avoid this bug on Python 3.12 or 3.13, they will have to upgrade patch versions.

[aclark4life]

In that case, we should include that info in the changelog e.g.:

Disabled TLS session reuse on Python 3.11 due to a CPython memory leak bug (python/cpython#116810 <https://github.com/python/cpython/issues/116810>). The bug was fixed in Python 3.12.7 and Python 3.13.1. Python 3.12.0 through 3.12.6 and 3.13.0 still contain the bug; users on those versions should upgrade Python. (PYTHON-6182)

[NoahStapp]

The changelog already says "The bug was fixed in Python 3.12.7 and Python 3.13.1", which implies that Python 3.12.0 through 3.12.6 and 3.13.0 contain the bug.

[aclark4life]

The changelog already says "The bug was fixed in Python 3.12.7 and Python 3.13.1", which implies that Python 3.12.0 through 3.12.6 and 3.13.0 contain the bug.

The changelog should explicitly state that we are not addressing the bug on 3.12 - 3.12.6 and 3.13 and that "Python 3.12.0 through 3.12.6 and 3.13.0 still contain the bug; users on those versions should upgrade Python." (PYTHON-6182)" if for no other reason than Copilot may not have flagged this had we done that to begin with 😂

In other words, "You can use 3.12 with PyMongo just make sure you upgrade your Python." Otherwise, the implicit warning to not use PyMongo with those versions is more likely to go unnoticed.

Ideally we would track patch versions of Python but that's a different issue and the least we can do here is be super-clear about what's going on. 🙏

[aclark4life]

Agree