I work on CPython free-threading memory safety: finding C-extension lifetime bugs, building controlled reproducers, and verifying fixes at exact revisions.

I build checks that can fail, and I try to keep what I claim under what I can show. When I report a concurrency bug, I try to bring an exact revision, a base-positive reproducer, negative and decoy controls, and an exact-head re-test of the eventual fix.

Code I authored and maintainers merged:

• cpython#159864 → cpython#159884 — reproduced a free-threaded syslog.openlog() crash when another thread changes sys.argv[0], and fixed the borrowed reference. A core developer merged it into main on 2026-10-20; the 3.15 backport (#159952) is merged and the 3.14 backport (#159954) is still open. Others had listed the problem before I reproduced it.
• zope.interface#380 → zope.interface#382 — contributed C cache-lifetime fixes and regression tests, revised with maintainers during review and merged on 2026-09-23.
• pybind11#6169 — added regression tests for syntax-error and traceback line numbers in evaluated Python source.
• Pillow#9953 — added regression tests for ImageChops.offset() on zero-sized images.
• python-rapidjson#236 — made the free-threading branch run CI, added a real 3.14t job with a fail-closed GIL assertion, and repaired the Python <3.13 collection failure.
• python-rapidjson#237 — restored stubtest, aligned constructor stubs with the C extension, and restricted wheel uploads to release tags.
• python-rapidjson#239 — fixed the rejected-argument allocation leak in RawJSON() and corrected its stub. It merged into master and shipped in v1.24.
• nox#1153 — fixed tests that only passed when uv happened to be installed.
• mutmut#546 — added ternary-condition mutation for a branch the mutation engine previously never touched.
• StringZilla#338 — stopped the 5.x wheels from installing a top-level test package into site-packages; merged on 2026-10-02.

Reports that led to upstream fixes:

• Pillow#9892 → Pillow#9893 — reported out-of-bounds reads caused by sequence-length handling. Another contributor, lazerg, authored the fix; Pillow's maintainers merged it on 2026-09-25.
• pybind11#6180 — reported unconstructed sibling-base storage surviving deprecated constructor dispatch, and re-tested two reproducer paths against the maintainer's fix at exact revisions. The maintainer authored and merged the fix.
• CodeQL#22305 → CodeQL#22310 — reported lost Python taint flow through list.extend, list.insert, and +=; the merged maintainer/bot PR closes extend and insert. The larger += gap is not claimed fixed.
• wrapt#347 — reported a free-threaded double-free. The maintainer made a broader repair and released 2.4.0rc1; I re-ran the same controlled harness at their request: 10/10 SIGSEGV → 0/10.
• StringZilla#328 — reported a stale-size borrowed-list-item crash; the maintainer fixed it in v5.1.1.
• confluent-kafka#2319 → confluent-kafka#2334 — reported a borrowed-reference use-after-free in Admin_create_topics. A Confluent engineer wrote the fix, initially merged into the free-threading preview branch. On 2026-09-21, the maintainer confirmed it had reached master and closed the issue. I re-ran my harness against their exact fix head at their request.

Merged fixes I reproduced or reviewed:

• Pillow#9917 → Pillow#9919 — contributed additional re-entrant and concurrent mutation cases, then tested the Python-side copy at commit 655298e on GIL and free-threaded builds. The report and fix PR were written by other contributors; Pillow's maintainers reviewed and merged it.

Status in this section was rechecked on 2026-10-21.

• cpython#157841 — reported a free-threaded 3.14 crash in concurrent functools.partial.__setstate__; open.

• pybind11#6177 — proposed removing an internals-lock acquisition from bound-call dispatch on free-threaded builds, with a regression test that checks whether dispatch can proceed while that lock is held. Open and not merged.

• python-rapidjson#235 — my free-threaded container-walk fix; open and not merged.

• tree#143 → tree#144 — reported and reproduced memory-safety faults in free-threaded dict traversal; the fix PR is open and was authored by another contributor.

• PyO3#5774 — closed without merging after maintainer feedback. CodSpeed measured an 11.6% regression.
• Pillow#9854 — proposed a critical section around the getlist() walk for Pillow#9852; closed without merging. A maintainer closed the issue as completed on 2026-09-04.

• firing-checks — includes negative controls and a --selftest that runs them for three of my own tools.
• scanner-false-negatives — explored where a scanner's false negatives cluster; the experiments did not support the hypothesis. All 61 labels are public; they were LLM-produced, have no independent human regrade, and the repository is a post-hoc export.

I am an external contributor, not a maintainer or core developer of the projects above. Open PRs are not merged contributions, and unresolved reports are not counted as fixes. Private reports are not public evidence and are deliberately omitted. My own tools still have no independent adopter, so their value outside my repositories remains unmeasured.

Reach me: espressolee1@gmail.com · ORCID 0009-0003-0423-6686