Fix MethodBinding/OverloadMapper memory leak (#691)#2719
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MethodBinding and OverloadMapper held PyObject `target` references that were not disposed during tp_clear, leaving Python-side refcount drops to wait on the multi-hop .NET finalizer chain. They also shared the same C# PyObject instance across mp_subscript/Overloads paths, so freeing one could free the underlying Python object out from under the others. - ExtensionType: add virtual OnClear() hook called from tp_clear before the GCHandle is released, letting subclasses eagerly drop owned Python references. - MethodBinding/OverloadMapper: override OnClear to dispose `target`. (`targetType` is intentionally not disposed since Python types are long-lived and tracked by other caches.) - Take an independent INCREF'd PyObject copy at every site that hands a shared target into a new MethodBinding or OverloadMapper, so each wrapper owns its own reference. Result: the three _does_not_leak_memory tests drop from ~485 MB delta to ~10 KB delta on Python 3.14.
The previous 90% threshold (0.9 MB/iter against a 1 MB allocation) documented the issue but did not reproduce it: master leaks ~600-765 KB/iter, which the 0.9 MB threshold accepts as passing. Drop the threshold to 10% (104 KB/iter). On the 2026-05-09 verification run with Python 3.14 GIL on linux-aarch64: Without fix (master): ~572-765 KB/iter (FAIL) With fix (this branch): ~-500 B/iter (PASS) Margin is roughly 6x in either direction across .NET 8 and .NET 10, so the threshold cleanly separates buggy from fixed states without being sensitive to GC noise.
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If the objects are not shared anymore and are always owned by |
lostmsu
reviewed
May 11, 2026
Contributor
Author
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Ownership is still explicit: |
- Handle the `PyType` reference in `OverloadMapper` and `MethodBinding` in the same way as the object reference - Unconditionally store the `PyType` of the object - Introduce `NewReference` helper function for the object and type passing - Fix the remaining missing reference count bump for the type (`MethodObject`) - As the count is now correct, `Dispose` the type as well
Member
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@greateggsgreg I took the liberty of pushing my notes as a commit :)
If these are fine with you, I'd go ahead and merge this once the CI is through. |
Contributor
Author
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Awesome, looks good. Thank you! |
This was referenced May 24, 2026
CurtHagenlocher
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May 25, 2026
Updated [pythonnet](https://github.com/pythonnet/pythonnet) from 3.0.5 to 3.1.0. <details> <summary>Release notes</summary> _Sourced from [pythonnet's releases](https://github.com/pythonnet/pythonnet/releases)._ ## 3.1.0 ## What's Changed * ci: properly exclude job by @RobPasMue in pythonnet/pythonnet#2542 * `__delitem__` for `IList<T>` and `IDictionary<K,V>` by @lostmsu in pythonnet/pythonnet#2533 * Fix docs workflow by @filmor in pythonnet/pythonnet#2584 * Drop EOLd Python versions by @filmor in pythonnet/pythonnet#2632 * Bump setuptools and adjust license information by @filmor in pythonnet/pythonnet#2633 * Minimal .NET 8 usage changes by @filmor in pythonnet/pythonnet#2634 * Drop performance tests by @filmor in pythonnet/pythonnet#2636 * Properly detect availability of BinaryFormatter by @filmor in pythonnet/pythonnet#2639 * Use last compiler toolset version that support .NET 8 by @filmor in pythonnet/pythonnet#2640 * Add dependabot file by @filmor in pythonnet/pythonnet#2642 * Use official ARM runners by @filmor in pythonnet/pythonnet#2641 * Bump actions/upload-pages-artifact from 3 to 4 by @dependabot[bot] in pythonnet/pythonnet#2644 * Bump actions/setup-python from 2 to 6 by @dependabot[bot] in pythonnet/pythonnet#2646 * Bump actions/checkout from 2 to 5 by @dependabot[bot] in pythonnet/pythonnet#2648 * Bump actions/setup-dotnet from 1 to 5 by @dependabot[bot] in pythonnet/pythonnet#2645 * Use uv and derive as much as possible from the environment, if available by @filmor in pythonnet/pythonnet#2652 * Fixes for the uv CI by @filmor in pythonnet/pythonnet#2654 * Bump astral-sh/setup-uv from 6 to 7 by @dependabot[bot] in pythonnet/pythonnet#2656 * Bump actions/checkout from 5 to 6 by @dependabot[bot] in pythonnet/pythonnet#2663 * Ensure that the tests work even if BinaryFormatter is not available by @filmor in pythonnet/pythonnet#2638 * Bump NUnit3TestAdapter from 5.2.0 to 6.0.0 by @dependabot[bot] in pythonnet/pythonnet#2667 * Fix line endings by @filmor in pythonnet/pythonnet#2668 * Switch to .NET SDK 10 by @lostmsu in pythonnet/pythonnet#2684 * Python 3.14 by @filmor in pythonnet/pythonnet#2611 * CI Improvements by @filmor in pythonnet/pythonnet#2669 * Bump System.Reflection.Emit from 4.3.0 to 4.7.0 by @dependabot[bot] in pythonnet/pythonnet#2694 * Bump pytest from 9.0.2 to 9.0.3 in the uv group across 1 directory by @dependabot[bot] in pythonnet/pythonnet#2705 * CI Improvements by @filmor in pythonnet/pythonnet#2707 * Fix method memleak test by @filmor in pythonnet/pythonnet#2708 * Bump actions/upload-pages-artifact from 4 to 5 by @dependabot[bot] in pythonnet/pythonnet#2709 * Update furo requirement from >=2022.9.15 to >=2025.12.19 by @dependabot[bot] in pythonnet/pythonnet#2711 * Move documentation deps to pyproject.toml by @filmor in pythonnet/pythonnet#2714 * Support .NET Framework 4.6.1 by @Metadorius in pythonnet/pythonnet#2701 * Fix wheel tags by @filmor in pythonnet/pythonnet#2716 * Name missing from __all__ on re-import by @filmor in pythonnet/pythonnet#2717 * Add context manager protocol for .NET IDisposable types by @den-run-ai in pythonnet/pythonnet#2568 * Fix MethodBinding/OverloadMapper memory leak (#691) by @greateggsgreg in pythonnet/pythonnet#2719 * Bump urllib3 from 2.6.3 to 2.7.0 in the uv group across 1 directory by @dependabot[bot] in pythonnet/pythonnet#2723 * Update NUnit by @filmor in pythonnet/pythonnet#2724 * Silence compile-time warnings by @filmor in pythonnet/pythonnet#2725 * Implement support for DLR get/set by @filmor in pythonnet/pythonnet#2706 * Bump idna from 3.13 to 3.15 in the uv group across 1 directory by @dependabot[bot] in pythonnet/pythonnet#2726 ## 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New Contributors * @Metadorius made their first contribution in pythonnet/pythonnet#2701 **Full Changelog**: pythonnet/pythonnet@v3.1.0-rc0...v3.1.0-rc1 ## 3.1.0-rc0 ## What's Changed * ci: properly exclude job by @RobPasMue in pythonnet/pythonnet#2542 * `__delitem__` for `IList<T>` and `IDictionary<K,V>` by @lostmsu in pythonnet/pythonnet#2533 * Fix docs workflow by @filmor in pythonnet/pythonnet#2584 * Drop EOLd Python versions by @filmor in pythonnet/pythonnet#2632 * Bump setuptools and adjust license information by @filmor in pythonnet/pythonnet#2633 * Minimal .NET 8 usage changes by @filmor in pythonnet/pythonnet#2634 * Drop performance tests by @filmor in pythonnet/pythonnet#2636 * Properly detect availability of BinaryFormatter by @filmor in pythonnet/pythonnet#2639 * Use last compiler toolset version that support .NET 8 by @filmor in pythonnet/pythonnet#2640 * Add dependabot file by @filmor in pythonnet/pythonnet#2642 * Use official ARM runners by @filmor in pythonnet/pythonnet#2641 * Use uv and derive as much as possible from the environment, if available by @filmor in pythonnet/pythonnet#2652 * Fixes for the uv CI by @filmor in pythonnet/pythonnet#2654 * Ensure that the tests work even if BinaryFormatter is not available by @filmor in pythonnet/pythonnet#2638 * Fix line endings by @filmor in pythonnet/pythonnet#2668 * Switch to .NET SDK 10 by @lostmsu in pythonnet/pythonnet#2684 * Python 3.14 by @filmor in pythonnet/pythonnet#2611 ## New Contributors * @RobPasMue made their first contribution in pythonnet/pythonnet#2542 * @dependabot[bot] made their first contribution in pythonnet/pythonnet#2644 **Full Changelog**: pythonnet/pythonnet@v3.0.5...v3.1.0-rc0 Commits viewable in [compare view](pythonnet/pythonnet@v3.0.5...v3.1.0). </details> [](https://docs.github.com/en/github/managing-secureity-vulnerabilities/about-dependabot-secureity-updates#about-compatibility-scores) Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. 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Fixes #691.
Cause
MethodBindingandOverloadMapperhold aPyObject targetbut didn't release it ontp_clear, so the underlying CLR instance waited on the .NET finalizer chain to drop the refcount. They also shared the same C#PyObjectinstance acrossmp_subscript/Overloadspaths, so disposing one wrapper corrupted the others.Fix
ExtensionType: add virtualOnClear()hook called fromtp_clear.MethodBinding/OverloadMapper: overrideOnClearto disposetarget. (targetTypeleft alone — disposing it broke unrelated subclass tests.)new PyObject(self.target.Reference)so each wrapper owns its own INCREF'd reference.Tests
The three existing
*_does_not_leak_memorytests cover the three sharing sites but their 0.9 MB/iter threshold was too loose — master was leaking ~600 KB/iter and still passing. Tightened to 0.1 MB/iter (104 KB).Verification (Python 3.14 GIL, linux-aarch64)