gh-108240: Fix a reference cycle in _socket.CAPI capsule #108241
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_socket.CAPI capsule contains a strong reference to _socket.socket type. The garbage collector cannot visit this reference and so cannot create the reference cycle involving _socket.socket (a heap type creates a reference cycle to inside, via MRO and methods). At Python, _PyImport_ClearModules() sets _socket attributes to None which works around the issue. If the module is cleared from sys.modules manually, _PyImport_ClearModules() cannot set _socket.CAPI to None and so the issue cannot be worked around. Change _socket.CAPI to use a borrowed reference instead of a strong reference to allow clearing _socket.socket in this case. Co-authored-by: Kirill Podoprigora <[email protected]>
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Storing a borrowed reference in the capsule object is unsafe if the capsule is used after the extension is finalized. But right now, it's not possible to configure a capsule object to visit its strong references. |
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If we agree that this approach is the correct one, I will write a NEWS entry and backport the change to stable versions. |
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The PR fix the issue. Example with reproducer attached to the issue: |
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Since the |
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I merged the safe approach instead: PR #108339. |
_socket.CAPI capsule contains a strong reference to _socket.socket type. The garbage collector cannot visit this reference and so cannot create the reference cycle involving _socket.socket (a heap type creates a reference cycle to inside, via MRO and methods). At Python, _PyImport_ClearModules() sets _socket attributes to None which works around the issue.
If the module is cleared from sys.modules manually, _PyImport_ClearModules() cannot set _socket.CAPI to None and so the issue cannot be worked around.
Change _socket.CAPI to use a borrowed reference instead of a strong reference to allow clearing _socket.socket in this case.