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[C API] Make PyGC_Head structure opaque, or even move it to the internal C API #84422
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Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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GitHub fields:
assignee = None
closed_at = <Date 2020-04-13.10:20:38.319>
created_at = <Date 2020-04-09.15:19:56.824>
labels = ['expert-C-API', '3.9']
title = '[C API] Make PyGC_Head structure opaque, or even move it to the internal C API'
updated_at = <Date 2020-06-02.10:09:35.483>
user = 'https://github.com/vstinner'bugs.python.org fields:
activity = <Date 2020-06-02.10:09:35.483>
actor = 'miss-islington'
assignee = 'none'
closed = True
closed_date = <Date 2020-04-13.10:20:38.319>
closer = 'vstinner'
components = ['C API']
creation = <Date 2020-04-09.15:19:56.824>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 40241
keywords = ['patch']
message_count = 12.0
messages = ['366064', '366067', '366092', '366094', '366095', '366170', '366291', '366296', '366300', '366353', '370600', '370601']
nosy_count = 3.0
nosy_names = ['vstinner', 'pablogsal', 'miss-islington']
pr_nums = ['19452', '19461', '19494', '19499', '19505', '20586', '20593']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue40241'
versions = ['Python 3.9']
Activity
vstinner commentedon Apr 9, 2020
Similarly to bpo-39573 (PyObject) and bpo-40170 (PyTypeObject), I propose to make the PyGC_Head structure opaque in the C API.
See https://bugs.python.org/issue39573#msg361513 for the rationale. In short, my plan is to hide all implementation details from the C API.
The PyGC_Head structure caused ABI issues recently: bpo-39599 "ABI breakage between Python 3.7.4 and 3.7.5: change in PyGC_Head structure". Making the structure opaque would reduce the risk of such ABI issue. In fact, the reporter of bpo-39599 really require to access PyGC_Head structure to write a profiler, so this issue doesn't fix all use cases, but it should benefit to most people ;-) PyGC_Head structure will remain accessible via the internal C API which doesn't provide any backward compatibility warranty.
vstinner commentedon Apr 9, 2020
See also bpo-40240: "Expose public spelling of _PyGC_FINALIZED and _PyGC_SET_FINALIZED?".
vstinner commentedon Apr 10, 2020
The following macros rely on PyGC_Head:
_testcapi uses _PyObject_GC_IS_TRACKED() and sizeof(PyGC_Head).
vstinner commentedon Apr 10, 2020
I already moved them to the internal C API in commit 1a6be91 of bpo-35081.
But Stefan Behnel reported that it breaks Cython:
"Making _PyGC_FINALIZED() internal broke Cython (cython/cython#2721). It's used in the finaliser implementation (https://github.com/cython/cython/blob/da657c8e326a419cde8ae6ea91be9661b9622504/Cython/Compiler/ModuleNode.py#L1442-L1456), to determine if an object for which tp_dealloc() is called has already been finalised or whether we have to do it. I'm not sure how to deal with this on our side now. Any clue?"
https://bugs.python.org/issue35081#msg330045
So I simply reverted (partially) this change: commit 3e21ad1.
vstinner commentedon Apr 10, 2020
_testcapi uses _PyObject_GC_IS_TRACKED(). This macro is exposed in Python as gc.is_tracked(). IMO the function should be available in the public C API. For example, PyObject_GC_IsTracked(obj).
Cython uses _PyGC_FINALIZED(). This macro is exposed in Python as gc.is_finalized(). So again, I consider that it should be exposed in a public C function as well, like PyObject_GC_IsFinalized(obj).
[-][C API] Make PyGC_Head structure opaque, or even more it to the internal C API[/-][+][C API] Make PyGC_Head structure opaque, or even move it to the internal C API[/+][-][C API] Make PyGC_Head structure opaque, or even more it to the internal C API[/-][+][C API] Make PyGC_Head structure opaque, or even move it to the internal C API[/+]pablogsal commentedon Apr 11, 2020
New changeset f13072b by Pablo Galindo in branch 'master':
bpo-40241: Add PyObject_GC_IsTracked and PyObject_GC_IsFinalized to the public C-API (GH-19461)
f13072b
vstinner commentedon Apr 13, 2020
New changeset 0135598 by Victor Stinner in branch 'master':
bpo-40241: Add pycore_gc.h header file (GH-19494)
0135598
vstinner commentedon Apr 13, 2020
Ok, this issue should now be closed. It was easier than expected ;-)
vstinner commentedon Apr 13, 2020
New changeset 0c13e1f by Victor Stinner in branch 'master':
bpo-40241: Add pycore_interp.h header (GH-19499)
0c13e1f
vstinner commentedon Apr 13, 2020
Oops, this change was for bpo-40268.
vstinner commentedon Jun 2, 2020
New changeset 337d310 by Victor Stinner in branch 'master':
bpo-40241: What's New in Python 3.9: opaque PyGC_Head (GH-20586)
337d310
miss-islington commentedon Jun 2, 2020
New changeset ff442f3 by Miss Islington (bot) in branch '3.9':
bpo-40241: What's New in Python 3.9: opaque PyGC_Head (GH-20586)
ff442f3