[3.7] clean extern PyGC_Head *_PyGC_generation0; in Include/objimpl.h

[egaudry]

BPO	38542
Nosy	@gvanrossum, @vstinner, @serhiy-storchaka, @pablogsal
Files	gc.diff: patch for exposing _PyGC_generation0 on all platforms

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2019-10-21.13:19:58.545>
created_at = <Date 2019-10-21.09:43:52.818>
labels = ['interpreter-core', 'type-feature', '3.7']
title = '[3.7] clean extern PyGC_Head *_PyGC_generation0; in Include/objimpl.h'
updated_at = <Date 2019-10-21.15:09:20.554>
user = 'https://bugs.python.org/egaudry'

bugs.python.org fields:

activity = <Date 2019-10-21.15:09:20.554>
actor = 'egaudry'
assignee = 'none'
closed = True
closed_date = <Date 2019-10-21.13:19:58.545>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2019-10-21.09:43:52.818>
creator = 'egaudry'
dependencies = []
files = ['48671']
hgrepos = []
issue_num = 38542
keywords = ['patch']
message_count = 8.0
messages = ['355034', '355036', '355058', '355065', '355066', '355068', '355077', '355078']
nosy_count = 5.0
nosy_names = ['gvanrossum', 'vstinner', 'egaudry', 'serhiy.storchaka', 'pablogsal']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue38542'
versions = ['Python 3.7']

[egaudry]

Hi

I would like to be able to get an handle on PyGC_Head*_PyGC_generation0 from a CPython extension.
This is possible when building Python on a Posix host, but not on Windows because of a missing PyAPI_DATA wrapping of the said object in the objimpl.h header.

Having the possibility to inspect the PyGC_Head here can be very useful when customizing the way a garbage-collectable object created from CPython extension would be cleaned/inspected, etc.

Any chance this patch could be added to get the same capabilities on Windows and Posix platforms ?

Thanks for your feedback,
Eloi

[vstinner]

This issue comes from a thread on python-ideas:
https://mail.python.org/archives/list/python-ideas@python.org/thread/7FC2NHAOMKBHONFTUKMAHO5F56UU5UUD/

I understand that it's specific to Python 2.7.

--

_PyGC_generation0 variable no longer exists in Python 3.7, it has been moved into _PyRuntime.gc.generation0:

Include/internal/mem.h:146:#define _PyGC_generation0 _PyRuntime.gc.generation0

In the master branch, _PyRuntime is exported with:

Include/internal/pycore_pystate.h: PyAPI_DATA(_PyRuntimeState) _PyRuntime;

In Python 3.7, _PyRuntime is also exported, but in a different header file:

Include/internal/pystate.h: PyAPI_DATA(_PyRuntimeState) _PyRuntime;

--

Python 2.7:

Modules/gcmodule.c:50:PyGC_Head *_PyGC_generation0 = GEN_HEAD(0);

Include/objimpl.h:276:extern PyGC_Head *_PyGC_generation0;

[egaudry]

Victor,

extern PyGC_Head *_PyGC_generation0;
is located at line 374 of Include/objimpl.h (ifndef Py_LIMITED_API).

I used it from a CPython extension we are developing as an handle to the collection of objects being garbage collected.

From what you explained, I should use the _PyRuntime exported symbol to get the handle this PyGC_Head. I'll do this.

I guess that cleaning the Include/objimpl.h is the only remaining action to do then.

Thanks for your feedback.

[serhiy-storchaka]

I think it is too late to add this feature in 2.7. See https://www.python.org/dev/peps/pep-0373/#release-schedule, the feature freeze was 9.5 years ago.

[egaudry]

Yes, the only thing that remains to be done is cleaning objimpl.h.