Deprecate _PyObject_GC_TRACK() in Python 3.6

[vstinner]

BPO	35229
Nosy	@vstinner, @miss-islington
PRs	[3.7] bpo-35229: Deprecate _PyObject_GC_TRACK() #10511 [3.6] bpo-35229: Deprecate _PyObject_GC_TRACK() (GH-10511) #10512

^{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 2018-11-13.14:01:22.146>
created_at = <Date 2018-11-13.11:49:14.251>
labels = ['interpreter-core', '3.7']
title = 'Deprecate _PyObject_GC_TRACK() in Python 3.6'
updated_at = <Date 2018-11-13.14:01:22.145>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2018-11-13.14:01:22.145>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2018-11-13.14:01:22.146>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2018-11-13.11:49:14.251>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 35229
keywords = ['patch']
message_count = 4.0
messages = ['329829', '329831', '329832', '329836']
nosy_count = 2.0
nosy_names = ['vstinner', 'miss-islington']
pr_nums = ['10511', '10512']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue35229'
versions = ['Python 3.6', 'Python 3.7']

[vstinner]

The _PyObject_GC_TRACK() and _PyObject_GC_UNTRACK() macros are documented:

• https://docs.python.org/dev/c-api/gcsupport.html#c.\_PyObject_GC_TRACK
• https://docs.python.org/dev/c-api/gcsupport.html#c.\_PyObject_GC_UNTRACK

The documentation says "It should not be used for extension modules."

These macros have been broken in Python 3.7 by Eric Snow's work on moving internal APIs to Include/internal/: _PyObject_GC_TRACK() access _PyRuntime.gc.generation0, whereas accessing _PyRuntime requires to include Include/internal/pycore_pystate.h (header previously called Include/internal/pystate.h in Python 3.7) which is not installed by "make install".

My PR 10507 moved these macros to Include/internal/ to clarify that they must not be used outside CPython internals.

I suggest to deprecate them in Python 3.6: just add a note in Python 3.6 and 3.7 documentation.

[vstinner]

These macros have been broken in Python 3.7 by Eric Snow's work on moving internal APIs to Include/internal/ (...)

Oh, I forgot to mention that IMHO it's perfectly fine :-)

[vstinner]

New changeset 55b4771 by Victor Stinner in branch '3.7':
bpo-35229: Deprecate _PyObject_GC_TRACK() (GH-10511)
55b4771

[miss-islington]

New changeset 60a6bae by Miss Islington (bot) in branch '3.6':
bpo-35229: Deprecate _PyObject_GC_TRACK() (GH-10511)
60a6bae