[issue36465] No longer enable Py_TRACE_REFS by default in debug build

[STINNER Victor]

STINNER Victor <vstinner at redhat.com> added the comment:

> Another more radical idea is to completely remove Py_TRACE_REFS special build.

I wrote PR 12614 to implement this idea. I wrote it to see which code depends on it:

commit 63509498761a0e7f72585a8cd7df325ea2abd1b2 (HEAD -> remove_trace_refs, origin/remove_trace_refs)
Author: Victor Stinner <vstinner at redhat.com>
Date:   Thu Mar 28 23:26:58 2019 +0100

    WIP: bpo-36465: Remove Py_TRACE_REFS special build
    
    Remove _ob_prev and _ob_next fields of PyObject when Python is
    compiled in debug mode to make debug ABI closer to the release ABI.
    
    Remove:
    
    * sys.getobjects()
    * PYTHONDUMPREFS environment variable
    * _PyCoreConfig.dump_refs
    * PyObject._ob_prev and PyObject._ob_next fields
    * _PyObject_HEAD_EXTRA and _PyObject_EXTRA_INIT macros
    * _Py_AddToAllObjects()
    * _Py_PrintReferenceAddresses()
    * _Py_PrintReferences()
    * _Py_ForgetReference(op) is replaced with _Py_INC_TPFREES(op)

I never used PYTHONDUMPREFS. I just tried in Python 3.7: Python does crash with this option... so this option doesn't sound popuplar. Otherwise, many users would complain.

$ PYTHONDUMPREFS=1 python3.7-dbg -c pass
...
0x7f7eae14aa90 [1] 'Thread-local dummy'
0x7f7eae19b448 [1] (<class '_thread._localdummy'>, <class 'object'>)
0x7f7eae14aa30 [1] {'__doc__': 'Thread-local dummy'}
0x7f7eae1356d8 [1] (<class 'object'>,)
0x7d0940 [2] <class '_thread._localdummy'>
0x7f7eae120d58 [1] Segmentation fault (core dumped)

I never used sys.getobjects() neither, but I can imagine that someone might want to use for very specific needs. So maybe it's safer to not immediately remove the feature. At least, a deprecation period would be needed.

I suggest to reject PR 12614 and not remove Py_TRACE_REFS.

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