Debug mode: check if objects are valid

[vstinner]

BPO	39873
Nosy	@vstinner, @methane, @serhiy-storchaka, @pablogsal
PRs	[WIP] bpo-39873: _PyObject_CAST() check if the object is valid #18803 bpo-39873: PyObject_Init() uses PyObject_INIT() #18804 bpo-39873: Cleanup _PyObject_CheckConsistency() #18807

^{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 2020-03-25.16:28:32.499>
created_at = <Date 2020-03-06.09:24:50.515>
labels = ['interpreter-core', '3.9']
title = 'Debug mode: check if objects are valid'
updated_at = <Date 2020-03-25.16:28:32.498>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2020-03-25.16:28:32.498>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2020-03-25.16:28:32.499>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2020-03-06.09:24:50.515>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 39873
keywords = ['patch']
message_count = 5.0
messages = ['363499', '363500', '363521', '363529', '364997']
nosy_count = 4.0
nosy_names = ['vstinner', 'methane', 'serhiy.storchaka', 'pablogsal']
pr_nums = ['18803', '18804', '18807']
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue39873'
versions = ['Python 3.9']

[vstinner]

One of the worst issue that I had to debug is a crash in the Python garbage collector. It is usually a crash in visit_decref(). See my notes:
https://pythondev.readthedocs.io/debug_tools.html#debug-crash-in-garbage-collection-visit-decref

My previous attempt to help debugging such issue failed: bpo-36389 "Add gc.enable_object_debugger(): detect corrupted Python objects in the GC". The idea was to check frequently if all objects tracked by the GC are valid. The problem is that even if the check looked trivial, checking all objects made Python way slower. Even when I tried to only check objects of the "young" GC generation (generation 0), it was still too slow.

Here I propose a different approach: attempt to only check objects when they are accessed. Recently, I started to replace explicit cast to (PyObject *) type with an indirection: a new _PyObject_CAST() macro which should be the only way to cast any object pointer to (PyObject *).

/* Cast argument to PyObject* type. */
#define _PyObject_CAST(op) ((PyObject*)(op))

This macro is used in many "core" macros like Py_TYPE(op), Py_REFCNT(op), Py_SIZE(op), Py_SETREF(op, op2), Py_VISIT(op), etc.

The idea here is to inject code in _PyObject_CAST(op) when Python is built in debug mode to ensure that the object is valid. The intent is to detect corrupted objects earlier than a garbage collection, to ease debugging C extensions.

The checks should be limited to reduce the performance overhead.

Attached PR implemnts this idea.

[vstinner]

See also bpo-38392: "Ensure that objects entering the GC are valid". I fixed this one with commit 1b18455.

[vstinner]

New changeset 1fb5a9f by Victor Stinner in branch 'master':
bpo-39873: PyObject_Init() uses PyObject_INIT() (GH-18804)
1fb5a9f

[vstinner]

New changeset 9a73705 by Victor Stinner in branch 'master':
bpo-39873: Cleanup _PyObject_CheckConsistency() (GH-18807)
9a73705

[vstinner]

This change was mostly an experimentation. But I'm not sure that it does solve any issue. The change is more intrusive than what I expected. I prefer to abandon PR 18803 and this issue.