Py_FatalError(): dump the list of extension modules

[vstinner]

BPO	42923
Nosy	@vstinner, @corona10
PRs	bpo-42923: Dump extension modules on fatal error #24207 bpo-42923: Add Py_FatalError() test in test_capi #24240 bpo-42923: Py_FatalError() avoids fprintf() #24242 bpo-42923: _Py_DumpExtensionModules() ignores stdlib ext #24254 bpo-42955: Add sys.modules_names #24238 bpo-42923: Fix _Py_DumpExtensionModules() for NULL sysdict #25262

^{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 2021-01-19.22:36:24.746>
created_at = <Date 2021-01-13.10:50:13.084>
labels = ['interpreter-core', '3.10']
title = 'Py_FatalError(): dump the list of extension modules'
updated_at = <Date 2021-04-07.21:13:01.803>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2021-04-07.21:13:01.803>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2021-01-19.22:36:24.746>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2021-01-13.10:50:13.084>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 42923
keywords = ['patch']
message_count = 12.0
messages = ['385013', '385016', '385020', '385161', '385206', '385207', '385221', '385222', '385303', '385306', '385403', '390475']
nosy_count = 2.0
nosy_names = ['vstinner', 'corona10']
pr_nums = ['24207', '24240', '24242', '24254', '24238', '25262']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue42923'
versions = ['Python 3.10']

[vstinner]

When Python cannot report an error and its is not possible to recover from the error, Py_FatalError() displays an error message and exit immediately Python.

It's common that Python crashes are coming from third party extension modules, rather than Python itself. Recent example: bpo-42891 crash reported in Python, but it was a bug in lsm-db third party extension module.

I propose to enhance Py_FatalError() to attempt to dump the list of extension modules, to ease debugging.

I'm working on a PR.

[vstinner]

Example using PR 24207, the new part the "Extension modules:" list at the end:
---------------

$ ./python -X faulthandler
Python 3.10.0a4+ (heads/master-dirty:2396614b89, Jan 13 2021, 12:09:15) 
>>> import ctypes
>>> ctypes.string_at(0)
Fatal Python error: Segmentation fault

Current thread 0x00007f0aabd09740 (most recent call first):
File "/home/vstinner/python/master/Lib/ctypes/init.py", line 517 in string_at
File "<stdin>", line 1 in <module>

Extension modules:

• sys
• builtins
• _imp
• _thread
• _warnings
• _weakref
• _io
• marshal
• posix
• time
• faulthandler
• _codecs
• _signal
• _abc
• _stat
• readline
• atexit
• itertools
• _operator
• _collections
• _functools
• _opcode
• _sre
• _locale
• _ctypes
• _struct
  ---------------

Here you can see that the _ctypes module is loaded.

[vstinner]

It's common that Python crashes are coming from third party extension modules, rather than Python itself. Recent example: bpo-42891 crash (...)

Another recent example: bpo-42879 "python: Objects/abstract.c:155: PyObject_GetItem: Assertion `(item != NULL) ^ (PyErr_Occurred() != NULL)' failed." was a bug in pygame.

[vstinner]

Example of a Pillow crash in Python 3.10:
https://mail.python.org/archives/list/python-dev@python.org/message/5DCR2R6POCOHTPFUOY4F5G7GTBYX6OPO/

[vstinner]

New changeset e232025 by Victor Stinner in branch 'master':
bpo-42923: Add Py_FatalError() test in test_capi (GH-24240)
e232025

[vstinner]

New changeset 314b878 by Victor Stinner in branch 'master':
bpo-42923: Py_FatalError() avoids fprintf() (GH-24242)
314b878

[vstinner]

New changeset 250035d by Victor Stinner in branch 'master':
bpo-42923: Dump extension modules on fatal error (GH-24207)
250035d

[vstinner]

I created https://bugs.python.org/issue42955 to add sys.modules_names tuple: names of stdlib modules. Once it will be merged, I will updated this PR to filter the list of modules (ignore stdlib modules).

[vstinner]

New changeset 66f77ca by Victor Stinner in branch 'master':
bpo-42923: _Py_DumpExtensionModules() ignores stdlib ext (GH-24254)
66f77ca

[vstinner]

More "real world" example using cinder which imports 220 modules (ctypes is used to simulate a crash):
---------------

$ ./python -m venv env
$ env/bin/python -m pip install wheel
$ env/bin/python -m pip install cinder
$ env/bin/python -X dev -c 'import cinder, ctypes, sys; print(f"{len(sys.modules)=}"); print(); ctypes.string_at(0)'
(...)
len(sys.modules)=220

Fatal Python error: Segmentation fault

Current thread 0x00007fc4a88e0740 (most recent call first):
File "/home/vstinner/python/master/Lib/ctypes/init.py", line 517 in string_at
File "<string>", line 1 in <module>

Extension modules: greenlet._greenlet, __original_module__thread, __original_module_select, __original_module_time, _cffi_backend (total: 5)
Erreur de segmentation (core dumped)
---------------

cinder only uses 2 third party extension modules (on a total of 220 modules): greenlet and cffi.

Note: __original_xxx modules are aliases of stdlib modules created by eventlet monkey patching.

So if cinder does crash, I suggest to look at Python, but *also* look at these 2 extensions ;-)

[vstinner]

See also bpo-42955 "Add sys.module_names: list of stdlib module names (Python and extension modules)".

[vstinner]

New changeset 3d55aa9 by Victor Stinner in branch 'master':
bpo-42923: Fix _Py_DumpExtensionModules() for NULL sysdict (GH-25262)
3d55aa9