Segmentation fault when overriding sys.stderr

[pablogsal]

BPO	43660
Nosy	@vstinner, @pablogsal, @miss-islington
PRs	bpo-43660: Fix crash when displaying exceptions with custom values for sys.stderr #25075 [3.9] bpo-43660: Fix crash when displaying exceptions with custom values for sys.stderr (GH-25075) #25083 [3.8] bpo-43660: Fix crash when displaying exceptions with custom values for sys.stderr (GH-25075) #25084 [3.7] bpo-43660: Fix crash when displaying exceptions with custom values for sys.stderr (GH-25075). #25085

^{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 2021-03-29.23:50:17.702>
created_at = <Date 2021-03-29.18:42:46.525>
labels = ['interpreter-core', '3.7', '3.8', '3.9', '3.10']
title = 'Segmentation fault when overriding sys.stderr'
updated_at = <Date 2021-03-29.23:50:17.702>
user = 'https://github.com/pablogsal'

bugs.python.org fields:

activity = <Date 2021-03-29.23:50:17.702>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2021-03-29.23:50:17.702>
closer = 'pablogsal'
components = ['Interpreter Core']
creation = <Date 2021-03-29.18:42:46.525>
creator = 'pablogsal'
dependencies = []
files = []
hgrepos = []
issue_num = 43660
keywords = ['patch']
message_count = 6.0
messages = ['389722', '389723', '389764', '389769', '389771', '389772']
nosy_count = 3.0
nosy_names = ['vstinner', 'pablogsal', 'miss-islington']
pr_nums = ['25075', '25083', '25084', '25085']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue43660'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

[pablogsal]

This code crashes (reported by the one and only Matt Wozniski):

import sys
class MyStderr:
    def write(self, s):
        sys.stderr = None
sys.stderr = MyStderr()
1/0

[1] 34112 segmentation fault ./python.exe lel.py

[pablogsal]

The problem is that https://github.com/python/cpython/blob/master/Python/pythonrun.c#L1074 doesn't take a strong reference to sys.stderr but code underneath can potentially release the GIL and remove the reference that keeps the object alive.

[pablogsal]

New changeset 09b90a0 by Pablo Galindo in branch 'master':
bpo-43660: Fix crash when displaying exceptions with custom values for sys.stderr (GH-25075)
09b90a0

[miss-islington]

New changeset ba7f863 by Miss Islington (bot) in branch '3.8':
bpo-43660: Fix crash when displaying exceptions with custom values for sys.stderr (GH-25075)
ba7f863

[pablogsal]

New changeset 2f01c56 by Pablo Galindo in branch '3.7':
[3.7] bpo-43660: Fix crash when displaying exceptions with custom values for sys.stderr (GH-25075). (GH-25085)
2f01c56

[pablogsal]

New changeset ff4715a by Miss Islington (bot) in branch '3.9':
bpo-43660: Fix crash when displaying exceptions with custom values for sys.stderr (GH-25075) (GH-25083)
ff4715a