subprocess with preexec_fn when fork() fails could corrupt PyErr state

[gpshead]

BPO	34658
Nosy	@gpshead, @miss-islington
PRs	bpo-34658: Fix rare subprocess prexec_fn fork error. #9255 [3.7] bpo-34658: Fix rare subprocess prexec_fn fork error. (GH-9255) #9256 [3.6] bpo-34658: Fix rare subprocess prexec_fn fork error. (GH-9255) #9292

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = 'https://github.com/gpshead'
closed_at = <Date 2018-09-14.05:24:27.467>
created_at = <Date 2018-09-13.10:57:27.885>
labels = ['extension-modules', '3.8', 'type-bug', '3.7']
title = 'subprocess with preexec_fn when fork() fails could corrupt PyErr state'
updated_at = <Date 2018-09-14.05:24:27.465>
user = 'https://github.com/gpshead'

bugs.python.org fields:

activity = <Date 2018-09-14.05:24:27.465>
actor = 'gregory.p.smith'
assignee = 'gregory.p.smith'
closed = True
closed_date = <Date 2018-09-14.05:24:27.467>
closer = 'gregory.p.smith'
components = ['Extension Modules']
creation = <Date 2018-09-13.10:57:27.885>
creator = 'gregory.p.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 34658
keywords = ['patch']
message_count = 3.0
messages = ['325238', '325239', '325265']
nosy_count = 2.0
nosy_names = ['gregory.p.smith', 'miss-islington']
pr_nums = ['9255', '9256', '9292']
priority = 'normal'
resolution = 'fixed'
stage = 'commit review'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34658'
versions = ['Python 3.7', 'Python 3.8']

[gpshead]

Found in the stdlib test suite while testing the fix for bpo-6721: When subprocess is called with a preexec_fn and os.register_at_fork has been used and the fork() system call fails, the Python error state is set before the "after fork in parent process" function calls that were registered were called. So they'd call back into Python code with a pending unreturned exception set.

boom. SystemError (or assertion failure in pydebug builds).

[miss-islington]

New changeset a20b6ad by Miss Islington (bot) (Gregory P. Smith) in branch 'master':
bpo-34658: Fix rare subprocess prexec_fn fork error. (GH-9255)
a20b6ad

[miss-islington]

New changeset b2ff9a9 by Miss Islington (bot) in branch '3.7':
bpo-34658: Fix rare subprocess prexec_fn fork error. (GH-9255)
b2ff9a9