curses module may call delwin() on original window before subwindows

[michaelforney]

BPO	44172
Nosy	@michaelforney
PRs	bpo-44172: Keep reference to original window in curses subwindow objects #26226

^{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 = None
created_at = <Date 2021-05-19.02:08:10.843>
labels = ['3.8', '3.9', '3.10', '3.11', 'extension-modules', '3.7', 'type-crash']
title = 'curses module may call delwin() on original window before subwindows'
updated_at = <Date 2021-05-19.02:24:53.341>
user = 'https://github.com/michaelforney'

bugs.python.org fields:

activity = <Date 2021-05-19.02:24:53.341>
actor = 'michaelforney'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Extension Modules']
creation = <Date 2021-05-19.02:08:10.843>
creator = 'michaelforney'
dependencies = []
files = []
hgrepos = []
issue_num = 44172
keywords = ['patch']
message_count = 1.0
messages = ['393916']
nosy_count = 1.0
nosy_names = ['michaelforney']
pr_nums = ['26226']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue44172'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10', 'Python 3.11']

[michaelforney]

When subwin() is used to create a subwindow for an original window, it does not keep a reference to the original window object. This can result in the original window getting deleted with delwin() before the subwindow.

According to the X/Open curses specification[0]:

The application must delete subwindows before deleting the main window.

This is also mentioned in the ncurses documentation[1]:

Subwindows must be deleted before the main window can be deleted.

When building the Python curses module against NetBSD's curses implementation, deleting the windows in the wrong order causes a double-free. This results in hanging or crashing when running test_curses.

To fix this, window objects for subwindows should keep a reference to the original window object.

[0] https://pubs.opengroup.org/onlinepubs/7908799/xcurses/delwin.html
[1] https://invisible-island.net/ncurses/man/curs_window.3x.html