Python mingw64 stdin error with subprocess pipe

[gedare]

Description / Steps to reproduce the issue

1. Create a subprocess with subprocess.Popen(cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE) in the usual way.
2. Repeatedly write to the pipe's stdin and read from its stdout.
3. At some point the pipe's stdin is unable to be written for no apparent reason. This seems to be a bug.

Reduced test case: test.txt

Compare the behavior with the msys2 python:

$ /usr/bin/python -V
Python 3.12.7
$ /usr/bin/python test.py

see Expected behavior

vs. failure on readline with mingw Python:

$ /mingw64/bin/python.exe -V
Python 3.11.10
$ /mingw64/bin/python.exe test.py

see Actual behavior

Expected behavior

0
b'test\n'
1
2
b'test\n'
...
17
b'test\n'
18
b'test\n'
19
b'test\n'
20
b'test\n'

Actual behavior

0
1
b'test\r\n'
Traceback (most recent call last):
File "", line 9, in
OSError: [Errno 22] Invalid argument
b''
Traceback (most recent call last):
File "C:/msys64/home/gedare/test.py", line 34, in
proc.stdin.flush()
OSError: [Errno 22] Invalid argument

Verification

• I have verified that my MSYS2 is up-to-date before submitting the report (see https://www.msys2.org/docs/updating/)

Windows Version

MSYS_NT-10.0-26100

MINGW environments affected

• MINGW64
• MINGW32
• UCRT64
• CLANG64
• CLANGARM64

Are you willing to submit a PR?

No response