Make the function flush_io accessible in the C-API

[Demi-Lune]

BPO	38829
Nosy	@encukou, @serhiy-storchaka, @Demi-Lune

^{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 2021-06-22.14:22:34.742>
created_at = <Date 2019-11-17.11:52:05.034>
labels = ['expert-C-API', 'type-feature', '3.8', 'invalid']
title = 'Make the function flush_io accessible in the C-API'
updated_at = <Date 2021-06-22.14:22:34.740>
user = 'https://github.com/Demi-Lune'

bugs.python.org fields:

activity = <Date 2021-06-22.14:22:34.740>
actor = 'petr.viktorin'
assignee = 'none'
closed = True
closed_date = <Date 2021-06-22.14:22:34.742>
closer = 'petr.viktorin'
components = ['C API']
creation = <Date 2019-11-17.11:52:05.034>
creator = 'Jean-Didier'
dependencies = []
files = []
hgrepos = []
issue_num = 38829
keywords = []
message_count = 3.0
messages = ['356801', '356803', '396341']
nosy_count = 3.0
nosy_names = ['petr.viktorin', 'serhiy.storchaka', 'Jean-Didier']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue38829'
versions = ['Python 3.8']

[Demi-Lune]

Hello,
when using an embedded python interpreter in a C++ program, which itself uses MPI, the embedded script's error messages are not flushed properly.
(see the whole discussion in this StackOverflow : https://stackoverflow.com/questions/29352485/python-print-not-working-when-embedded-into-mpi-program/49076389#49076389).

The current preferred solution involves adding a few calls to flush_io in the C++.
However, flush_io is a static function in pythonrun.c, and is not visible in the C-API.

Would it be possible to remove its static attribute, and add a reference to it in pythonrun.h?
Or maybe there is another method to flush IO from the C-API?

[serhiy-storchaka]

PyObject_CallMethod(file, "flush", NULL)

[encukou]

To follow up on the StackOverflow discussion: A call to PyErr_Print should invoke sys.excepthook. Unless sys.excepthook was changed, this should print the message *and* flush standard output.
If it doesn't, I recommend investigating why.

If that does not work, you can flush stderr with two function calls:

    sys_stderr = PySys_GetObject("stderr");
    PyObject_CallMethodNoArgs(sys_stderr, "flush");

plus the necessary error handling if any returns NULL, of course.

And/or the same thing with stdout. The flush_io function does exactly this (for both stdout and stderr).
I don't think this is general enough to expose as an API -- IMO it's better to be explicit and call the flush method as above.