Expose _PyErr_GetTopmostException

[jd]

BPO	40545
Nosy	@scoder, @vstinner, @jd, @markshannon
PRs	bpo-40545: Expose _PyErr_GetTopmostException in the public C API #19977 bpo-40545: Export _PyErr_GetTopmostException() function #19978

^{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 2022-01-06.16:49:34.997>
created_at = <Date 2020-05-07.09:29:58.433>
labels = ['expert-C-API', 'type-feature', '3.9']
title = 'Expose _PyErr_GetTopmostException'
updated_at = <Date 2022-01-06.16:49:34.996>
user = 'https://github.com/jd'

bugs.python.org fields:

activity = <Date 2022-01-06.16:49:34.996>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2022-01-06.16:49:34.997>
closer = 'iritkatriel'
components = ['C API']
creation = <Date 2020-05-07.09:29:58.433>
creator = 'jd'
dependencies = []
files = []
hgrepos = []
issue_num = 40545
keywords = ['patch']
message_count = 10.0
messages = ['368327', '368328', '368332', '368338', '368339', '368340', '368341', '368343', '368345', '368348']
nosy_count = 4.0
nosy_names = ['scoder', 'vstinner', 'jd', 'Mark.Shannon']
pr_nums = ['19977', '19978']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40545'
versions = ['Python 3.9']

[jd]

The function _PyErr_GetTopmostException() is not exported and is useful to get access to exceptions info from a PyThreadState.

[vstinner]

The function was added by:

commit ae3087c
Author: Mark Shannon <mark@hotpy.org>
Date: Sun Oct 22 22:41:51 2017 +0100

Move exc state to generator. Fixes bpo-25612 (bpo-1773)

Move exception state information from frame objects to coroutine (generator/thread) object where it belongs.

Python 3.9 is now built with -fvisibility=hidden, bpo-11410:

commit 0b60f64
Author: Vinay Sajip <vinay_sajip@yahoo.co.uk>
Date: Tue Oct 15 08:26:12 2019 +0100

bpo-11410: Standardize and use symbol visibility attributes across POSIX and Windows. (GH-16347)

It means that functions which are not explicitly exported are no longer exported.

So yeah, _PyErr_GetTopmostException() symbol is not properly exported.

But the function remains private. IMHO if someone wants to add a *public* function, it should return a *strong* reference to the 3 variables (exc type, exc value, exc tb).

See also bpo-39947: "[C API] Make the PyThreadState structure opaque (move it to the internal C API)". I added multiple public getters for PyThreadState structure in Python 3.9.

[jd]

I would definitely love to have a public API for this.

Would PyErr_GetTopmostException be a good candidate for this?

(I can open a different bug report if needed)

[vstinner]

New changeset 8963a7f by Victor Stinner in branch 'master':
bpo-40545: Export _PyErr_GetTopmostException() function (GH-19978)
8963a7f

[vstinner]

I would definitely love to have a public API for this.

I'm not sure if it's a good idea. It's kind of semi-internal.

Stephan Behnel and Mark Shannon: What do you think of exposing PyThreadState exception stack (exc_state)? Especially the "top" exception. If I recall correctly, Mark designed it and Stephan "reimplemented" it in Cython.

I also recall a minor issue in greenlet:
python-greenlet/greenlet#132

[markshannon]

What's wrong with PyErr_GetExcInfo() for accessing the current exception?

Victor, would you revert #19978 until we decide what to do. Thanks.

[jd]

PyErr_GetExcInfo() does not allow to specify a PyThreadState as argument.

[markshannon]

Why do you need to specify a PyThreadState?

[vstinner]

Why do you need to specify a PyThreadState?

From past issues, I understood that Julien's use case is a profiler which inspects memory without calling functions nor switching to other threads. It's like "dump the state of all Python threads".

I'm fine with exposing _PyErr_GetTopmostException() as a *private* function. Moreover, the function was already exposed in 3.8. It was not exposed because of the visibility change which was made in 3.9.