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API function PySignal_SetWakeupFd is not exported and unusable #89364
Description
| BPO | 45201 |
|---|---|
| Nosy | @vstinner |
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 2021-09-15.06:03:15.175>
labels = ['expert-C-API', 'type-bug', '3.9']
title = 'API function PySignal_SetWakeupFd is not exported and unusable'
updated_at = <Date 2021-09-29.08:18:20.534>
user = 'https://bugs.python.org/cy'bugs.python.org fields:
activity = <Date 2021-09-29.08:18:20.534>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['C API']
creation = <Date 2021-09-15.06:03:15.175>
creator = 'cy'
dependencies = []
files = []
hgrepos = []
issue_num = 45201
keywords = []
message_count = 2.0
messages = ['401810', '402828']
nosy_count = 2.0
nosy_names = ['vstinner', 'cy']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45201'
versions = ['Python 3.9']
Activity
cy commentedon Sep 15, 2021
I want to wait on curl_multi_wait which messes up python's signal handling since libcurl doesn't know to exit the polling loop on signal. I created a pipe that curl could read from when python registered a signal, and PySignal_SetWakeupFd would write to the pipe, thus letting curl leave its polling loop, letting my C module return an exception condition.
Except PySignal_SetWakeupFd cannot be used.
In Modules/signalmodule.c, it says:
when it should say:
This probably isn't a problem for most, since gcc has visiblity=public by default, but I guess Gentoo's process for building python sets -fvisibility=hidden, so I can't access it, and all Microsoft users should have no access to PySignal_SetWakeupFd, since Microsoft does have hidden visibility by default.
vstinner commentedon Sep 29, 2021
See also bpo-45316: [C API] Functions not exported with PyAPI_FUNC().
NicolasT commentedon Jul 2, 2024
Also running into this. The function is documented, but not exported by various builds of
libpython(likely because of symbol-visibilityhiddenby default). ApplyingPyAPI_FUNCseems to fix this.If the function isn't intended to be public, maybe the docs should be removed? Otherwise, the symbol should be available 😄
Given this function's prototype has (to my knowledge) not changed since its introduction in Python 2.6, any chance this could, then, become part of the limited API as well?
vstinner commentedon Jul 3, 2024
Would you mind to elaborate your use case? Also, since the function is not usable, how do you work around this issue? Import the signal module and call the
set_wakeup_fd()function?NicolasT commentedon Jul 8, 2024
Sure! We're working on a framework combining Python and OCaml, including integration with a library called Async and its scheduler/event-loop (also integrating PEP-492-style coroutines with it). Being able to promptly react to signals delivered to such process (which are handled by the Python runtime, since that's the "main" entry-point and semantics we want to provide to users), even when the
Asyncmain-loop is running (i.e., pure OCaml code is executing) is important. I'd like to register asocketpair(or self-pipe) usingPySignal_SetWakeupFdbefore entering the main-loop, and have this then select on the socket (among other things, of course), such that we can then usePyErr_CheckSignalwhenever the socket became readable (i.e., a (C) signal handler wrote a byte on the socket) and return control to the Python runtime if appropriate.I created a custom build of
libpythonwithPyAPI_FUNCapplied to the function's declaration, and this did result in the symbol being exported by the library (unlike the default).We could use
signal.set_wakeup_fd, though then we'll need to call into Python code etc., whilst a simple C function is a lot easier (and cheaper) to call.If
PySignal_SetWakeupFdisn't meant to be part of the (public, exported) CPython C API, then maybe it should not be documented, and shouldn't be declared throughPython.h(frompyerrors.h), and we'll usesignal.set_wakeup_fdby importing/calling it using the applicable C APIs. The C function seems useful though for anyone integrating with some "foreign" event loop.pythongh-89364: Export PySignal_SetWakeupFd() function
vstinner commentedon Jul 9, 2024
I created PR gh-121537 to export the function.
gh-89364: Export PySignal_SetWakeupFd() function (#121537)
pythongh-89364: Export PySignal_SetWakeupFd() function (pythonGH-121537)
vstinner commentedon Jul 10, 2024
Fixed by change ca0fb34.
[3.13] gh-89364: Export PySignal_SetWakeupFd() function (GH-121537) (#…
pythongh-89364: Export PySignal_SetWakeupFd() function (python#121537)
pythongh-89364: Export PySignal_SetWakeupFd() function (python#121537)