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Calling pselect/ppoll/epoll_pwait

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Ian Pilcher

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Dec 2, 2022, 3:00:49 PM12/2/22
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Does Python provide any way to call the "p" variants of the I/O
multiplexing functions?

Looking at the documentation of the select[1] and selectors[2] modules,
it appears that they expose only the "non-p" variants.

[1] https://docs.python.org/3/library/select.html
[2] https://docs.python.org/3/library/selectors.html

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Barry

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Dec 2, 2022, 7:01:13 PM12/2/22
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> On 2 Dec 2022, at 20:03, Ian Pilcher <arequ...@gmail.com> wrote:
>
> Does Python provide any way to call the "p" variants of the I/O
> multiplexing functions?
>
> Looking at the documentation of the select[1] and selectors[2] modules,
> it appears that they expose only the "non-p" variants.
>
> [1] https://docs.python.org/3/library/select.html
> [2] https://docs.python.org/3/library/selectors.html

Can you use signalfd and select/poll/epoll?

Barry

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Barry

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Dec 4, 2022, 4:56:06 AM12/4/22
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On 3 Dec 2022, at 13:13, Weatherby,Gerard <gweat...@uchc.edu> wrote:



Signalfd and select could probably be made to work if one codes around
the race conditions pselect is provided to avoid. I’m not sure if using
the GIL (for CPython) is sufficient or if a dedicated concurrency
control (e.g. lock, mutex, semaphore) is necessary.

An alternative is to call the C library function via a wrapper. I had
need to use setfsuid on a project a couple of years ago and found this
example:

[1]https://gist.github.com/atdt/ebafa299e843a767139b

 

I read this on SO when researching your question.
Search for epoll and signal.

(I would post the link but ipad turns the link into an image…)

I assume that the lack of pepoll means you can use epoll and

signalfd without race conditions. The trick seems to be setting

the signal to ignore.

Barry

 

From: Python-list <python-list-bounces+gweatherby=uchc...@python.org>
on behalf of Barry <ba...@barrys-emacs.org>

Date: Friday, December 2, 2022 at 7:02 PM
To: Ian Pilcher <arequ...@gmail.com>
Cc: pytho...@python.org <pytho...@python.org>
Subject: Re: Calling pselect/ppoll/epoll_pwait

*** Attention: This is an external email. Use caution responding,
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> On 2 Dec 2022, at 20:03, Ian Pilcher <arequ...@gmail.com> wrote:
>
> Does Python provide any way to call the "p" variants of the I/O
> multiplexing functions?
>
> Looking at the documentation of the select[1] and selectors[2]
modules,
> it appears that they expose only the "non-p" variants.
>
> [1]
[2]https://urldefense.com/v3/__https://docs.python.org/3/library/select.html__;!!Cn_UX_p3!hClFFo4XdiwQAhHxDbHA5zFr490Of9uheHSf84V9cREMHyw1kX-baG5HzXWMt-hFLP30q6DpSUb2G_OD5qBjeA$
> [2]
[3]https://urldefense.com/v3/__https://docs.python.org/3/library/selectors.html__;!!Cn_UX_p3!hClFFo4XdiwQAhHxDbHA5zFr490Of9uheHSf84V9cREMHyw1kX-baG5HzXWMt-hFLP30q6DpSUb2G_MwNjgO8A$

Can you use signalfd and select/poll/epoll?

Barry

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Idiocracy
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References

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1. https://gist.github.com/atdt/ebafa299e843a767139b
2. https://urldefense.com/v3/__https:/docs.python.org/3/library/select.html__;!!Cn_UX_p3!hClFFo4XdiwQAhHxDbHA5zFr490Of9uheHSf84V9cREMHyw1kX-baG5HzXWMt-hFLP30q6DpSUb2G_OD5qBjeA$
3. https://urldefense.com/v3/__https:/docs.python.org/3/library/selectors.html__;!!Cn_UX_p3!hClFFo4XdiwQAhHxDbHA5zFr490Of9uheHSf84V9cREMHyw1kX-baG5HzXWMt-hFLP30q6DpSUb2G_MwNjgO8A$
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Ian Pilcher

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Dec 13, 2022, 1:51:00 PM12/13/22
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On 12/2/22 14:00, Ian Pilcher wrote:
> Does Python provide any way to call the "p" variants of the I/O
> multiplexing functions?

Just to close this out ...

As others suggested, there's no easy way to call the "p" variants of the
I/O multiplexing functions, but this can be worked around by "mapping"
signals to file descriptors.

There are a few ways to accomplish this.

1. Use a Linux signalfd. There's at least one library out there that
provides signalfd support to Python.

2. Use signal.set_wakeup_fd()[1]. I didn't really explore this, as it
appears that there isn't any way to filter the signals that will be
reported.

3. Roll your own. This turned out to be really simple for my use case,
which is simply to set an exit flag and wake my program up if it
receives SIGINT or SIGTERM.

_sel = selectors.DefaultSelector()
_exit_flag = False
_sig_pipe_r, _sig_pipe_w = os.pipe2(os.O_NONBLOCK | os.O_CLOEXEC)

def _sig_handler(signum, frame):
global _exit_flag
_exit_flag = True
os.write(_sig_pipe_w, b'\x00')

_sel.register(_sig_pipe_r, selectors.EVENT_READ)
# register other file descriptors of interest
signal.signal(signal.SIGINT, _sig_handler)
signal.signal(signal.SIGTERM, _sig_handler)
while not _exit_flag:
ready = _sel.select()
# handle other file descriptors

[1] https://docs.python.org/3/library/signal.html#signal.set_wakeup_fd
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