selectors.EpollSelector.select raises exception when nothing to select.

[AlexeyPoryadin]

BPO	23009
Nosy	@gvanrossum, @vstinner, @1st1
Files	epoll_01.patch epoll_02.patch

^{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 2014-12-08.17:24:07.288>
created_at = <Date 2014-12-08.10:36:45.366>
labels = ['type-bug', 'expert-asyncio']
title = 'selectors.EpollSelector.select raises exception when nothing to select.'
updated_at = <Date 2015-01-22.08:36:20.569>
user = 'https://bugs.python.org/AlexeyPoryadin'

bugs.python.org fields:

activity = <Date 2015-01-22.08:36:20.569>
actor = 'python-dev'
assignee = 'none'
closed = True
closed_date = <Date 2014-12-08.17:24:07.288>
closer = 'yselivanov'
components = ['asyncio']
creation = <Date 2014-12-08.10:36:45.366>
creator = 'Alexey.Poryadin'
dependencies = []
files = ['37387', '37388']
hgrepos = []
issue_num = 23009
keywords = ['patch']
message_count = 13.0
messages = ['232300', '232307', '232309', '232310', '232311', '232314', '232316', '232317', '232318', '232320', '232779', '234480', '234484']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'vstinner', 'neologix', 'python-dev', 'yselivanov', 'Alexey.Poryadin']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue23009'
versions = ['Python 3.4', 'Python 3.5']

[AlexeyPoryadin]

When is called select, but registered no one fileobject rise exception:
...
polled = self._selector.select(timeout)
File "/usr/lib/python3.4/selectors.py", line 424, in select
fd_event_list = self._epoll.poll(timeout, max_ev)
ValueError: maxevents must be greater than 0, got 0

Of course, it makes no sense to call select if there is nothing to watch. But "select.epol.poll" is not raises exception in the same case if max_ev=-1. And this simplifies the application code and is useful if need just sleep for a timeout.

So, I suggest a small fix:

• fd_event_list = self._epoll.poll(timeout, max_ev)
  + fd_event_list = self._epoll.poll(timeout, max_ev or -1)

[1st1]

Patch attached. Tests pass on Linux.

[vstinner]

+ s.select(timeout=0)

I suggest to ensure that the result is an empty list.

[1st1]

I agree. Please see another one.

[vstinner]

It's a little bit surprising to call epoll_wait() without any FD subscribed, but select([], [], [], delay) is a known way to sleep 'delay' seconds, so why not using epoll in a similar way? :-)

epoll_02.patch looks good to me. Can you please also apply the patch to the Tulip project?

(By the way, test_selectors.py to Tulip is completly different, we may just reuse the file from CPython and drop the code from Tulip.)

[gvanrossum]

Please add a comment explaining the complaint from epoll.poll() we're
trying to avoid here.

I presume Tulip never gets into this state because of the self-pipe.

On Mon, Dec 8, 2014 at 8:01 AM, STINNER Victor <report@bugs.python.org>
wrote:

STINNER Victor added the comment:

It's a little bit surprising to call epoll_wait() without any FD
subscribed, but select([], [], [], delay) is a known way to sleep 'delay'
seconds, so why not using epoll in a similar way? :-)

epoll_02.patch looks good to me. Can you please also apply the patch to
the Tulip project?

(By the way, test_selectors.py to Tulip is completly different, we may
just reuse the file from CPython and drop the code from Tulip.)

----------

---

Python tracker <report@bugs.python.org>
<http://bugs.python.org/issue23009\>

---

[python-dev]

New changeset b2ee06684b6a by Yury Selivanov in branch '3.4':
selectors: Make sure EpollSelecrtor.select() works when no FD is registered.
https://hg.python.org/cpython/rev/b2ee06684b6a

New changeset 202995833ef4 by Yury Selivanov in branch 'default':
selectors: Make sure EpollSelecrtor.select() works when no FD is registered.
https://hg.python.org/cpython/rev/202995833ef4

[1st1]

Please add a comment explaining the complaint from epoll.poll() we're
trying to avoid here.

Good point! Committed.

[neologix]

Thanks for taking care of this.

[python-dev]

New changeset 8f1be68dfcab by Yury Selivanov in branch '3.4':
NEWS: Add news entry for issue bpo-23009.
https://hg.python.org/cpython/rev/8f1be68dfcab

New changeset d36711410f48 by Yury Selivanov in branch 'default':
NEWS: Add news entry for issue bpo-23009.
https://hg.python.org/cpython/rev/d36711410f48

[python-dev]

New changeset 1e985daa7be6 by Victor Stinner in branch 'default':
Issue bpo-23009: Skip test_selectors.test_empty_select() on Windows
https://hg.python.org/cpython/rev/1e985daa7be6

[python-dev]

New changeset d3a27a27e008 by Victor Stinner in branch '3.4':
Issue bpo-23009: Skip test_selectors.test_empty_select() on Windows
https://hg.python.org/cpython/rev/d3a27a27e008