[issue31334] select.poll.poll fails on BSDs with arbitrary negative timeouts

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:


New changeset 27b951c63353345cdf7a9a8c4c8133a5dafd6a80 by Serhiy Storchaka 
(Riccardo Coccioli) in branch '2.7':
[2.7] bpo-31334: Fix timeout in select.poll.poll() (GH-3277) (#4034)
https://github.com/python/cpython/commit/27b951c63353345cdf7a9a8c4c8133a5dafd6a80


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[issue31334] select.poll.poll fails on BSDs with arbitrary negative timeouts

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue31334] select.poll.poll fails on BSDs with arbitrary negative timeouts

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:


New changeset 97abcabc195b87d6a5562dbb867a469fac27d3f6 by Serhiy Storchaka 
(Miss Islington (bot)) in branch '3.6':
[3.6] bpo-31334: Fix timeout in select.poll.poll() (GH-3277) (#4033)
https://github.com/python/cpython/commit/97abcabc195b87d6a5562dbb867a469fac27d3f6


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[issue31334] select.poll.poll fails on BSDs with arbitrary negative timeouts

[Riccardo Coccioli]

Change by Riccardo Coccioli :


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[issue31334] select.poll.poll fails on BSDs with arbitrary negative timeouts

[Roundup Robot]

Change by Roundup Robot :


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[issue31334] select.poll.poll fails on BSDs with arbitrary negative timeouts

[STINNER Victor]

STINNER Victor  added the comment:


New changeset 6cfa927ceb931ad968b5b03e4a2bffb64a8a0604 by Victor Stinner 
(Riccardo Coccioli) in branch 'master':
bpo-31334: Fix timeout in select.poll.poll() (GH-3277)
https://github.com/python/cpython/commit/6cfa927ceb931ad968b5b03e4a2bffb64a8a0604


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[issue31334] select.poll.poll fails on BSDs with arbitrary negative timeouts

[Riccardo Coccioli]

Riccardo Coccioli  added the comment:

Although it's documented as -1 in Linux man page [1], from my quick tests I was 
not able to get any error with negative values different from -1 and it seems 
to wait indefinitely as expected. Looking also at its implementation in [2], it 
doesn't seem to differentiate between negative values. It could be argued that 
is implementation dependent at the moment and the behaviour might change in the 
future.
But, on a related note, the Python documentation doesn't say much either as 
what are acceptable values for the timeout parameter, see [3].

So at the moment there isn't any discrepancy between the Python documentation 
and the current behaviour IMHO, but I'm happy to open a separate task and send 
a PR if you think this should be improved/fixed too.

[1] http://man7.org/linux/man-pages/man2/epoll_wait.2.html
[2] http://elixir.free-electrons.com/linux/latest/source/fs/eventpoll.c#L1754
[3] https://docs.python.org/3.7/library/select.html

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[issue31334] select.poll.poll fails on BSDs with arbitrary negative timeouts

[STINNER Victor]

Change by STINNER Victor :


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[issue31334] select.poll.poll fails on BSDs with arbitrary negative timeouts

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Shouldn't epoll_poll() be fixed too? Only -1 is documented as an infinity 
timeout in epoll_wait (2).

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[issue31334] select.poll.poll fails on BSDs with arbitrary negative timeouts

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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stage:  -> patch review
versions:  -Python 3.3, Python 3.4, Python 3.5

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[issue31334] select.poll.poll fails on BSDs with arbitrary negative timeouts

[Riccardo Coccioli]

Riccardo Coccioli added the comment:

This can actually be reproduced with Python 2.7 too (thanks @thiell to let me 
know). At first I thought that it was not because it doesn't repro with the 
stock macOS-shipped Python 2.7.10 on macOS Sierra 10.12.6, where the 
select.poll() is not available at all, see below.

Updated list of version where I was able to reproduce the error:
- macOS Sierra 10.12.6 with those Python versions: 2.7.10, 2.7.13, 3.3.6, 
3.4.6, 3.5.3, 3.6.2, 3.7.0a0 (heads/master:2ef37607b7)
- FreeBSD 11.1 with those Python versions: 2.7.13, 3.7.0a0 
(heads/master:2ef37607b7)

For reference, the repro code executed with the stock macOS-shipped Python:
#--
$ /usr/bin/python
Python 2.7.10 (default, Feb  7 2017, 00:08:15)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.34)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import select
>>> p = select.poll()
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: 'module' object has no attribute 'poll'
#--

If the PR 3277 that I've sent against the master branch with the fix will be 
accepted, I'm ready to send additional PRs to backport the fix in all affected 
versions.

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[issue31334] select.poll.poll fails on BSDs with arbitrary negative timeouts

[Riccardo Coccioli]

Changes by Riccardo Coccioli :


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[issue31334] select.poll.poll fails on BSDs with arbitrary negative timeouts

[Riccardo Coccioli]

New submission from Riccardo Coccioli:

According to the Python documentation for the 'poll.poll([timeout])' method in 
the 'select' module, any negative value for the 'timeout' parameter is valid 
and should have the same behaviour [1]:
"If timeout is omitted, negative, or None, the call will block until there 
is an event for this poll object."

Unfortunately, unlike the Linux, on many other OSes, including, but not limited 
to, macOS and {Free,Open,Net}BSD, the 'poll()' system call requires that the 
'timeout' parameter is a non-negative integer or exactly -1 (sometimes defined 
as INFTIM). Any other negative value throws an error, see [2], [3], [4] and [5].

This is a snippet of code to reproduce the error:
#-
import select

p = select.poll()
p.poll(-100)
#-

Expected behaviour: block until there is an event for the poll object, in this 
case block indefinitely
Current behaviour on macOS and FreeBSD: OSError: [Errno 22] Invalid argument

I was able to reproduce the error on:
- macOS Sierra 10.12.6 with those Python versions: 3.3.6, 3.4.6, 3.5.3, 3.6.2, 
3.7.0a0 (heads/master:2ef37607b7)
- FreeBSD 11.1 with Python 3.7.0a0 (heads/master:2ef37607b7)

On Linux this doesn't happen because the 'poll()' system call accept any 
negative value to wait indefinitely, see [6].
To adhere with the Python documentation described behaviour, I'm sending a pull 
request to propose to force the 'timeout' value passed to the 'poll()' system 
call to be exactly -1 (or INFTIM where defined) when a negative value is given.
This will not change the current behaviour on Linux and will have the behaviour 
described in the documentation on other OSes where is currently failing with an 
error.

[1] https://docs.python.org/3/library/select.html#poll-objects
[2] https://www.freebsd.org/cgi/man.cgi?poll
[3] https://man.openbsd.org/poll.2
[4] http://netbsd.gw.com/cgi-bin/man-cgi/man?poll
[5] From macOS 'man poll': "If timeout is greater than zero, it specifies a 
maximum interval (in milliseconds) to wait for any file descriptor to become 
ready. If timeout is zero, then poll() will return without blocking. If the 
value of timeout is -1, the poll blocks indefinitely."
[6] http://man7.org/linux/man-pages/man2/poll.2.html

--
components: Extension Modules, FreeBSD, macOS
messages: 301202
nosy: Riccardo Coccioli, koobs, ned.deily, ronaldoussoren
priority: normal
severity: normal
status: open
title: select.poll.poll fails on BSDs with arbitrary negative timeouts
type: behavior
versions: Python 3.3, Python 3.4, Python 3.5, Python 3.6, Python 3.7

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