[issue29386] select.epoll.poll may behave differently if timeout = -1 vs timeout = None

[Berker Peksag]

Change by Berker Peksag :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed
versions:  -Python 3.6, Python 3.7

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[issue29386] select.epoll.poll may behave differently if timeout = -1 vs timeout = None

[Berker Peksag]

Berker Peksag  added the comment:


New changeset b690b9b04729ba3d91c59bff1bb11c3dcc1b50fc by Berker Peksag in 
branch 'master':
bpo-29386: Pass -1 to epoll_wait() when timeout is < -1 (GH-9040)
https://github.com/python/cpython/commit/b690b9b04729ba3d91c59bff1bb11c3dcc1b50fc


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[issue29386] select.epoll.poll may behave differently if timeout = -1 vs timeout = None

[Berker Peksag]

Change by Berker Peksag :


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keywords: +patch
pull_requests: +8502
stage:  -> patch review

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[issue29386] select.epoll.poll may behave differently if timeout = -1 vs timeout = None

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

See similar issue31334. It may be worth to make the code similar. Only -1 is 
documented as a special timeout value for epoll_wait().

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[issue29386] select.epoll.poll may behave differently if timeout = -1 vs timeout = None

[Gabriel McManus]

Gabriel McManus  added the comment:

I don't know of any other OS that implements epoll, so this is issue is likely 
no longer a problem. Although it is strange to convert -1 to -1000 (as though 
from seconds to milliseconds), it may not be worth changing.

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[issue29386] select.epoll.poll may behave differently if timeout = -1 vs timeout = None

[Berker Peksag]

Berker Peksag  added the comment:

Thanks for the report.

Looking at the Kernel source code, there doesn't seem to be any difference 
between -1, -100, or -255: 
https://github.com/torvalds/linux/blob/9bd553929f68921be0f2014dd06561e0c8249a0d/fs/eventpoll.c#L1747-L1761

Do you know any other OS that implements or mimicks epoll() other than Illumos? 
Since 
https://github.com/joyent/illumos-joyent/commit/d21b3b2e1bbefbd2f6158ed5d329cd58f86677ab,
 Illumos follows Linux's behavior, so I'm not sure whether we should do 
something similar to 
https://github.com/python/cpython/commit/6cfa927ceb931ad968b5b03e4a2bffb64a8a0604
 for epoll.poll().

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nosy: +berker.peksag
versions: +Python 3.8 -Python 3.5

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[issue29386] select.epoll.poll may behave differently if timeout = -1 vs timeout = None

[Gabriel McManus]

Gabriel McManus added the comment:

As mentioned in [1], Illumos will be fixed to match Linux's behaviour, so this 
problem will go away. It may still be worth changing epoll to just send -1 
though, in case this causes similar issues in other operating systems.

[1] https://github.com/joyent/illumos-joyent/issues/136

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[issue29386] select.epoll.poll may behave differently if timeout = -1 vs timeout = None

[Gabriel McManus]

New submission from Gabriel McManus:

The select module epoll.poll method takes a "timeout" parameter which is 
documented as having a default value of -1 [1]. If no timeout (or None) is 
passed to epoll.poll, then a value of -1 is passed to the epoll_wait system 
call. But if a timeout of -1 is passed to epoll.poll, then a value of -1000 is 
passed to epoll_wait. This is because the timeout is converted from seconds to 
milliseconds.

Before Python 3.5, if a negative timeout was passed to epoll.poll then -1 was 
passed to epoll_wait [2].

The Linux epoll_wait documentation doesn't specify the behaviour if timeout < 
-1. Linux itself behaves the same for all negative timeouts: never time out. 
But on Illumos, timeout < -1 currently times out immediately, and only timeout 
== -1 never times out [3].

Some code does pass -1 to select.epoll.poll expecting it to never time out. For 
example, selectors.EpollSelector [4].

I suggest restoring the pre-3.5 behaviour: epoll.poll should use -1 if the 
given timeout is negative.

I discovered this because ipython3 uses selectors.EpollSelector on Illumos,
and it was using 100% cpu while waiting for input because epoll_wait was 
returning immediately.

To demonstrate the issue you can run:

strace python3.5 -c 'import select; select.epoll().poll(timeout=-1)' &

On Illumos this completes immediately, and the output contains the -1000 
timeout:

epoll_wait(3, [], 1023, -1000)  = 0

On Linux, it will block. If you then kill the python process with SIGTERM, 
strace should print the interrupted epoll_wait call, revealing the -1000 
timeout:

epoll_wait(3, 
...
299a070, 1023, -100)= -1 EINTR (Interrupted system call)

[1] 
https://github.com/python/cpython/blob/b9e40ed1bcce127893e40dd355087cda7187ac27/Modules/selectmodule.c#L1489
[2] 
https://github.com/python/cpython/commit/02e27d1301ea680dce9c3013010e3befedf9628a
[3] https://github.com/joyent/illumos-joyent/issues/136
[4] 
https://github.com/python/cpython/blob/8228a2b306844a213eddb4fb908c1925840ff67e/Lib/selectors.py#L428

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components: Extension Modules
messages: 286431
nosy: Gabriel McManus
priority: normal
severity: normal
status: open
title: select.epoll.poll may behave differently if timeout = -1 vs timeout = 
None
type: behavior
versions: Python 3.5, Python 3.6, Python 3.7

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