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
--
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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<http://bugs.python.org/issue29386>
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