Fix handling of sizehint=-1 in select.epoll()

[taleinat]

BPO	32568
Nosy	@vstinner, @taleinat, @giampaolo, @benjaminp, @serhiy-storchaka
PRs	bpo-32568: make select.epoll() and its docs consistent #7840 bpo-31938: Convert selectmodule.c to Argument Clinic #4265 [3.7] bpo-32568: make select.epoll() and its docs consistent (GH-7840) #8024 [3.6] bpo-32568: make select.epoll() and its docs consistent (GH-7840) #8025

^{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 2018-06-30.13:16:38.061>
created_at = <Date 2018-01-16.10:48:25.552>
labels = ['extension-modules', '3.8', 'type-bug', '3.7']
title = 'Fix handling of sizehint=-1 in select.epoll()'
updated_at = <Date 2018-06-30.13:16:38.061>
user = 'https://github.com/taleinat'

bugs.python.org fields:

activity = <Date 2018-06-30.13:16:38.061>
actor = 'taleinat'
assignee = 'none'
closed = True
closed_date = <Date 2018-06-30.13:16:38.061>
closer = 'taleinat'
components = ['Extension Modules']
creation = <Date 2018-01-16.10:48:25.552>
creator = 'taleinat'
dependencies = []
files = []
hgrepos = []
issue_num = 32568
keywords = ['patch']
message_count = 9.0
messages = ['310072', '310073', '310075', '320236', '320238', '320252', '320774', '320776', '320778']
nosy_count = 5.0
nosy_names = ['vstinner', 'taleinat', 'giampaolo.rodola', 'benjamin.peterson', 'serhiy.storchaka']
pr_nums = ['7840', '4265', '8024', '8025']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32568'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[taleinat]

In 2fb9ae9, select.epoll() was changed to reject sizehint=-1, despite that still being documented as the default value. Would it be possible to reverse this change, making -1 a special value signaling select.epoll() to use sizehint=FD_SETSIZE-1, as was before this change?

Additionally, in the same commit the docs were changed to say that the sizehint parameter is deprecated and has no effect, but later changes have restored use of sizehint in certain cases without updating the documentation. This inconsistency should be fixed.

For reference, this came up as part of bpo-31938.

[taleinat]

Also see discussion in GitHub PR 4265 for bpo-31938.

[serhiy-storchaka]

Also sizehint=0 was rejected before 2fb9ae9. Now it is accepted.

[vstinner]

*flags* is deprecated and completely ignored.

What if tomorrow, Linux accepts a new flag, passing the flag would just be ignored? Maybe it's fine. Especially usually Python developers only use flags exposed in Python, and currently, only EPOLL_CLOEXEC is exposed and this flag is useless in the specific case of Python (Python always uses the flag).

[vstinner]

Also sizehint=0 was rejected before 2fb9ae9. Now it is accepted.

DESCRIPTION
epoll_create() creates a new epoll(7) instance. Since Linux 2.6.8, the
size argument is ignored, but must be greater than zero; see NOTES
below.

(...)

NOTES
In the initial epoll_create() implementation, the size argument
informed the kernel of the number of file descriptors that the caller
expected to add to the epoll instance. The kernel used this informa‐
tion as a hint for the amount of space to initially allocate in inter‐
nal data structures describing events. (If necessary, the kernel would
allocate more space if the caller's usage exceeded the hint given in
size.) Nowadays, this hint is no longer required (the kernel dynami‐
cally sizes the required data structures without needing the hint), but
size must still be greater than zero, in order to ensure backward com‐
patibility when new epoll applications are run on older kernels.

[taleinat]

> Also sizehint=0 was rejected before 2fb9ae9. Now it is accepted.

Please note that by "Now it is accepted" I meant in the current codebase, before PR 7840. In this regard, that PR changes nothing.

Given the documentation quoted by Victor, perhaps it would be wise to review that earlier change.

[taleinat]

New changeset 0cdf5f4 by Tal Einat in branch 'master':
bpo-32568: make select.epoll() and its docs consistent (bpo-7840)
0cdf5f4

[taleinat]

New changeset fd1c092 by Tal Einat (Miss Islington (bot)) in branch '3.7':
bpo-32568: make select.epoll() and its docs consistent (GH-7840) (GH-8024)
fd1c092

[taleinat]

New changeset db7ac30 by Tal Einat in branch '3.6':
[3.6] bpo-32568: make select.epoll() and its docs consistent (GH-7840) (GH-8025)
db7ac30