selects.KqueueSelector behaves incorrectly when no fds are registered

[njsmith]

BPO	29255
Nosy	@gvanrossum, @njsmith, @russelldavis
PRs	bpo-29255: Wait in KqueueSelector when no fds are registered #19508

^{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 2020-04-15.18:57:42.078>
created_at = <Date 2017-01-12.23:35:06.834>
labels = ['3.7', 'library']
title = 'selects.KqueueSelector behaves incorrectly when no fds are registered'
updated_at = <Date 2020-04-15.18:57:42.077>
user = 'https://github.com/njsmith'

bugs.python.org fields:

activity = <Date 2020-04-15.18:57:42.077>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2020-04-15.18:57:42.078>
closer = 'gvanrossum'
components = ['Library (Lib)']
creation = <Date 2017-01-12.23:35:06.834>
creator = 'njs'
dependencies = []
files = []
hgrepos = []
issue_num = 29255
keywords = ['patch']
message_count = 3.0
messages = ['285352', '366162', '366540']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'njs', 'russelldavis']
pr_nums = ['19508']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue29255'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7']

[njsmith]

When calling kevent(), selectors.KqueueSelector.select sets the "maxevents" argument to len(self._fd_to_key). So if there are no fds registered, it passes maxevents=0.

It turns out that the kevent() API has a special case behavior for maxevents=0: it returns immediately, ignoring timeout. I have no idea why kevent() works this way, but it's specifically called out in the man page:

The nevents argument determines the size of eventlist. When
nevents is zero, kevent() will return immediately even if there is a
timeout specified unlike select(2).

https://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2

It happens that asyncio doesn't run into this because asyncio always has at least one fd registered with its selector, but this causes problems for other users of the selectors module, e.g.:
dabeaz/curio#156

I believe fix would just be to add some code like: "if max_ev == 0: max_ev = 1" to selectors.KqueueSelector.select.

[russelldavis]

This looks like it's the cause of https://bugs.python.org/issue25680

[gvanrossum]

New changeset ba1bcff by Russell Davis in branch 'master':
bpo-29255: Wait in KqueueSelector.select when no fds are registered (GH-19508)
ba1bcff