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From: bugzilla-noreply@freebsd.org
To: python@FreeBSD.org
Subject: maintainer-feedback requested: [Bug 222112] lang/python36
selectors.select() does not block on named pipes / mkfifo
Date: Thu, 07 Sep 2017 01:18:19 +0000
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Jeff Kletsky <jeff+freebsd@wagsky.com> has reassigned Bugzilla Automation
<bugzilla@FreeBSD.org>'s request for maintainer-feedback to FreeBSD Python
<python@FreeBSD.org>:
Bug 222112: lang/python36 selectors.select() does not block on named pipes /
mkfifo
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D222112
--- Description ---
An instance of selectors.select() blocks as expected until data is ready on=
a
named pipe or a regular file. On FreeBSD 11.1-RELEASE-p1 it does not block
after that data has been read from a named pipe (but does continue to block
with a regular file).=20
This is both unexpected and inconsistent with at least behavior under Mac O=
S X,
which continues to block as expected with both a named pipe and a regular f=
ile.
Observed behavior:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
* my_selectors.select() blocks as expected until data is written to the pipe
* the data is read from the pipe
* my_selectors.select() no longer blocks
Expected behavior:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
* my_selectors.select() would block until there was more data written to the
pipe
Same behavior is seen with the "default" selectors.KqueueSelector as well as
with explicitly using a selectors.PollSelector (Mac OS X default).
Impact:
=3D=3D=3D=3D=3D=3D=3D
* No clear way to use select from Python to "read when ready" from a named =
pipe
* Programs that rely on select may have very unexpected behavior
Workarounds:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
(none at this time)
Environment:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
FreeBSD 11.1-RELEASE-p1
python3-3_3
python36-3.6.1_4
jailed or host system, same behavior
venv or not, same behavior
To replicate:
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
(warning, this will spew to the terminal until interrupted if commands-in i=
s a
named pipe)
$ rm commands-in
$ mkfifo commands-in
(run the program, below)
$ echo "Something" >> commands-in # ">" also fails
using 'touch' to obtain a regular file will demonstrate the regular-file
behavior
key_event_list is retained for visibility in debugger
timeout=3DNone is the default, made explicit to confirm not the issue
"If timeout is None, the call will block until a monitored file object beco=
mes
ready.=E2=80=8B"
See also https://forums.freebsd.org/threads/62377/
Example Python 3 code follows:
import logging
import selectors
def test():
command_pipe =3D 'commands-in'
logging.basicConfig(level=3Dlogging.INFO)
logger =3D logging.getLogger()
logger.info(f"Opening command pipe: '{command_pipe}'")
with open(command_pipe, 'r') as cp:
select_cp =3D selectors.DefaultSelector()
select_cp.register(cp, selectors.EVENT_READ)
while True:
key_event_list =3D select_cp.select(timeout=3DNone)
line =3D cp.readline()
logger.info(f"Read: '{line}'")
if __name__ =3D=3D '__main__':