https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=222112
Bug ID: 222112
Summary: lang/python36 selectors.select() does not block on
named pipes / mkfifo
Product: Ports & Packages
Version: Latest
Hardware: Any
OS: Any
Status: New
Severity: Affects Some People
Priority: ---
Component: Individual Port(s)
Assignee: pyt...@freebsd.org
Reporter: jeff+free...@wagsky.com
Assignee: pyt...@freebsd.org
Flags: maintainer-feedback?(pyt...@freebsd.org)
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).
This is both unexpected and inconsistent with at least behavior under Mac OS X,
which continues to block as expected with both a named pipe and a regular file.
Observed behavior:
==================
* 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:
==================
* 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:
=======
* 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:
============
(none at this time)
Environment:
============
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:
=============
(warning, this will spew to the terminal until interrupted if commands-in is 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=None is the default, made explicit to confirm not the issue
"If timeout is None, the call will block until a monitored file object becomes
ready."
See also https://forums.freebsd.org/threads/62377/
Example Python 3 code follows:
import logging
import selectors
def test():
command_pipe = 'commands-in'
logging.basicConfig(level=logging.INFO)
logger = logging.getLogger()
logger.info(f"Opening command pipe: '{command_pipe}'")
with open(command_pipe, 'r') as cp:
select_cp = selectors.DefaultSelector()
select_cp.register(cp, selectors.EVENT_READ)
while True:
key_event_list = select_cp.select(timeout=None)
line = cp.readline()
logger.info(f"Read: '{line}'")
if __name__ == '__main__':
--
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