Input still available after EPOLLHUP from pipe

	benji reported
Trac ID	trac#7932
Type	defect
Created	2015-06-08 19:32:18Z

I have an app that uses the epoll reactor to read input from a subprocess. Infrequently the EPOLLHUP event will arrive before the first EPOLLIN event, even though input is available.

Here is a self-contained test script which will generate a test failure about every 100 runs on my machine: http://pastebin.ubuntu.com/11655018/. Save it as repro.py and run "trial -u repro.py". It may take a while to fail. It seems to fail more frequently on slower machines.

Seeing that http://man7.org/linux/man-pages/man2/epoll_ctl.2.html says of EPOLLHUP:

Note that when reading from a channel such as a pipe or a stream
socket, this event merely indicates that the peer closed its end
of the channel. Subsequent reads from the channel will return 0
(end of file) only after all outstanding data in the channel has
been consumed.

I added this hack to our app:

reactor._POLL_IN |= EPOLLHUP

Which causes the reactor to consider the channel readable on EPOLLHUP events. This completely rectified the issue for the test script and our app.

I'm not certain this is actually enough to solve it generally though; my reading of the above man page suggests that it may take multiple post-HUP reads to completely drain the available input.

I was using Twisted 15.2.1 with Python 2.7.8 running on Ubuntu 14.04.2.

trac-id__7932 7932
type__defect defect
reporter__benji benji
priority__normal normal
milestone__ 
branch__ 
branch_author__ 
status__assigned assigned
resolution__None None
component__core core
keywords__ 
time__1433791938007367 1433791938007367
changetime__1469268134345035 1469268134345035
version__None None
owner__sparkiegeek sparkiegeek
cc__@acollard

Save the linked script as repro.py and run trial -u repro.py to reproduce the intermittent failure. Start by tracing the epoll reactor's handling of EPOLLHUP and EPOLLIN, then verify that available pipe input is consumed after EPOLLHUP and that the reproducer no longer fails.