Cygwin: processEnded never called for spawned process

[Erik Bray]

This issue can be reproduced with a simple test case like:

import os
from twisted.internet import reactor
from twisted.internet import utils

def printResults(*args):
    print(args)
    reactor.stop()


d = utils.getProcessOutputAndValue('echo', ['hello'], env=os.environ)
d.addCallback(printResults)
reactor.run()

The printResults callback is never run and the reactor (specifically PollReactor which is the default on Cygwin) blocks at poll().

This can also be seen with the tests:

python -m twisted.trial twisted.internet.test.test_process.ProcessTestsBuilder_PollReactorTests.test_processEnded
python -m twisted.trial twisted.internet.test.test_process.ProcessTestsBuilder_PollReactorTests.test_processExited

and possibly others, which hang due to this issue.

The problem, as I understand it, has to do with the forceReadHack flag of ​`ProcessWriter`, which Process always passes True.

Cygwin has a bug(?) that I documented ​here. The technical details in Cygwin are unimportant but in short poll()-ing for readability on the write end of a pipe does not set any useful bits (much less POLLERR). So we get stuck polling for the write end of the stdin pipe to be ready which never happens on Cygwin.

It would be better to explicitly disable the "read hack" on Cygwin, and manually call Process.closeStdin when the child process is already known to have exited.

PR: ​https://github.com/twisted/twisted/pull/925

[Adi Roiban]

Thanks for working on this. The ticket description is great and it was very easy to review the PR. Great work!

See my review comments ​https://github.com/twisted/twisted/pull/925#pullrequestreview-79033710

I started a conversation on the mailing list to see what can be done for patches sent for unsupported platforms. See ​https://twistedmatrix.com/pipermail/twisted-python/2017-November/031746.html

[Erik Bray]

Thanks for the review. I'm also having a look at the (interesting) discussion on the mailing list. Should I follow up more here or on GitHub?