Hi, I encountered this bug through a hang in Twisted, but narrowed it down to a simple example (in Python here, but the same would work in C).
Basically, in Twisted's system process runner, there's a "hack" [1] which basically assumes readability (as in a poll() loop) of the writable end of a pipe indicates that the pipe is closed and/or there's an error. Although the "hack" is disabled by default, in practice it is always forced to be enabled [2]. One could maybe consider this a bug in Twisted since I can't find anything in POSIX which states this behavior (though I could be missing it). Nevertheless, on Linux this *is* the behavior: >>> import os >>> import select >>> r, w = os.pipe() >>> poller = select.poll() >>> os.close(r) >>> poller.register(w, select.POLLIN) >>> print(poller.poll(1000)) [(4, 8)] where 8 indicates that POLLERR is set on fd 4, indicating in this case that if we tried to write to the pipe we would get a broken pipe error. However, on Cygwin the same code returns an empty list. I don't know if this *should* be fixed, but it would be nice. It's slightly tricky though. In Cygwin's poll there's a line [3] that does something similar for sockets--if the socket is not connected it sets POLLERR in the results. One could do something similar for pipes, but there isn't an existing internal API to do this conveniently. What one might want is something that calls NtQueryInformationFile like in pipe_data_available [4], and checks the NamedPipeState flag. But that something doesn't exist yet. Any ideas? Erik [1] https://github.com/twisted/twisted/blob/bb371b3b602b1547c6e603d3b234bb18ded6d67c/src/twisted/internet/process.py#L139 [2] https://github.com/twisted/twisted/blob/bb371b3b602b1547c6e603d3b234bb18ded6d67c/src/twisted/internet/process.py#L764 [3] https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/poll.cc;h=ea45b70adacaca96edbc46d7b8ffe1cd8a94270e;hb=HEAD#l122 [4] https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/select.cc;h=f5a993850408477a65d9ecda444785d0db1f35a8;hb=HEAD#l567 -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
