When someone runs "bin/trial --reactor=select 
twisted.internet.test.test_process.ProcessTestsBuilder_PollReactor.test_
childConnectionLost" on cygwin, using the cygwin version of Python, 
either manually (which I've done on two cygwin installations), or from 
the twisted buildbot -- 
http://buildbot.twistedmatrix.com/builders/cygwin-py2.5-select -- then 
once the child process closes its fd #2, it subsequently receives no 
further bytes from its fd #1, even though the parent process has written 
some bytes to that pipe.

I can reproduce this by running the Twisted unit tests, like this:

step 1: get twisted:

svn co svn://svn.twistedmatrix.com/svn/Twisted/trunk twisted

step 2: cd into the top-level directory, and run trial:

cd twisted && python ./bin/trial --reactor=select 
twisted.internet.test.test_process.ProcessTestsBuilder_PollReactor.test_
childConnectionLost

On other platforms that cygwin (see the Twisted buildbot -- 
http://buildbot.twistedmatrix.com/supported?branch=trunk -- then this 
test exits quickly with success. On cygwin, using cygwin python.exe, 
this test instead enter an infinite loop as the child waits to receive 
further bytes on its stdin, and the parent waits for the child to 
respond to those bytes. (This doesn't happen on cygwin with the standard 
Win32 build of python.exe, of course, anymore than it happens on a 
Windows system without cygwin installed when using that executable.)

I've opened a ticket about this on the twisted issue tracker as well: 

http://twistedmatrix.com/trac/ticket/3529 # closing stdout in a child 
process on cygwin means that process doesn't receive bytes from stdin 
anymore. I think.