lib/poll.c, win32 SIGHUP on pipes

[Erik Faye-Lund]

I've recently looked into replacing the poll-emulation in Git for
Windows with the code from gnulib, due to the improved pipe-support in
gnulib.

However, I've noticed that win32_compute_revents() never returns
SIGHUP for FILE_TYPE_PIPE, which is something the code I'm trying to
port currently depends on. What seems to happen is the following: one
of the pipe-ends is closed, and thus PeekNamedPipe() fails.
win32_compute_revents() mistakenly believes that the failure means
that the pipe can be written because NtQueryInformationFile succeeds
and returns 8192 for both fpli.WriteQuotaAvailable and
fpli.OutboundQuota.

In my case, a simple "happened |= POLLHUP;"-hack in the error-case
solves the issue in this particular case, but this is obviously not
correct in the general case.

My guess is that a proper solution would be to find device a robust
way of checking if a pipe is closed or not. I haven't looked much into
this yet, but at least GetNamedPipeInfo() doesn't seem to error out on
my pipe.

Thoughts?