bug#9767: 24.0.90; gdb initialization on Cygwin

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 19 Oct 2011 16:00:09 -0400
> From: Ken Brown <kbrown@cornell.edu>
> CC: 9767@debbugs.gnu.org
> 
> After M-x gdb finishes its initialization, emacs goes into its command 
> loop.  read_char calls sit_for with a timeout of 30 seconds, and sit_for 
> calls wait_reading_process_output, which calls select.  The call to 
> select fails immediately with EINTR.  I don't understand the command 
> loop well enough to know what's interrupting the select call.

EINTR means that some signal arrived (assuming that Cygwin's `select'
is Posix-ish enough).  The question is, which signal?  Does Cygwin
provide any tools to see which signals were delivered to a program?

Also, the fact that `select' is interrupted doesn't necessarily mean
that the input arrival is ignored, does it?  Doesn't
wait_reading_process_output loop around and examines the input
descriptors again?  If not, why not?  IOW, why should EINTR become a
failure?

> The code in keyboard.c is full of alarms and timers, presumably related 
> to polling for keyboard input.  Could this polling be doing something 
> that interrupts the select call under some circumstances?

Atimers (those which are responsible for the "busy cursor" display)
could deliver SIGALRM, yes.  But again, I don't see why this should
fail the loop that waits for input, and then only in this particular
case.  Something else is at work here.