bug#9767: 24.0.90; gdb initialization on Cygwin

[Ken Brown <kbrown@cornell.edu>]

On 10/19/2011 4:26 PM, Eli Zaretskii wrote:
>> 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?

There's a program called strace that produces lots of debugging 
information like this.  I'll give it a try.

> 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?

No, wait_reading_process_output treats EINTR as though it meant there's 
no input available.  Here's the relevant code from process.c, line 4638.

       if (nfds < 0)
	{
	  if (xerrno == EINTR)
	    no_avail = 1;

Even worse, xg_select (which is what the Cygwin build uses) deliberately 
returns -1 and sets errno = EINTR whenever the actual select call 
returns 0.  Here's the code from xg_select.c, line 141:

       /* To not have to recalculate timeout, return like this.  */
       if (retval == 0)
         {
           retval = -1;
           errno = EINTR;
         }

I don't know the rationale for treating EINTR the same as no input 
available, but I agree that it doesn't seem right.

>> 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.