bug#48629: 28.0.50; GUI emacsclient frames stop accepting keyboard input around recv

[Eli Zaretskii]

> From: "Basil L. Contovounesios" <contovob@tcd.ie>
> Cc: schwab@linux-m68k.org,  48629@debbugs.gnu.org
> Date: Wed, 26 May 2021 19:10:30 +0100
> 
> > IOW, when you find Emacs stuck in pselect, type "finish" and let Emacs
> > exit the pselect call.
> 
> I did this, and it returned zero after a while:

So it found no input available.

> > Is the keyboard descriptor in the set of descriptors pselect waits
> > for?
> 
> How do I determine what the keyboard descriptor is?

Look at fd_callback_info[desc].flags for values of DESC from zero to
max_desc, and see if any of them have the FOR_READ and KEYBOARD_FD
flags set.  Then see whether those descriptors, which have those flags
set, have their bit set in readfds set passed to pselect.