bug#50043: 28.0.50; USABLE_SIGOI undef code paths do not work correctly

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 17 Nov 2021 09:19:27 -0500
> Cc: larsi@gnus.org, 50043@debbugs.gnu.org
> From: Ken Brown <kbrown@cornell.edu>
> 
> > What is that more general problem, and when does it rear its ugly
> > head?
> 
> The problem is that whenever wait_reading_process_output needs to repeatedly 
> check for input (i.e., read_kbd || !NILP (wait_for_cell) == true), it can get 
> stuck in select for up to 2 seconds on systems without USABLE_SIGIO.  (2 arises 
> here because it's the default value of polling-period.)

But that can only happen if select is called with a long enough
timeout, right?

Anyway, if the problem is that on these systems select doesn't return
when there are input events waiting, I guess it's okay to do the
change there.  But maybe it will be better to do it the same way we
decrease the timeout when a timer is expected to expire before the
timeout: we decrease the timeout, but still remember its value, and
don't return from waiting before the timeout unless there really was
some input.  This would avoid affecting unrelated features such as
accept-process-output.