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

[Eli Zaretskii]

> Date: Mon, 15 Nov 2021 10:19:32 -0500
> Cc: 50043@debbugs.gnu.org
> From: Ken Brown <kbrown@cornell.edu>
> 
> x_get_foreign_selection (Lisp_Object selection_symbol, Lisp_Object 
> target_type,
>                        Lisp_Object time_stamp, Lisp_Object frame)
> {
> [...]
>    wait_reading_process_output (secs, nsecs, 0, false,
>                              reading_selection_reply, NULL, 0);
> 
> I think wait_reading_process_output gets stuck for 2 seconds in a call to 
> select 
> (actually xg_select because I'm testing a gtk build).  This is independent of 
> the fact that x-selection-timeout is 2 seconds; it happens even if 
> x-selection-timeout is 0.  select returns after 2 seconds because the 
> poll_timer 
> fires.

Sorry, I don't understand: select waits for up to 2 seconds because
that's what we ask it to do, and those 2 sec do come from
x-selection-timeout.  If x-selection-timeout is zero, select is not
supposed to wait at all, so why does it?  What am I missing?

> On systems with SIGIO, select returns as soon as X events occur, because 
> SIGIO is signaled.

Which X event is that? something related to Emacs and selections, or
just a random event which simply happens at that time?

Anyway, AFAIU, the wait is supposed to end because XTread_socket reads
a SelectionNotify event, and that modifies the cell for which we
wait.  What I'm not sure I understand is how are we supposed to call
XTread_socket when we are stuck inside select all the time?

> We certainly don't want to always skip the select call, but would it make 
> sense 
> to use a very short timeout for select in that case?  Or maybe someone has a 
> better idea.

Making timeout shorter might be the solution, but I'd like to
understand the problem better first.

Thanks.