bug#69561: 30.0.50; Freeze from M-x gnus on macOS

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Gerd Möllmann <gerd.moellmann@gmail.com> writes:

> (CC'd to Alan.)
>
> As a reminder, the freezes are especially annoying because one cannot
> C-g out of them. I read the NS code today, and I think I at least have a
> strong suspicion why that is.
>
> Let's just simplify things and say that the NS code has a queue named
> hold_events_q of struct input_event (global variable). The queue is
> filled when EmacsView receives NS events from the system, for example
> C-g. NS events are processed by calling [NSApp run] with some
> ornamention around it to make sure the calls returns. ns_select and
> ns_read_socket do that.
>
> The input_events in hold_events_q are given to Emacs in ns_read_socket,
> which is installed for terminal type as read_socket_hook. That's how
> normally a C-g is recognized by kdb_store_event and Vquit_flag is set.
>
> But hold_events_q are _not_ kbd_store'd in ns_select. Instead we have
>
>   if (hold_event_q.nr > 0 && !run_loop_only)
>     {
>       /* We already have events pending.  */
>       raise (SIGIO);
>       errno = EINTR;
>       return -1;
>     }
>
> So, ns_select returns -1 to wait_reading_process_out which loops,
> AFAICT.
>
>       if (nfds < 0)
> 	{
> 	  if (xerrno == EINTR)
> 	    no_avail = 1;
> ...
>       if (no_avail || nfds == 0)
> 	continue;
>
> And Vquit_flag is never changing because the C-g is still in
> hold_events_q, and maybe_quit does nothing.
>
> Does that make sense? 

I'm now using the below change. This should also lead to NS events being
processed.

1 file changed, 6 insertions(+), 5 deletions(-)
src/nsterm.m | 11 ++++++-----

modified   src/nsterm.m
@@ -4739,12 +4739,13 @@ Function modeled after x_draw_glyph_string_box ().
   check_native_fs ();
 #endif
 
-  if (hold_event_q.nr > 0 && !run_loop_only)
+  /* If there are input events pending, store them so that
+     Emacs can recognize C-g.  */
+  if (hold_event_q.nr > 0)
     {
-      /* We already have events pending.  */
-      raise (SIGIO);
-      errno = EINTR;
-      return -1;
+      for (int i = 0; i < hold_event_q.nr; ++i)
+        kbd_buffer_store_event_hold (&hold_event_q.q[i], NULL);
+      hold_event_q.nr = 0;
     }
 
   eassert (nfds <= FD_SETSIZE);