bug#60144: 30.0.50; PGTK Emacs crashes after signal

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> That code has problems signalling errors, unless it is okay for
>> unblock_input to signal.
>
> I don't understand this part.  Why and how is unblock_input part of
> the picture?

Because unblock_input can call process_pending_signals, and in doing so
handle_async_input, which calls gobble_input (and thus the
read_socket_hook.)  As a result, it is not safe for any read_socket_hook
to signal as long as it is not ok for unblock_input to signal as well.

> So in the X/GTK build we have the same problem as with PGTK?  If so,
> why not change that as well, to work as I described, i.e. enqueue
> events to our own event queue, which we will then read and process in
> safe context?
>
> AFAIU, w32 already works like that.  Does it not?

It doesn't, see how w32_note_mouse_movement is called from
w32_read_socket.

> Yes, understood.  But it just tells me that we need to change the
> architecture so that the events delivered by the window-system are not
> processed in callbacks we install to be called by the window-system,
> they should be processed in our own safe context.

The problem is note_mouse_highlight is simply not supposed to signal.
It is a function called directly while handling async input as far back
as Emacs 19, much like expose_frame.  (IIRC back then there was a
slightly different implementation in each of the *term.c files.)

Moving note_mouse_highlight out of handle_one_xevent would lead to other
bugs, since mouse movement must be processed in order wrt to other X
events.  For example, if an XI_Motion event arrives and is queued, and
then a subsequent XI_Leave event arrives before that event has a chance
to be processed ``in our own safe context'', note_mouse_highlight will
be called after the mouse has left the frame, leading to stuck mouse
highlight.