bug#70311: Crash when dnd-indicate-insertion-point is non-nil

[Eli Zaretskii <eliz@gnu.org>]

> From: Po Lu <luangruo@yahoo.com>
> Cc: Greg Sexton <gregsexton@gmail.com>,  70311@debbugs.gnu.org
> Date: Sat, 13 Apr 2024 19:58:25 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> From: Greg Sexton <gregsexton@gmail.com>
> >> Date: Tue, 9 Apr 2024 11:26:41 +0100
> >> 
> >> Emacs (29.1.90) can crash on Mac OS X when dnd-indicate-insertion-point
> >> is non-nil.
> >> 
> >> I'm not able to deterministically reproduce this, but I have a high
> >> success rate dragging a file over the window and just randomly moving
> >> it around without releasing the click.
> >> 
> >> This said, I attached a debugger and found the root cause.
> >> 
> >> There is code that looks like this in dnd-handle-movement:
> >> 
> >>   (when dnd-indicate-insertion-point
> >>      (ignore-errors
> >>        (goto-char (posn-point posn))))
> >> 
> >> posn-point can return nil, which causes goto-char to call
> >> wrong_type_argument, this reaches signal_or_quit (eval.c), which calls
> >> emacs_abort as waiting_for_input is true.
> >> 
> >> A quick fix might be to when-let the result of posn-point, but I'm
> >> unsure if waiting_for_input being true points to a deeper issue.
> >
> > Po Lu, any comments or suggestions?
> 
> As regards suggestions, none at present.  It's not clear to me why
> waiting_for_input should continue to exist, but neither am I especially
> satisfied with the present arrangements for calling drag-and-drop
> callbacks in nsterm.m: some means should be devised of returning to the
> event loop during a DND session, rather than calling Lisp and parts of
> redisplay from non-reentrant input handlers that the toolkit can call at
> will.
> 
> Come to think of it, at the time this kludge was written, I never quite
> understood why DND events were never received on Mac OS, when the same
> was smooth sailing on GNUstep.

So you think the suggested bad-aid is the best we can currently do to
prevent crashes in those cases?