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

From: Po Lu <[email-masked]>
Date: Sat, 13 Apr 2024 19:58:25 +0800

Eli Zaretskii <[email-masked]> writes:

>> From: Greg Sexton <[email-masked]>
>> 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.