bug#39962: 27.0.90; Crash in Emacs 27.0.90

[Eli Zaretskii]

> From: Pip Cet <pipcet@gmail.com>
> Date: Thu, 12 Mar 2020 10:32:16 +0000
> Cc: pieter-l@vanoostrum.org, 39962@debbugs.gnu.org, eggert@cs.ucla.edu
> 
> On Wed, Mar 11, 2020 at 7:34 PM Eli Zaretskii <eliz@gnu.org> wrote:
> > > > Did you audit all the users of this function, both direct and
> > > > indirect?  Some of them are outside of GC.
> > >
> > > Thanks for the comment; I just re-checked, and they look fine to me.
> >
> > ??? Fine in what way?
> 
> It doesn't affect visible behavior of any callers, except in the case
> where the previous behavior was buggy.

I guess we have different notions of "visible" and "buggy".

> What confused me is that live_buffer and live_buffer_p both exist and
> do wildly different things.

They do very similar things, AFAICT.

> I'm most certainly not changing the semantics of live_buffer, if
> that's what you're worried about. I am changing the semantics of
> live_buffer_p, which is an internal function, and my initial patch
> also changed the return value of valid_lisp_object_p, to another value
> that would be treated equivalently. If there are objections to that,
> we can easily distinguish the two cases.

I actually don't understand why we need to make such a change.

> And I think "so we don't collect reachable objects" is a fairly good
> reason, generally.

I didn't say it wasn't good, I said it didn't justify the proposed
solution.

How about if you tell more about the root cause of the crash you are
trying to solve, and why disregarding the fact that a buffer is killed
is the way to solve it?

> > The problem you are trying to solve is rare
> 
> I think it would become much less rare with lexical binding in effect,
> at least when the code's byte-compiled.

That remains to be seen.

> > since this code was with us since 20 years ago without
> > anyone bumping into it,
> 
> That we know of. They might have just accrued it to random Emacs crashes.

Then again, they might not.  We don't really have any evidence to that
effect, all we know is that the code survived virtually intact since
the day it was written.