Re: last_marked array is now ifdef'ed away

[Eli Zaretskii <eliz@gnu.org>]

> From: Mattias Engdegård <mattias.engdegard@gmail.com>
> Date: Tue, 17 Sep 2024 12:15:16 +0200
> Cc: emacs-devel@gnu.org,
>  Andrea Corallo <acorallo@gnu.org>
> 
> 15 sep. 2024 kl. 16.21 skrev Eli Zaretskii <eliz@gnu.org>:
> 
> > There's no need for any evidence to keep the code which we always had.
> 
> Well, we didn't have it in Emacs 29, so one might be excused for thinking it wasn't a strict necessity.

Because it was silently disabled.  This change should have been
discussed in advance.

> > That won't help because GC crashes are seldom if ever reproducible.
> > So if the trace is off, the information is gone and cannot be
> > recovered in practice.
> 
> My experience is rather that in such cases the crash was already gone because there was no core dump or debugger attached anyway, but we then take the necessary steps to catch the bug next time – turn on core dumps (if possible), run with a debugger, enable checking, etc – and we always end up trapping the gremlin eventually.

At least some (many?) users have core dumps available, even though
current systems foolishly disable them by default.  I see this all the
time on debbugs: users report bugs and tell they have core files or
the crashed Emacs in a debugger.

> Anyway, I'm going to re-enable the mark trace buffer for the sake of development peace; since it's important to you, that's also worth something. I shall add a configuration option for disabling it, with its trade-off clearly documented, so that users can make an informed decision, but the buffer will be enabled by default.

Thank you.  Having a knob to disable it is fine, as long as the
default is to have it enabled.