bug#21428: 24.5; Crash of emacs on OS X, installed via homebrew cask

[Rainer M Krug <Rainer@krugs.de>]

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Eli Zaretskii <eliz@gnu.org> writes:

>> From: Rainer M Krug <Rainer@krugs.de>
>> Cc: 21428@debbugs.gnu.org
>> Date: Mon, 26 Oct 2015 08:59:39 +0100
>> 
>> the bug does not affect to many users (strange config? OS X?).
>
> It's not really true that it didn't affect others.  We have in the bug
> tracker a few similar bug reports where an invalid face ID was the
> culprit, I believe they were caused by the same problem.
>
> It's true that these problems are rare.  That's because, for this
> situation to happen, you need to have some to enable feature that can
> create new faces or change existing faces in code that is invoked by
> redisplay, for example some Lisp form that gets evaluated while
> displaying the mode line.  When a face is created or changed, Emacs
> forgets all the cached faces (because a new/changed face could
> potentially require fresh realization of the faces that depend on the
> changed face, and Emacs doesn't track face dependencies and this
> cannot know which ones, if any, need to be recomputed).  If this
> happens, and if Emacs also needs to redraw the portions of the display
> that used one of the "forgotten" faces, it crashes.  So this requires
> a relatively rare combination of factors, and so went under the radar
> for a long time.  I think it was exposed more lately because we
> consistently introduced more and more redisplay optimizations, which
> increased the probability of these rare situations because they allow
> Emacs to avoid examining faces in larger areas of its display, thus
> failing to recompute them after discarding the cached faces.

Thanks for the explanations - makes sense.

Cheers,

Rainer



-- 
Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation Biology, UCT), Dipl. Phys. (Germany)

Centre of Excellence for Invasion Biology
Stellenbosch University
South Africa

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