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Re: Crash with --enable-checking and some glyphs

From: Eli Zaretskii
Subject: Re: Crash with --enable-checking and some glyphs
Date: Sun, 28 Nov 2021 09:21:37 +0200

> Date: Sat, 27 Nov 2021 22:01:05 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: Eli Zaretskii <eliz@gnu.org>, rpluim@gmail.com, emacs-devel@gnu.org
> I very much doubt that this is the root cause of this bug.

What do you mean by "this"?

The fix that I installed didn't pretend to fix any root causes,
because those are not known.  I just verified that the code in that
part does its job and works according to what the font metrics
returned by the font backend feeds it.  Since the fonts are identical,
the root cause could be in the font backend (which is not our code) or
maybe elsewhere.  You are welcome to investigate further, but my
suggestion is to start from xftfont.c and its ilk, tracing the
information it returns for the font in question.

The problem is also extremely rare, because otherwise everyone who
builds Emacs with --enable-checking would see frequent crashes long

In any case, the installed fix just does what other branches of that
function already did for glyphs that are not simple character glyphs,
that's all.  It "corrects" the ascent and descent values after they
have been already used for the glyph metrics on our display, so its
effect on the display should be minimal at best, and usually nil.  But
if you have evidence to the contrary, by all means present it, and
let's see what we should and can do about that.

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