bug#25279: 26.0.50; Slowdown/crash on certain characters

[Richard Copley <rcopley@gmail.com>]

On 26 December 2016 at 20:25, Eli Zaretskii <eliz@gnu.org> wrote:
>> From: Richard Copley <rcopley@gmail.com>
>> Date: Mon, 26 Dec 2016 20:09:16 +0000
>>
>> >From emacs -Q:
>> Insert MUSIC FLAT SIGN or RIGHTWARDS DOUBLE ARROW in a buffer.
>> Move point around in the buffer or edit the buffer text.
>> Emacs gets very slow, consuming a lot of CPU.
>> Sometimes it completely grinds to a halt.
>
> Doesn't happen here.
>
>> MUSIC FLAT SIGN and RIGHTWARDS DOUBLE ARROW are examples
>> that cause this problem for me. MUSIC SHARP SIGN and
>> RIGHTWARDS ARROW are examples that do not cause a problem.
>>
>> Below are the contents of the describe-char buffer for these
>> characters (with the character itself asterisked out in each
>> case so as not to crash my Emacs while I edit this mail).
>>
>> Note the categories. They seem illogical. Are they supposed
>> to be like that? Why?
>
> Because you don't have Symbola installed, I guess.  The fonts Emacs
> finds for displaying these characters all have non-Unicode registry
> fields, and that causes Emacs to desperately look for a Unicode font
> each time it needs to display one of these characters.

OK, thanks, but I don't quite follow, sorry. Unless you're saying there's a
non-desperate mechanism that's usually used but which fails unless a
font with a Unicode registry field is found for the character?

> Symbola is referenced in the default fontset with Unicode registry.
> You could also customize the fontset with the fonts you have, giving
> them iso10646-1 as the registry instead of what you have now, and that
> might also fix the problem.  But installing Symbola is better, IMO.

Installing Symbola works a treat. Thanks very much.
I wish I could see how to make that information easy to discover.

> Alternatively, setting inhibit-compacting-font-caches to a non-nil
> value will probably work around the problem.
>
>> Note the fonts. Could there be a bug in "Malgun Gothic"?
>> As far as I know it's a Korean font installed by default with Windows.
>> Could there be a bug in "Consolas"? Why does Emacs find the MUSIC
>> SHARP SIGN glyph but not the MUSIC FLAT SIGN glyph from Consolas?
>
> You will need to look into the coverage of these fonts to answer those
> questions, I think.  On Windows, Emacs generally examines fonts in the
> alphabetical order, looking for the first font that supports the
> character, and that's after it tried to use the default face's font.

Commendably thorough, but causes the editor to grind to a halt and crash
in some circumstances.

>> I asked about this on IRC and there exist Windows Emacs users who
>> don't have the issue, so it may be influenced by environmental
>> factors.
>
> Those factors are the fonts they have installed, I think.

Seems so. Thanks again!