bug#73752: 29.4; Ligatures are randomly rendered with extra spaces

[Visuwesh <visuweshm@gmail.com>]

[திங்கள் நவம்பர் 04, 2024] Eli Zaretskii wrote:

>> From: Tim Ruffing <dev@real-or-random.org>
>> Cc: dev@real-or-random.org, xuan@xlk.me, 73752@debbugs.gnu.org
>> Date: Mon, 04 Nov 2024 01:11:39 +0100
>> 
>> This fixes the problem:  
>
> Thanks.  But do you understand why?
>
> And how did you arrive to the conclusion that this is the change which
> might help?
>
>> diff --git a/src/ftfont.c b/src/ftfont.c
>> index 882d3eec256..2be443108f1 100644
>> --- a/src/ftfont.c
>> +++ b/src/ftfont.c
>> @@ -2994,9 +2994,8 @@ fthbfont_begin_hb_font (struct font *font, double
>> *position_unit)
>>    struct font_info *ftfont_info = (struct font_info *) font;
>>  
>>    *position_unit = 1.0 / (1 << 6);
>> -  if (! ftfont_info->hb_font)
>> -    ftfont_info->hb_font
>> -      = hb_ft_font_create_referenced (ftfont_info->ft_size->face);
>> +  ftfont_info->hb_font
>> +    = hb_ft_font_create_referenced (ftfont_info->ft_size->face);
>>    return ftfont_info->hb_font;
>>  }
>>  
>> That is, it makes at least the bug with Yixuan's script disappear. I
>> haven't yet verified if this fixes what I often see during my daily
>> usage.
>
> That code was written by a leading HarfBuzz developer, so it's hard
> for me to believe that it is so incorrect.
>
> OTOH, I see that ftcrhbfont is the _only_ HarfBuzz-based font backend
> which implements the end_hb_font method, and I think you both told me
> that only the Cairo build has this problem?  If so, I think the code
> to look at is the end_hb_font method and what it does to the hb_font
> object.  The end_hb_font method is called each time the shaper is
> called, so whatever it does to the hb_font object is inherited by the
> next call to the shaper.
>
> Visuwesh, do the problems you see also disappear when you install the
> change in fthbfont_begin_hb_font which removes the
> !ftfont_info->hb_font condition for calling
> hb_ft_font_create_referenced?

I cannot seem to reproduce the msialignment with that change either.
Likewise for the change Tim posted further down the thread which changes
ftcrhbfont_end_hb_font.