bug#33798: 27.0.50; font-lock-ensure fontifies the whole buffer

bug#33798: 27.0.50; font-lock-ensure fontifies the whole buffer

[Juri Linkov]

X-Debbugs-CC: Stefan Monnier <monnier@iro.umontreal.ca>

The simplest test case to reproduce this is to eval in emacs -Q:

(with-temp-buffer
  (insert "#include <stdio.h>\n\nmain()\n{\n printf(\"Hello, world\");\n}")
  (c-mode)
  (font-lock-ensure 21 (point-max))
  (get-text-property (point-min) 'face))

that returns
=> font-lock-preprocessor-face

This means that `font-lock-ensure' fontifies not only the given region,
but also the whole buffer.

OTOH,

(with-temp-buffer
  (insert "#include <stdio.h>\n\nmain()\n{\n printf(\"Hello, world\");\n}")
  (c-mode)
  (font-lock-fontify-region 21 (point-max))
  (get-text-property (point-min) 'face))

returns `nil' that means that it fontifies only within the specified boundary.

This difference is essential for bug#33567: Syntactic fontification of diff hunks
where fontifying the whole buffer might slow down fontification of diff hunks
that are small parts of the whole buffer.

bug#33798: 27.0.50; font-lock-ensure fontifies the whole buffer

[Stefan Monnier]

Hi Jury,

> (with-temp-buffer
>   (insert "#include <stdio.h>\n\nmain()\n{\n printf(\"Hello, world\");\n}")
>   (c-mode)
>   (font-lock-ensure 21 (point-max))
>   (get-text-property (point-min) 'face))

Is this testcase representative of what diff-mode does?


        Stefan

bug#33798: 27.0.50; font-lock-ensure fontifies the whole buffer

[Juri Linkov]

>> (with-temp-buffer
>>   (insert "#include <stdio.h>\n\nmain()\n{\n printf(\"Hello, world\");\n}")
>>   (c-mode)
>>   (font-lock-ensure 21 (point-max))
>>   (get-text-property (point-min) 'face))
>
> Is this testcase representative of what diff-mode does?

I can see the same effect in internal buffers created by diff-mode:
switching to these buffers I see that font-lock-ensure fontifies
the whole buffer, whereas using font-lock-fontify-region I see only
small parts of the source buffer fontified corresponding to diff hunks.

bug#33798: 27.0.50; font-lock-ensure fontifies the whole buffer

[Stefan Monnier]

> I can see the same effect in internal buffers created by diff-mode:
> switching to these buffers I see that font-lock-ensure fontifies
> the whole buffer, whereas using font-lock-fontify-region I see only
> small parts of the source buffer fontified corresponding to diff hunks.

I installed a patch into `master` which should fix this.
Could you check it works for your cases?


        Stefan

bug#33798: 27.0.50; font-lock-ensure fontifies the whole buffer

[Juri Linkov]

>> I can see the same effect in internal buffers created by diff-mode:
>> switching to these buffers I see that font-lock-ensure fontifies
>> the whole buffer, whereas using font-lock-fontify-region I see only
>> small parts of the source buffer fontified corresponding to diff hunks.
>
> I installed a patch into `master` which should fix this.
> Could you check it works for your cases?

I checked that it works now in all cases and fontifies only needed hunks,
so I updated diff-mode to always use font-lock-ensure.