bug#74738: 31.0.50; Freezes in Python-mode on some Python file when searching or scrolling

[rehan malak]

thanks kobarity, with your patch, I confirm that :

- I can open gef.py and scroll
- I get the correct font-lock-string-face in the f-string after a "#" in the format

On Wed, Dec 11, 2024 at 3:24 PM kobarity <kobarity@gmail.com> wrote:

rehan malak wrote:
> Deleting all the "#" in the f-string of the gef.py file removes the problem on my side.
>  sed -i -e 's/#0{align/0{align/g' gef.py
>  sed -i -e 's/#0{width/0{width/g' gef.py
>  sed -i -e 's/#07x}/07x}/g' gef.py
>  sed -i -e 's/#06x}/06x}/g' gef.py
>  sed -i -e 's/#04x}/04x}/g' gef.py
>  sed -i -e 's/#4x}/4x}/g' gef.py
>  sed -i -e 's/#8x}/8x}/g' gef.py
>  sed -i -e 's/#10x}/10x}/g' gef.py
>  sed -i -e 's/#x}/x}/g' gef.py
>
> Your smaller example Eli also contains a f-string with the "#".
>
> This python f-string format specifier is described here:
> https://docs.python.org/3/library/string.html#format-specification-mini-language
>
> Is this format specifier supported from the beginning ?
>
> For example :
>
> value = 0xab
> print(f"{value:x} is a value")
>
> => color syntax : is a value    has the color defined by font-lock-string-face
>
> while
>
> value = 0xab
> print(f"{value:#x} is a value")
>
> => color are messed up... is has color defined by font-lock-keyword-face and a value has color defined by default face

The mechanism of the hang is still unclear, but I found the cause of
the font-lock malfunction: when using `up-list' to find the
corresponding closing brace for an opening brace in an f-string, if
the format specifier contains "#", the rest of the string is
considered a comment and the search fails.  Therefore, it can be
worked around by temporarily binding `parse-sexp-ignore-comments' to
nil, as in the attached patch.

Hangs on gef.py can also be avoided with this patch, but it assumes
that the braces are properly closed.  If you remove the closing braces
of expressions containing "#", it will eventually hang.