bug#66732: tree-sitter fontification doesn't update multi-line syntax reliably

[Dmitry Gutov <dmitry@gutov.dev>]

On 12/12/2023 09:50, Yuan Fu wrote:

> I feel that font-lock and syntax-ppss have different problems and 
> requires different solutions. For font-lock, we need to mark affected 
> range unfontified; we just need to make sure we do that after 
> jit-lock-fontify-now. For syntax-ppss, we need to force a reparse before 
> entering syntax-ppss so that syntax text properties are up-to-date when 
> syntax-ppss do its work.

font-lock *could* have a different solution, but I don't see why any 
solution that works for syntax-ppss properly, shouldn't work just as 
well for font-lock. And if it's possible - why not do that, saving on 
variations in code. We also save on having to handle two differently 
callbacks from the parser.

The proposed alternative is not without problems. For example, that 
unfontifying a region inside a timer, you trigger a second fontification 
rather than having the job finished in one go. Possibly introducing a 
extra "blink" as well. That is a minor inconvenience by itself, but 
ideally we'd avoid it.

>> Here's a draft solution based on *-extend-region-functions, attached.
>>
>> Alas, while it works fine in python-ts-mode (for both syntax and 
>> font-lock), making it behave better than python-mode, in c-ts-mode it 
>> doesn't quite have the same effect: when you backspace over the 
>> closing "/", the highlighting is properly updated only after you make 
>> the next edit (any edit), or select another window. I'm not sure, 
>> though, if it's due to my own problems with Emacs's failure to 
>> redisplay (reported elsewhere), so more testing is welcome.
>>
>> But that might also be related to the use of 
>> c-ts-mode--emacs-set-ranges: printing a backtrace calls inside 
>> treesit--font-lock-notifier shows that the last notification comes 
>> also during font-lock but after treesit--font-lock-extend-region, 
>> inside c-ts-mode--emacs-set-ranges. I don't quite understand this 
>> design where the ranges are applied inside the font-lock code.
> c-ts-mode--emacs-set-ranges is registered as a range rule, so many 
> tree-sitter function calls it before doing anything to make sure range 
> is up-to-date. treesit-font-lock-fontify-region calls 
> treesit-update-ranges at the beginning of its body, and 
> treesit-update-ranges calls c-ts-mode--emacs-set-ranges.

That seems to mean that any feature accessing the parse tree should call 
treesit-update-ranges first. Including syntax-propertize-functions and 
*-extend-region-functions, which we currently don't do.

But which boundaries is it supposed to use? Should 
treesit--syntax-extend-region call treesit-update-ranges, when wait for 
the parser updates, then possibly call treesit-update-ranges again on 
the extended boundaries, and so on, until the parser stops sending 
notifications? Will it stop?

Anyway, could you try my patch? Like I said, I'm not sure if the 
insufficient fontification I'm observing in c-ts-mode is due to the 
problem with the solution, or due to the other redisplay-related 
problems on my system.