Re: Selective font-locking?

[Stefan Monnier]

>> On Apr 12, 2021, at 10:07 PM, Stefan Monnier <monnier@iro.umontreal.ca> 
>> wrote:
>> 
>> Calling `font-lock-flush` or `font-lock-ensure` from font-lock-keywords
>> is quite odd.  I'd call something like `font-lock-fontify-region` instead.
>
> Using `font-lock-fontify-region` instead causes Emacs to become mostly
> unresponsive.  Sending a USR2 reveals:
>
> Debugger entered--entering a function:
> * #f(compiled-function () #<bytecode 0x1fed77d1f329>)()
>   font-lock-default-fontify-region(188 189 nil)
>   font-lock-fontify-region(188 189)
>   #f(compiled-function (fun) #<bytecode 
> 0x1fed77d1f2f9>)(font-lock-fontify-region)
>   run-hook-wrapped(#f(compiled-function (fun) #<bytecode 0x1fed77d1f2f9>) 
> font-lock-fontify-region)
>   jit-lock--run-functions(188 189)
>   jit-lock-fontify-now(188 688)
>   jit-lock-function(188)
>   redisplay_internal\ \(C\ function\)()  

That's not giving enough info to figure out what's going on, sorry.
My crystal ball suggests that maybe your matching function simply
forgets to return nil, so font-lock calls it again and again thinking
we're keeping matching "more stuff" without ever reaching limit.

But it does remind me that rather than mess with `font-lock-keywords`,
you could do a similar dance with `font-lock-fontify-region-function`
(and internally call `font-lock-default-fontify-region`).
It might prove simpler and more reliable.

>> There's one thing with which you might want to be careful, tho, which is
>> the `syntax-ppss` state.  You might want to `narrow-to-region` around
>> the call to `font-lock-fontify-region` (maybe narrow to 
>> pmark...(point-max)?).
>> 
>> This is because in a shell buffer, some of the past interactions may
>> have been truncated (e.g. by `comint-truncate-buffer`), so you may end
>> up with (point-min) being in the middle of a string or something.
>> [ Similar problems can occur if the prompt itself contains funny characters
>>  like unmatched quotes.  or if past interactions include output which
>>  is not lexically valid Python code.  ]
>
> Well hmm, this is a bummer.  l tested for this issue by inserting an entirely 
> unmatched quote:
>
> In [12]: print(chr(39))
> '
>
> and this does affect the syntax (everything is a string).  But unfortunately 
> narrowing as follows doesn’t seem to fix this:
>
>           (save-restriction
>             (narrow-to-region pmark (point-max))
>             (with-syntax-table python-mode-syntax-table
>               (font-lock-flush start limit)
>               (font-lock-ensure start limit)))))))
>
> I’m not sure it’s the same thing, but I found a related issue with 
> `indent-for-tab-command'.  In attempting to ignore the prompt for computing 
> indentation, I narrowed to a region which excluded it, but indent.el calls 
> `indent--funcall-widened’, which undoes my narrowing!  
>
> Is there any way to specify "narrow to this region and don’t let anybody 
> widen it(!)"?

It's called `font-lock-dont-widen`.


        Stefan