bug#25672: Font-locking issues when font-lock-fontified is nil (Was: What to do when font-lock-flush + font-lock-ensure do less than font-lock-fontify-buffer? on emacs-devel)

[Clément Pit--Claudel]

X-Debbugs-CC: monnier@iro.umontreal.ca

Hi bug-gnu-emacs,

Sorry for the long delay in opening this.  This is a follow-up to the 
Emacs-devel thread "What to do when font-lock-flush + font-lock-ensure do less 
than font-lock-fontify-buffer?", where Stefan asked me to open a bug report.

My original question was this:

> IIUC the proper way in Emacs 25 to signal changes that require a
> refontification to font-lock is to call font-lock-flush. This doesn't always
> do as much as font-lock-fontify-buffer does, though.  In particular, when the
> font-lock-fontified variable is nil (which does happen, though I don't know
> why; I don't know what this variable is for), then calling font-lock-fontify
> buffer does refontify the whole buffer, but font-lock-flush and
> font-lock-ensure don't seem to do anything.  What's the proper way to mark a
> buffer for refontification (and refontify the visible portion) immediately in
> 25?

And here's is my original test case:

    (with-current-buffer (get-buffer-create "temp")
      (erase-buffer)
      (setq-default prettify-symbols-unprettify-at-point 'right-edge)
      (emacs-lisp-mode)
      (prettify-symbols-mode)
      (insert "lambda\n\nlambda")
      (pop-to-buffer (current-buffer)))

    Moving around in this buffer unprettifies and reprettifies ‘lambda’s into 
‘λ’s as the point moves in and out of them. Setting ‘font-lock-fontified’ to 
nil breaks that mechanism (the ‘λ’s are unprettified but no reprettified), and 
no series of ‘font-lock-flush’ and ‘font-lock-ensure’ will reprettify them.

Eli remarked the following:

> However, please explain why font-lock-ensure-function calls
> font-lock-DEFAULT-fontify-buffer.  If it's supposed to be good for any mode, 
> why
> does it call the default fontification?
>
> As a matter of fact, if I replace the font-lock-default-fontify-buffer
> call with a call to font-lock-fontify-buffer, the problem described by
> the OP goes away.  That is why I said what I said.

And Stefan answered:

> I haven't seen the beginning of the thread, but if that fixes the
> problem, then I think it's a good change.
>
> To go back to the previous question.  I guess I used
> font-lock-default-fontify-buffer because the whole point is to get rid
> of all the misuses of font-lock-fontify-buffer, but I think in this case
> it's indeed the right thing to do.

Then Stefan said:

> > Should I open a bug report about this?
> Yes.  And put me in X-Debbugs-Cc.

Done :)

Thanks,
Clément.

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