font-lock-fontify-buffer removes highlighting in non font-lock buffers

[Juri Linkov]

font-lock-fontify-buffer is an interactive command and when it is invoked
as a buffer not intended for font-lock fontification, this breaks
highlighting in all other non font-lock buffers.

A simple way to reproduce:

C-h i M-x font-lock-fontify-buffer RET

and after this to see that highlighting is broken in all other buffers:

M-x list-colors-display RET
M-x list-faces-display RET
...

That's because font-lock-default-fontify-buffer doesn't make the variable
font-lock-keywords buffer-local, and its global value gets compiled to
`(t nil)'.

After creating new buffers, font-lock-default-function checks
font-lock-keywords for nil, and since its global value is now
`(t nil)', it calls font-lock-mode-internal which removes all
highlighting from every non font-lock buffer.

This bug was responsible for removing highlighting from Gnus buffers,
and now it is avoided but not calling font-lock-fontify-buffer
from hi-lock mode.  But I think it should be fixed no matter
if it called from hi-lock or not.

I see at least three solutions:

* check font-lock-keywords in font-lock-default-function for the compiled
  empty value `(t nil)' and don't call font-lock-mode-internal for this
  value;

* make the variable font-lock-keywords automatically buffer-local;

* in font-lock-default-fontify-buffer call font-lock-set-defaults
  regardless of the value of font-lock-mode, or depending on some other
  condition, e.g. if (local-variable-p 'font-lock-keywords).

-- 
Juri Linkov
http://www.jurta.org/emacs/