bug#11303: Whitespace mode causes auto-save to malfunction

[Lars Ingebrigtsen]

Kelly Dean <kellydeanch@yahoo.com> writes:

> Start Emacs with no options.
> Open a file, press space, wait for the timeout, press C-g, and wait
> for the timeout again. Notice that Emacs does write the auto-save file
> again, even though the buffer hasn't changed since the last
> auto-save. This is not the correct behavior.

This problem is still present in Emacs 28.

Poking around a bit at this, it seems that it's the call to

(jit-lock-refontify (point-min) (point-max))

that makes `M-x do-auto-save' auto-save again (even if nothing has
changed except the text properties in the buffer).

I.e., doing

(put-text-property 1 3 'face 'bold)

will make the buffer eligible for auto-saving again, even if putting
those text properties into the buffer doesn't switch from unmodified to
modified.

That's pretty inconsistent behaviour, at least?  I'm guessing this stems
from text properties upping the BUF_MODIFF, but not changing the buffer
modification status.

Does this sound like the correct analysis?  If so, what can we do about
it?  (`with-silent-modifications' doesn't seem to have any effect here.)

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no