bug#68006: 30.0.50; Image-mode speed

[Stefan Kangas <stefankangas@gmail.com>]

Manuel Giraud via "Bug reports for GNU Emacs, the Swiss army knife of
text editors" <bug-gnu-emacs@gnu.org> writes:

> And here is a new version of the patch.  Maybe, we need a NEWS entry for
> this too.

If we have an option, then yes we should add it to NEWS too.

But I'm not sure about this one:

Flushing the image cache all the time is fine for images that are 4KB in
size, but these days we routinely work with images that are several
megabytes in size (to give an idea, searching online tells me that
Twitter allows uploading photos up to 2 MB, Facebook up to 15 MB).
Furthermore, my experience with image-dired has taught me that Emacs is
terribly slow when compared to all other programs capable of viewing
images out there.  So I think we should think a bit more about this one.

Taking a step back, why are images treated differently from other
buffers?  If the risk is that the image changes on disk without us
noticing, then users should need to either run `revert-buffer' or enable
`auto-revert-mode'.  If we are talking about images that are inline in a
buffer, the cache should be flushed only when the buffer itself is
reverted.  What am I missing?

Lastly, I'm not sure about the "hash the first 4 KB of an image"
heuristic, for starters because images can be several megabytes in size.
Would it not be both more accurate and faster to check the mtime and/or
the size of the file?  Or do we need different heuristic for different
image formats?  (Maybe JPEG changes the first 4 KB on save, but I don't
think all bitmap formats do.)  But wouldn't it be even better to use the
notify system when possible, like with `auto-revert-use-notify'?