bug#68006: 30.0.50; Image-mode speed

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Kangas <stefankangas@gmail.com>
> Date: Sat, 30 Dec 2023 15:57:28 -0800
> Cc: 68006@debbugs.gnu.org
> 
> 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?

See my explanation of the purpose of this particular cache here:

  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=68006#14

In a nutshell, this cache is ephemeral anyway, and will be flushed at
some arbitrary time whether we want it or not, because its purpose is
not what you think it is.

In any case, if you intend to not flush the cache at all, then what
does that mean for Emacs sessions running for days and weeks, let
alone months, on end? will they keep these images in memory forever?
Or should GC sometimes evict those images from the cache, and if so,
under what conditions?

As for the differences between images and other buffers, there are
some:

  . many buffers are smaller than images nowadays
  . buffers with images are in many cases showing text we are
    interested in temporarily, like Web pages (program source code
    rarely if ever includes images, right?)