bug#33275: 27.0.50; Image cache pruning

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 33275@debbugs.gnu.org
> Date: Mon, 05 Nov 2018 18:48:33 +0100
> 
> Anyway, there's code that tries to keep the cache small:
> 
>         /* If the number of cached images has grown unusually large,
>            decrease the cache eviction delay (Bug#6230).  */
>         delay = XFIXNUM (Vimage_cache_eviction_delay);
>         if (nimages > 40)
>           delay = 1600 * delay / nimages / nimages;
> 
> But it works solely on the number of images in the cache, and not the
> size of the images.  So it's a heuristic that could be tweaked pretty
> easily, I think?

I don't see why we should tweak our cache management for the benefit
of exceptional use case.  Last time we did it (with fonts) we are
still leaking the wounds.

I think it should be enough to catch excess memory usage and signal an
error.  I hope this is possible.