Re: scratch/igc: Implications of MPS being asynchronous

[Gerd Möllmann]

Pip Cet <pipcet@protonmail.com> writes:

>> I skipped weak structures for learning MPS, so I just left this out. For
>> MPS, some other weak structure would be needed, If one wants that. Maybe
>> a weak hash table could be used for the font cache?
>
> While weak hash tables may continue to be necessary (I'm not sure about
> this), for internal data structures we control, finalizers are
> better:
>
> When you allocate a font structure, allocate an extra cons cell which
> you always use to refer, via XCAR, to the font structure.  Except that
> in the hash table, you use the car directly.  When the cons cell is no
> longer reachable (the font structure still is, because it's in the hash
> table), the finalizer will eventually run, and then you can remove the
> hash table entry from the hash table, eventually freeing the font
> structure, too..
>
> Note that this works with MPS finalizers (slow) and struct
> Lisp_Finalizer finalizers (even slower).  We should prefer the former as
> Lisp finalizers don't behave precisely like MPS finalizers; right now,
> we just use MPS behavior for Lisp finalizers, but that doesn't match the
> documentation in every detail.

Something like that, yes. Or maybe just clear the font caches completely
when some criterion is met? Time, size, something else, no idea.