Re: Ephemerons, self-referentality in weak hashtables

[Christine Lemmer-Webber <cwebber@dustycloud.org>]

Maxime Devos <maximedevos@telenet.be> writes:

> [[PGP Signed Part:Undecided]]
> Christine Lemmer-Webber schreef op wo 08-09-2021 om 12:18 [-0400]:
>> Maxime Devos <maximedevos@telenet.be> writes:
>> 
>> > [[PGP Signed Part:Undecided]]
>> > Maxime Devos schreef op zo 20-06-2021 om 17:01 [+0200]:
>> > > Christopher Lemmer Webber schreef op di 18-05-2021 om 11:46 [-0400]:
>> > > > Hello,
>> > > > 
>> > > > I'm finally taking some time to port Goblins to Guile, in-between other
>> > > > tasks anyway.  In Goblins there is a weak hashtable that maps current
>> > > > actor references to their current behavior.  I found that for
>> > > > self-referential actors, I needed ephemerons for GC stuff to work right.
>> > > 
>> > > [bla bla on how this could be implemented in Guile]
>> > 
>> > This doesn't work because there is nothing preventing the
>> > 'value' from being freed. Trying to fix that now, using
>> > the example implementation of "weak maps" in libgc.
>> > 
>> > Greetings,
>> > Maxime.
>> 
>> I fell off the radar on replying to this, but did path turn out to work?
>
> The example implementation was a bit complicated and not well-documented.
> The ‘disclaimer’ support is disabled by default, a configuration flag
> needs to be set while compiling libgc to use disclaimers.  Guix (the distro I
> use) doesn't set it.  I needed to be careful in libguile/weak-table.c to not
> protect too much (otherwise it wouldn't be an ephemeral weak hash table) or
> too little (otherwise ‘freed’ objects would be re-used).
>
> I think it can be made to work, but I didn't succeed, and moved on to other
> things.  If someone would like to implement this, I would recommend starting
> with something like ‘ephemeral pairs’ before moving to ephemeral hash tables.
>
> Greetings,
> Maxime.

Okay... thank you for the work you put into it! :)