wingolog

Consider guardians. Compared to finalizers, in which the cleanup procedures are run concurrently with the mutator, by the garbage collector, guardians allow the mutator to control concurrency. See Guile’s documentation for more notes. Java’s PhantomReference / ReferenceQueue seems to be similar in spirit, though the details differ.

questions

If we want guardians, how should we implement them in Whippet? How do they relate to ephemerons and finalizers?

It would be a shame if guardians were primitive, as they are a relatively niche language feature. Guile has them, yes, but really what Guile has is bugs: because Guile implements guardians on top of BDW-GC’s finalizers (without topological ordering), all the other things that finalizers might do in Guile (e.g. closing file ports) might run at the same time as the objects protected by guardians. For the specific object being guarded, this isn’t so much of a problem, because when you put an object in the guardian, it arranges to prepend the guardian finalizer before any existing finalizer. But when a whole clique of objects becomes unreachable, objects referred to by the guarded object may be finalized. So the object you get back from the guardian might refer to, say, already-closed file ports.

The guardians-are-primitive solution is to add a special guardian pass to the collector that will identify unreachable guarded objects. In this way, the transitive closure of all guarded objects will be already visited by the time finalizables are computed, protecting them from finalization. This would be sufficient, but is it necessary?

answers?

Thinking more abstractly, perhaps we can solve this issue and others with a set of finalizer priorities: a collector could have, say, 10 finalizer priorities, and run the finalizer fixpoint once per priority, in order. If no finalizer is registered at a given priority, there is no overhead. A given embedder (Guile, say) could use priority 0 for guardians, priority 1 for “normal” finalizers, and ignore the other priorities. I think such a facility could also support other constructs, including Java’s phantom references, weak references, and reference queues, especially when combined with ephemerons.

Anyway, all this is a note for posterity. Are there other interesting mutator-exposed GC constructs that can’t be implemented with a combination of ephemerons and multi-priority finalizers? Do let me know!