On 10/2/2017 2:52 PM, Matthew Flatt wrote:
If we change `ffi/unsafe/alloc` so that a custodian shutdown runs all deallocators, that would work in many cases. It would create a problem, however, if there are objects to deallocate where the deallocation function references other objects that themselves must be deallocated. That is, an allocated value A might depend on an allocated value B up until the point that A has been deallocated. That dependency can be expressed to `ffi/unsafe/alloc` (via the garbage collector) by including a reference to B in the deallocation closure for A. Setting up dependencies that way is not common, but I'm pretty sure I've used that pattern once or twice. Meanwhile, there's no similar way to order callbacks that are registered with a custodian.
IIRC, the JVM executes finalizers in reverse order of registration - expecting that, in your example, A would have been allocated before B. Is that something that would work here? [He asks naively.]
I'm tempted to say that `ffi/unsafe/alloc` should only be used for objects where the deallocation order doesn't matter, so that deallocators can be triggered by a custodian shutdown. But an additional constraint is needed: values to be deallocated cannot be used by any other custodian-shutdown callbacks. That additional constraint seems too limiting. Maybe we need to add some sort of ordering constraints to custodians, but I'm not immediately sure of the right way to do that.
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