[foreign-memaccess+abi] RFR: 8263018: Improve API for lifecycle of native resources [v10]

[Maurizio Cimadamore]

On Mon, 15 Mar 2021 18:36:36 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> > Less sure about `SegmentAllocator.of(scope)` -> `ResourceScope.toAllocator()`, FWIW I noticed this is a subset of `malloc`:
> > ResourceScope scope = ...
> > SegmentAllocator fixedScopeAllocator = SegmentAllocator.malloc(() -> scope);
> > There might also be some interconnections with `ScopedSegmentAllocator`? Seems like we either want to compose through inheritance or have some bi-direction between the two abstractions.
> 
> Good point - if we had a ScopedSegmentAllocator abstraction, I believe creating an allocator from a scope would not be as important (if clients need both an allocator and a scope, well they can just allocate a ScopedSegmentAllocator).

I made an attempt at this - captured in the following javadoc:

http://cr.openjdk.java.net/~mcimadamore/panama/resourceScope-javadoc_v6_exp/

There is a new ScopedMemoryAllocator - and some of the existing factory now return that. Note that now the difference between SegmentAllocator.malloc and SegmentAllocator.scoped is cleared, as only the latter gives you a ScopedSegmentAllocator.

All the arena allocators are also ScopedSegmentAllocators.

The recycling allocator has been moved off to the side in MemorySegment::toAllocator.

Is this an improvement? While more verbose than the old native scope, I think there's a lot more composition going on here, as now users can select full spectrum of { shared, confined } x { bounded vs. unbounded allocation } x { explicit vs. implicit close } - the former NativeScope used to be a point (although common) in this space: confined x { bounded vs. unbounded allocation } x explicit close.

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PR: https://git.openjdk.java.net/panama-foreign/pull/466