[foreign-memaccess+abi] RFR: 8264176: ResourceScope handles should keep parent scope reachable

[Maurizio Cimadamore]

On Thu, 25 Mar 2021 13:12:22 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> While ResourceScope handles are handy to ensure that a scope is not closed too soon, they don't do much in order to help preventing the scope to become unreachable. This means that if a scope supports both deterministic and implicit deallocation, there might be cases where using a try-with-resources with a resource scope handle is not enough (as the scope might become unreachable inside the TWR block).
> 
> The solution is to add reachability fences in the various handle implementations, so that the parent scope is kept reachable.
> 
> I've refactored the implementation a bit, to remove the "closeable" flag - which was generally not used (only default scopes and global scope are non closeable). So, I have created a new class which <: SharedScope, `NonCloseableScope`. Both global scope and default scope are subclasses of that. This class recycles the handles it generates - e.g. only one is created, after which the same handle instance is returned.
> 
> An alternative would have been to make NonCloseableScope a subtype of both MemoryScope and ResourceScope.Handle - but that's difficult since both are AutoCloseable and feature a `close` method with different semantics.

src/jdk.incubator.foreign/share/classes/jdk/internal/foreign/ConfinedScope.java line 124:

> 122:                 released = true;
> 123:                 lockCount--;
> 124:                 Reference.reachabilityFence(ConfinedScope.this);

Not sure this is needed - I added it as we were worried that possible optimization (e.g. scalarization) might break reachability (this class is an innner class and keeps a strong reference to its enclosing, after all).

src/jdk.incubator.foreign/share/classes/jdk/internal/foreign/SharedScope.java line 187:

> 185:                     }
> 186:                 } while (!STATE.compareAndSet(jdk.internal.foreign.SharedScope.this, value, value - 1));
> 187:                 Reference.reachabilityFence(SharedScope.this);

Not sure this is needed - I added it as we were worried that possible optimization (e.g. scalarization) might break reachability (this class is an innner class and keeps a strong reference to its enclosing, after all).

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PR: https://git.openjdk.java.net/panama-foreign/pull/476