[foreign-memaccess+abi] RFR: 8265751: MemoryAddress should have a scope accessor [v2]

[Maurizio Cimadamore]

On Thu, 22 Apr 2021 11:46:55 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Following the recent API changes, I think `MemoryAddress` has been left in an inconsistent state. On the one hand, we clearly want `MemoryAddress` to have a link to a `ResourceScope`; we clearly state so in the javadoc for:
>> 
>> * `CLinker::upcallStub`
>> * `MemorySegment::address`
>> 
>> (and we don't say, but we probably should in `VaList::address`).
>> 
>> This link is crucial, as it prevents scope closure, if an address is passed to a native function and the scope is implicit.
>> 
>> However, this dependency is, currently, ad-hoc, and not cleanly stated in the API.
>> 
>> This patch rectifies that; now all memory address have a scope (which can be the "global scope" for addresses obtained from native, or from `long` values).
>> 
>> There are not many changes in terms of implementation, but by putting the association between addresses and scopes front and center we can make the javadoc clearer.
>> 
>> Also, this allows us to move the scope check from `MemorySegment::address` (that has always been a bit odd) to `MemoryAddress::toRawLongAddress` - that is, closer to where the address value is actually used.
>> 
>> This allows us to, effectively, make calls to downcall method handles *safer*, since now a scope check will take place before the raw long value is extracted.
>> 
>> Note that this doesn't negatively affect performance: the check was occurring even before, albeit at a different level (closer to the user code). I've run all the downcall benchmarks we have and found no regressions.
>> 
>> I've added a test which makes sure that passing "closed" segments/addresses to native function is no longer possible.
>
> Maurizio Cimadamore has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:
> 
>  - Merge branch 'foreign-memaccess+abi' into scoped_memory_address
>  - Fix javadoc/impl of MemoryAddress::segmentOffset
>  - Add API changes
>    Fix various javadoc issues in MemorySegment
>  - Initial push
>    * move scope check inside toRawLongValue

I have also fixed several references to "this segment is not alive" which were still present in the MemorySegment javadoc

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PR: https://git.openjdk.java.net/panama-foreign/pull/513