[foreign-memaccess+abi] RFR: 8264386: LibraryLookup should be more friendly with implicit unloading [v2]

[Paul Sandoz]

On Mon, 29 Mar 2021 15:45:06 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Quoting from the PR:
>> 
>> Dereferencing a segment from a LibraryLookup is possible, in the following form:
>> 
>> LibraryLookup libLookup = ...
>> MemorySegment segment = libLookup.lookup("foo").asSegmentRestricted(....)
>> 
>> The above code has a problem: there is no link between the produced segment and the original library lookup - meaning that, in principle, it's possible for the library to be unloaded when dereference occurs.
>> 
>> After having discussed many alternatives with Jorn, the best way to address this conundrum would be to add an overload of `LibraryLookup::lookup` which takes a lyout and returns a segment.
>> 
>> As part of this change I have also removed the LibraryLookup.Symbol API which seems unnecessary - now you can lookup an address or a segment. Of course, looking up a segment is creating a restricted segment (so it should be treated as unsafe).
>> 
>> I've added a test to make sure that the new API works as expected.
>
> 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 three additional commits since the last revision:
> 
>  - Add @NativeAccess annotations and @CallerSensitive check on LibraryLookup::lookup(String, MemoryLayout)
>  - Merge branch 'foreign-memaccess+abi' into resourceScope+libraries
>  - Initial push

Summarizing so i have the correct understanding.

The core fix is to update `LibraryLookupImpl` to hold a memory segment that is sliced to the address of symbols, thereby keeping a ref-chain for returned segments **and** addresses of symbols.

Separately, the `Symbol` abstraction is not worth it, and we can lean on returning `MemorySegment` for a symbol covering some region of memory (like some constant).

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Marked as reviewed by psandoz (Committer).

PR: https://git.openjdk.java.net/panama-foreign/pull/480