FFM API summer update

[Maurizio Cimadamore]

Hi all,
As you know, the FFM API has been a Preview API since Java 19 [1]. The 
API was then re-previewed in 20 [2] and 21 [3]. Some of enhancements 
delivered in this timeframe were:

* Unification of memory segments and memory addresses [4]
* Treat arenas as "capabilities", so that memory segments can be safely 
shared [5, 6]
* Allow for custom arenas [5, 6]
* Add ways to customize linkage requests [7]
* Add fallback linker, to facilitate porting FFM API to all platforms [8]

We now believe that the FFM API is largely stable, and that, modulo 
disasters :-), it should exit preview in the next Java SE release:

https://openjdk.org/jeps/8310626

We have already started accumulating some changes in the panama 
repository which aim at polishing some minor edges in the API:

* Add better support for "wide" native strings [9]
* Make var handles derived from layouts more composable [10]
* Add a linker method that can be used to discover platform-dependent 
mapping between C types and layouts [11]

These changes are fairly minor, and should not have a big impact (if any 
at all) on existing code using the FFM API.

Of course, the fact that FFM API is headed for finalization doesn't mean 
that development on FFM API will stop. There are several enhancements 
(aside from "regular" performance fixes) that we have been discussing 
over the years which can be delivered as follow-up JEPs:

* Support for unsigned/complex types (needs Valhalla)
* Mapping between structs and records [12]
* Integration between Linker and JNI [13]
* Structured arenas (depends on finalization of StructuredTaskScope) [14]
* Pinning of heap segments

(Of course, we have made sure that all the above enhancement can be 
delivered in an additive fashion, e.g. without the need to make 
incompatible changes to the FFM API)

Finally, we'd like to thank all the developers who, over the years, have 
taken the time to take the API for a spin, try to build something with 
it and report feedback. This has been a long journey, and we wouldn't be 
where we are now without your help.

Cheers
Maurizio

[1] - https://openjdk.org/jeps/424
[2] - https://openjdk.org/jeps/434
[3] - https://openjdk.org/jeps/442
[4] - https://cr.openjdk.org/~mcimadamore/panama/segment_address.html
[5] - https://cr.openjdk.org/~mcimadamore/panama/session_arenas.html
[6] - https://cr.openjdk.org/~mcimadamore/panama/scoped_arenas.html
[7] - https://git.openjdk.org/panama-foreign/pull/734
[8] - https://git.openjdk.org/panama-foreign/pull/770
[9] - https://git.openjdk.org/panama-foreign/pull/836
[10] - https://github.com/openjdk/panama-foreign/pull/840
[11] - https://git.openjdk.org/panama-foreign/pull/839
[12] - https://github.com/openjdk/panama-foreign/pull/833
[13] - https://mail.openjdk.org/pipermail/panama-dev/2023-May/019079.html
[14] - https://cr.openjdk.org/~mcimadamore/panama/session_arenas.html