RFR: 8291897: TerminatingThreadLocal(s) not registered from virtual thread(s) [v2]

[Peter Levart]

> This is an attempt to fix inconsistent behavior of TerminatingThreadLocal(s) when used internally in JDK for per-carrier-thread caches of native ByteBuffer(s) and NativeBuffer(s). If used from virtual thread, such TerminatingThreadLocal(s) are not registered with the carrier thread for the cleanup.
> The fix introduces an internal CarrierThreadLocal subclass of ThreadLocal which rewires its API methods to use current thread's carrier thread as the source of ThreadLocalMap instead of current thread itself (if it is a VirtualThread for example). TerminatingThreadLocal is now a subclass of CarrierThreadLocal. It seems only per-carrier-thread caching is a usecase for it. The uses of JavaLangAccess in various places to access a carrier-thread value of given ThreadLocal has been replaced by public API calls and the use of CarrierThreadLocal instead of plain ThreadLocal. JavaLangAccess is still used to dispatch the CarrierThreadLocal API methods, but only for that. Would someone be tempted to use JavaLangAccess methods directly, they now require CarrierThreadLocal argument to guard against missuses.
> The REGISTRY of TerminatingThreadLocal(s) that tracks which of them have values bound to a particular carrier thread is now implemented conveniently  with a CarrierThreadLocal.
> The test is expanded with a case that demonstrates a situation where a carrier thread is terminated. Since it must wait for 30 seconds for that to happen, only one of the test cases is performed in this mode. The correct logic of TerminatingThreadLocal is still verified with all test-cases using platform threads that can be terminated more rapidly.

Peter Levart has updated the pull request incrementally with one additional commit since the last revision:

  Removed unused JLA, SharedSecrets, added @enablePreview test annotation

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9743/files
  - new: https://git.openjdk.org/jdk/pull/9743/files/fa2078e4..2339b87f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9743&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9743&range=00-01

  Stats: 10 lines in 2 files changed: 3 ins; 3 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/9743.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9743/head:pull/9743

PR: https://git.openjdk.org/jdk/pull/9743