[jdk21u-dev] RFR: 8342905: Thread.setContextClassloader from thread in FJP commonPool task no longer works after JDK-8327501 redux

[Francisco Ferrari Bihurriet]

Hi,

I would like to propose a [JDK-8342905](https://bugs.openjdk.org/browse/JDK-8342905 "Thread.setContextClassloader from thread in FJP commonPool task no longer works after JDK-8327501 redux") (openjdk/jdk23u at 872ae1347198408ffd62559ca8c1a7420c4a8108) backport from 23u to 21u.

The backport is not clean, for the following reasons:

* The `jdk.internal.access.JavaLangAccess` import context in `ForkJoinPool.java` has changed from 21u to 23u, due to [JDK-8288899](https://bugs.openjdk.org/browse/JDK-8288899 "java/util/concurrent/ExecutorService/CloseTest.java failed with "InterruptedException: sleep interrupted"") (openjdk/jdk at 667cca9d7aef1ff4abe630cefaac34c0b1646925)
* The `jdk.internal.access.JavaLangAccess::allowSecurityManager` method didn't exist in 21u, so I picked it from [JDK-8296244](https://bugs.openjdk.org/browse/JDK-8296244 "Alternate implementation of user-based authorization Subject APIs that doesn’t depend on Security Manager APIs") (openjdk/jdk at d32746ef4a0ce6fec558274244321991be141698)
    * We don't need [JDK-8296244](https://bugs.openjdk.org/browse/JDK-8296244 "Alternate implementation of user-based authorization Subject APIs that doesn’t depend on Security Manager APIs"), just the access to the `System::allowSecurityManager` private method
    * I don't see [JDK-8296244](https://bugs.openjdk.org/browse/JDK-8296244 "Alternate implementation of user-based authorization Subject APIs that doesn’t depend on Security Manager APIs") as a possible dependency candidate, given it is a much broader change with its own CSR affecting the Java SE API and only approved for 23

This pull request depends on @martinuy's #1181.

#### Testing

As part of our testing, we observed all the tests pass in the following categories:

* `jdk:tier1` (see [GitHub Actions run](https://github.com/franferrax/jdk21u-dev/actions/runs/12126257318))
* `jdk/java/util/concurrent/forkjoin`
* `jdk/java/util/concurrent/tck`

Additionally, an internal test for [JDK-8237117](https://bugs.openjdk.org/browse/JDK-8237117 "Better ForkJoinPool behavior") was executed without issues.

-------------

Depends on: https://git.openjdk.org/jdk21u-dev/pull/1181

Commit messages:
 - Backport 872ae1347198408ffd62559ca8c1a7420c4a8108

Changes: https://git.openjdk.org/jdk21u-dev/pull/1192/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1192&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342905
  Stats: 18 lines in 3 files changed: 15 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1192.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1192/head:pull/1192

PR: https://git.openjdk.org/jdk21u-dev/pull/1192