RFR: 8332154: Memory leak in SynchronousQueue [v3]

[Viktor Klang]

> Local testing seems to indicate that this fix (which mirrors what's done in the FIFO mode) addresses the problem.
> 
> But with that said, I haven't come up with a decent way of adding some form of regression test. Suggestions are most welcome. /cc @DougLea

Viktor Klang has updated the pull request incrementally with one additional commit since the last revision:

  Moving the memory leak test for SynchronousQueue into its own test and runs only for JDK20+, using VirtualThreads

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19271/files
  - new: https://git.openjdk.org/jdk/pull/19271/files/f365c5f0..cc0a2014

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19271&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19271&range=01-02

  Stats: 181 lines in 3 files changed: 116 ins; 65 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19271.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19271/head:pull/19271

PR: https://git.openjdk.org/jdk/pull/19271