[jdk21u-dev] RFR: 8334719: (se) Deferred close of SelectableChannel may result in a Selector doing the final close before concurrent I/O on channel has completed

[Goetz Lindenmaier]

I backport this for parity with 21.0.6-oracle.

Clean backport, but I had to adapt the Syntax of the test to 21.

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Commit messages:
 - Adapt test syntax to Java 21
 - Backport 9bb675f89dd1eeec423ca96cb3f96d29f5de477c

Changes: https://git.openjdk.org/jdk21u-dev/pull/1027/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1027&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334719
  Stats: 705 lines in 7 files changed: 705 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1027.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1027/head:pull/1027

PR: https://git.openjdk.org/jdk21u-dev/pull/1027