RFR: 8376308: java/net/httpclient/CancelRequestTest.java fails intermittently with "Expected CancellationException not received"

[Daniel Fuchs]

When using async mode, and if the "wrong" test/client thread gets suspended at the wrong time there's a small time window in which the server might be able to send its reply before the request is cancelled.
This can be avoided by having the server handler wait on a semaphore until the cancellation exception has been propagated to the caller on the client side.

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Commit messages:
 - Copyright year + unused imports
 - 8376308: java/net/httpclient/CancelRequestTest.java fails intermittently with "Expected CancellationException not received"

Changes: https://git.openjdk.org/jdk/pull/29415/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29415&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376308
  Stats: 96 lines in 1 file changed: 53 ins; 1 del; 42 mod
  Patch: https://git.openjdk.org/jdk/pull/29415.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29415/head:pull/29415

PR: https://git.openjdk.org/jdk/pull/29415