RFR: 8297424: java/net/httpclient/AsyncExecutorShutdown.java fails in AssertionError due to misplaced assert [v2]

[Daniel Fuchs]

> The `ResponseSubscribers.HttpResponseInputStream` class has an assert that has been observed firing (rarely) when running the `java/net/httpclient/AsyncExecutorShutdown.java` test.
> 
> After a thorough analysis of the code and the log failure I am convinced that the issue is due to a misplaced assert.
> If cancellation happens asynchronously while the subscriber is being subscribed with the lower layers of the stack, the `HttpResponseInputStream` might get closed and the `LAST_LIST` buffer might be offered after `closed == false` has been observed by the `onSubscribed` method, but before the assertion has been checked. The assertion assumes that the queue must be empty, but it might not if `close` has been called and the `LAST_LIST` buffer has been offered.
> 
> Moving the assert from within the synchronized block, to ensure that the observed value of `closed` is consistent with the state of the queue, should fix it. I have tagged the bug as `noreg-hard`, I'm not sure it would be fair to say that `AsyncExecutorShutdown.java` can be used to verify the fix.

Daniel Fuchs has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'master' into AsyncExecutorShutdown-8297424
 - Revert assignement of pendingResponseSubscriber to null to avoid race conditions with ternary operators
 - 8297424

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11332/files
  - new: https://git.openjdk.org/jdk/pull/11332/files/b4e462b4..71b59be3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11332&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11332&range=00-01

  Stats: 8939 lines in 305 files changed: 5754 ins; 2091 del; 1094 mod
  Patch: https://git.openjdk.org/jdk/pull/11332.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11332/head:pull/11332

PR: https://git.openjdk.org/jdk/pull/11332