RFR: 8294916: Cancelling a request must eventually cause its response body subscriber to be unregistered [v8]

[Daniel Fuchs]

> When [JDK-8277969](https://bugs.openjdk.org/browse/JDK-8277969) was implemented, a list of outstanding response subscribers was added to `HttpClientImpl`. A body subscriber is added to the list after being created and is removed from the list when it is completed, either successfully or exceptionally.
> 
> It appears that in the case where the subscription is cancelled before the subscriber is completed, the subscriber might remain registered in the list forever, or at least until the HttpClient gets garbage collected. This can be easily reproduced using streaming subscribers, such as BodySubscriber::ofInputStream. In the case where the input stream is closed without having read all the bytes, Subscription::cancel will be called. Whether the subscriber gets unregistered or not at that point becomes racy.
> 
> Indeed, the reactive stream specification doesn't guarantee whether onComplete or onError will be called or not after a subscriber cancels its subscription. Any cleanup that would have been performed by onComplete/onError might therefore need to be performed when the subscription is cancelled too.

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 ten additional commits since the last revision:

 - Merge branch 'master' into inputstream-leak-8294916
 - More review feedback
 - Update copyright years
 - Incorporated review feedback
 - More cleanup to the test
 - Minor cleanup to the test again
 - Minor cleanup to the test
 - Merge branch 'master' into inputstream-leak-8294916
 - 8294916

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10659/files
  - new: https://git.openjdk.org/jdk/pull/10659/files/984e7e1d..74c2c702

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10659&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10659&range=06-07

  Stats: 18865 lines in 895 files changed: 12135 ins; 3469 del; 3261 mod
  Patch: https://git.openjdk.org/jdk/pull/10659.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10659/head:pull/10659

PR: https://git.openjdk.org/jdk/pull/10659