RFR: 8316580: HttpClient with StructuredTaskScope does not close when a task fails [v6]

[Daniel Fuchs]

> Hi, 
> 
> Please find below a fix for 8316580: HttpClient with StructuredTaskScope does not close when a task fails.
> 
> The problem here is a subtle reference release issue: Interrupting the VirtualThread means that the CompletableFuture returned by sendAsync is eligible for GC after having been cancelled, which in turn means that some of the intermediate operations that would have been completed before that CF was completed get eligible for being GC’ed too. One of these intermediate operations is the action that decrements the ref counting. Since the refcount isn’t decremented properly, the client won’t exit. 
> 
> Holding onto the CompletableFuture returned to the caller by HttpClient::sendAsync until that CompletableFuture gets completed from upstream fixes the issue.

Daniel Fuchs has updated the pull request incrementally with one additional commit since the last revision:

  improved logic in whenCancelled

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15883/files
  - new: https://git.openjdk.org/jdk/pull/15883/files/3966a882..0df2a888

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15883&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15883&range=04-05

  Stats: 16 lines in 1 file changed: 0 ins; 2 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/15883.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15883/head:pull/15883

PR: https://git.openjdk.org/jdk/pull/15883