RFR: 8262027: Improve how HttpConnection detects a closed channel when taking/returning a connection to the pool [v6]

[Daniel Fuchs]

> Hi,
> 
> Please find here a fix for:
> 8262027: Improve how HttpConnection detects a closed channel when taking/returning a connection to the pool
> 
> While writing a new test to verify that it was possible to handle proxy *and* server authentication manually when both proxy and server required authentication, I stumbled on a race condition where the next request after receiving 407 would manage to retrieve the connection from the pool just before the connection close was receive from the proxy. Since the test was using POST, and POST are not retried by default, this caused the test to fail randomly and intermittently.
> 
> This fix proposes to add a checkOpen() method to HttpConnection, which we can call just after retrieving a connection from the pool. This method will attempt to read 1 byte from the channel. Because the connection was in the pool, it should not have received anything, and because the channel is non-blocking, the `read` should always return 0, unless the channel has been closed. This forces an early check on the channel state, rather then waiting for the selector to wake up the Selector Manager Thread - which might happen too late.
> 
> This is not a 100% silver bullet, but it drastically reduced the number of failures I was observing (to 0 after several 100 loops of testing on all machines). The only other failures I observed was on windows, where apparently closing the socket on the server side can cause a reset, even when SO_LINGER and TCP_NODELAY are specified. I solved that by adding a small delay between socket.shutdownOutput() and socket.close() in the test proxy - when running on windows.

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 eight additional commits since the last revision:

 - Incorporoted review comments
 - Merge branch 'master' into proxy-server-auth-8262027
 - Reworked ProxyServer to reuse the connection in case it can detect that there will be no request body
 - Merge branch 'master' into proxy-server-auth-8262027
 - Added blank line
 - Fixed trailing white spaces
 - Remove commented code in test
 - 8262027: Improve how HttpConnection detects a closed channel when taking/returning a connection to the pool

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2649/files
  - new: https://git.openjdk.java.net/jdk/pull/2649/files/d37e7bdb..51c65067

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2649&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2649&range=04-05

  Stats: 1699 lines in 84 files changed: 1141 ins; 340 del; 218 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2649.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2649/head:pull/2649

PR: https://git.openjdk.java.net/jdk/pull/2649