Downgrade pattern doesn't work with "Shared Reentrant Read Write Lock"

Issue: Downgrade pattern doesn't block already existed WRITE lock waiters.

Scenario:

• First thread takes a WRITE lock and waits
• Second thread starts and it is trying to obtain a WRITE lock. It waits because the WRITE lock is hold by the first thread.
• First thread does downgrade:
  • Takes the READ lock
  • Releases the WRITE lock

At this point the first thread still holds the READ lock but the second one will get the WRITE lock.

It is reproducible for:

• 2 threads use the same CuratorFramework instance
• 2 threads use the different sessions
• 2 separate JVMs

There is attached test class: DowngradeTest.java and zoo.cfg

Quote from Shared Reentrant Read Write Lock description
http://curator.apache.org/curator-recipes/shared-reentrant-read-write-lock.html

Lock Downgrading Re-entrancy also allows downgrading from the write lock to a read lock, by acquiring the write lock, then the read lock and then releasing the write lock. However, upgrading from a read lock to the write lock is not possible.