RFR: 7161229 - PriorityBlockingQueue keeps hard reference to last removed element

[David Holmes]

webrev:

http://cr.openjdk.java.net/~dholmes/7161229/webrev/

When removing the last element from the PBQ the "sift down" logic would 
store it back in as array[0]. The simple fix is for the sift-down to be 
a no-op if the queue size is zero.

The fix has been contributed by Doug Lea. We are taking this opportunity 
to synchronize PBQ with Doug's latest version at:

http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/main/java/util/concurrent/PriorityBlockingQueue.java?view=log

So in addition to the above functional fix there are a number of 
miscellaneous code and comment cleanups. The only non-trivial, but still 
very simple, change is to the drainTo method to make it more robust if 
the add() on the destination collection throws an exception.

I am of course a Reviewer for this.

I have provided the update to the LastElement test (as this is certainly 
related to the last element) but there are some reservations about 
examining the PBS internals this way. Unfortunately there is no good way 
to test for these kinds of retention issues. You either need a whitebox 
test like this, or need to rely on non-guaranteed GC and finalization 
behaviour.

Thanks,
David Holmes