ramkrishna.s.vasudevan updated HBASE-16643:
    Attachment: HBASE-16643.patch

Patch for 2.0. This makes MemstoreScanner to create either a KeyValueHeap or 
ReversedKeyValueHeap based on the type of scan we are doing. Since 
MemstoreScanner is now a Collection of SegmentScanner we need to apply the 
logic of backWardSeek or seekToLastRow as we do for the actual StoreScanners 
before we add on to the heap. 
Also removed some unused imports in related test cases.
Made MemstoreScannerType as a first class enum. While creating the 
memstoreScanner this type is passed on to decide the type of heap to be 
created. Removes the unwanted code that was closing a heap and creating a 
backward heap that was causing the actual problem mentioned in the JIRA title.
Ping [~anastas], [~eshcar], [~anoop.hbase] for reviews. 

> Reverse scanner heap creation may not allow MSLAB closure due to inproper ref 
> counting of segments
> --------------------------------------------------------------------------------------------------
>                 Key: HBASE-16643
>                 URL: https://issues.apache.org/jira/browse/HBASE-16643
>             Project: HBase
>          Issue Type: Bug
>            Reporter: ramkrishna.s.vasudevan
>            Assignee: ramkrishna.s.vasudevan
>            Priority: Critical
>         Attachments: HBASE-16643.patch
> In the reverse scanner case,
> While doing 'initBackwardHeapIfNeeded' in MemstoreScanner for setting the 
> backward heap, we do a 
> {code}
> if ((backwardHeap == null) && (forwardHeap != null)) {
>         forwardHeap.close();
>         forwardHeap = null;
>         // before building the heap seek for the relevant key on the scanners,
>         // for the heap to be built from the scanners correctly
>         for (KeyValueScanner scan : scanners) {
>           if (toLast) {
>             res |= scan.seekToLastRow();
>           } else {
>             res |= scan.backwardSeek(cell);
>           }
>         }
> {code}
> forwardHeap.close(). This would internally decrement the MSLAB ref counter 
> for the current active segment and snapshot segment.
> When the scan is actually closed again we do close() and that will again 
> decrement the count. Here chances are there that the count would go negative 
> and hence the actual MSLAB closure that checks for refCount==0 will fail. 
> Apart from this, when the refCount becomes 0 after the firstClose if any 
> other thread requests to close the segment, then we will end up in corrupted 
> segment because the segment could be put back to the MSLAB pool. 

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