Re: [PR] HBASE-29433: Resolve exception in AsyncTable for coprocessor services when a region merges/splits [hbase]

["krconv (via GitHub)" <gi...@apache.org>]

krconv commented on PR #7131:
URL: https://github.com/apache/hbase/pull/7131#issuecomment-3144330202

   > I think there are scenarios where we do not know how to retry? For example, we have finished a request to region, and when sending request to the next region, these two regions are merged. Then what is the correct way to do retrying here? Sending request again to the merged region? Or just jump to the next region?
   
   Sorry for the delay. In that case, this new logic would fail the request because ya, there is no way to return a response that doesn't double-count rows; in the code I am calling this an "overlappingAssignments"
    1. Run on Parent Region A, succeeds
    2. Region A and B merge to C
    3. Run on Region B (which is what was in our client's meta cache), fails due to RegionNoLongerExistsException
    4. Relocate the rows that Region B covered, find out that Region C now exists
    5. Attempt to "claim assignment" of the new range for Region C, find out that it overlaps with a region already processed, fail
    
    This is a less-likely race condition in this already unlikely scenario. The more common case I'd expect is for A and B to be merged sometime before the coprocessor service is called. In that scenario:
    1. <start with a warmed meta cache>
    2. Region A and B merge to C
    3. Run on Region A, fails due to RegionNoLongerExistsException 
    4. Relocate the regions that Region A covered, find out that Region C now exists
    5. Attempt to "claim assignment", succeeds because it is a range not already processed
   
   In this situation we can recover the request and maintain correctness; without this PR, `AsyncTable` fails in this scenario and `HTable` returns incorrect results (it would run the coprocessor service twice on Region C, and in cases that we are counting rows and returning the results, would dual-count those rows)


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