[GitHub] keith-turner opened a new pull request #539: Fix WAL race condition between zookeeper and metadata table

[GitBox <gi...@apache.org>]

keith-turner opened a new pull request #539: Fix WAL race condition between zookeeper and metadata table
URL: https://github.com/apache/accumulo/pull/539
 
 
   I noticed this race condition while looking in to #535.  This seems like a very low probability event because multiple conditions would need to be met in a small time window for data loss to occur as a result of this race.
   
    * Only tablet A references WAL_X in its otherLogs set
    * Thread T1 one clears otherLogs for tablet A 
    * Thread T2 calls markUnusedWALs() removing WAL_X from zookeeper
    * Tablet server dies (tablet data that was in WAL_X is not in metadata table or recovery data)
    * Thread T1 would have updated the metadata table with a new file if tserver had not died
   
   This change acquires a lock so that the call to markUnusedWALs() will block until the file is added to metadata table.  This lock prevents zookeeper updates from happening before metadata table updates.
   
   This bug only affects Accumulo 1.8.0 and later.

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