[GitHub] keith-turner opened a new pull request #866: Fix race condition when getting WALs for dead tserver

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

keith-turner opened a new pull request #866: Fix race condition when getting WALs for dead tserver
URL: https://github.com/apache/accumulo/pull/866
 
 
   When a tablet server dies the master gets its WALs from ZK.  In ZK there
   is a list of WALs per tserver. Each WAL in ZK has state that is either
   OPEN, CLOSED, or UNREFERENCED.  The master needs a list of OPEN and
   CLOSED logs for dead tservers.  While the master is trying to obtain
   this list its possible that the Accumulo GC may delete an UNREFERENCED
   WAL.  If this happened then the code before this commit would return an
   empty list of WALs. This could result in OPEN and CLOSED logs being
   ignored for recovery which could result in data loss.  This patch fixes
   the race condition.
   
   This bug was observed while looking into an exception I noticed while
   writing test for #860.  In the IT I was frequently calling another
   WalStateManager function and noticed NoNodeExceptions.  As a result I
   examined how the entire class handled NoNode race conditions and found
   this bug.  I noticed some other possible NoNode race condition, but do
   not think this these would occur with the current way the code is called.

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