Concurrent invocations of MetricsTableAggregateSourceImpl.getOrCreateTableSource may return unregistered MetricsTableSource

2018-09-01 21:09:38,174 WARN  [member: 'hw13463.attlocal.net,49623,1535861370108' subprocedure-pool6-thread-1] snapshot.                                                          RegionServerSnapshotManager$SnapshotSubprocedurePool(348): Got Exception in SnapshotSubprocedurePool
java.util.concurrent.ExecutionException: java.lang.NullPointerException
  at java.util.concurrent.FutureTask.report(FutureTask.java:122)
  at java.util.concurrent.FutureTask.get(FutureTask.java:192)
  at org.apache.hadoop.hbase.regionserver.snapshot.RegionServerSnapshotManager$SnapshotSubprocedurePool.waitForOutstandingTasks(RegionServerSnapshotManager.java:324)
  at org.apache.hadoop.hbase.regionserver.snapshot.FlushSnapshotSubprocedure.flushSnapshot(FlushSnapshotSubprocedure.java:173)
  at org.apache.hadoop.hbase.regionserver.snapshot.FlushSnapshotSubprocedure.insideBarrier(FlushSnapshotSubprocedure.java:193)
  at org.apache.hadoop.hbase.procedure.Subprocedure.call(Subprocedure.java:189)
  at org.apache.hadoop.hbase.procedure.Subprocedure.call(Subprocedure.java:53)
  at java.util.concurrent.FutureTask.run(FutureTask.java:266)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
  at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
  at org.apache.hadoop.hbase.regionserver.MetricsTableSourceImpl.updateFlushTime(MetricsTableSourceImpl.java:375)
  at org.apache.hadoop.hbase.regionserver.MetricsTable.updateFlushTime(MetricsTable.java:56)
  at org.apache.hadoop.hbase.regionserver.MetricsRegionServer.updateFlush(MetricsRegionServer.java:210)
  at org.apache.hadoop.hbase.regionserver.HRegion.internalFlushCacheAndCommit(HRegion.java:2826)
  at org.apache.hadoop.hbase.regionserver.HRegion.internalFlushcache(HRegion.java:2444)
  at org.apache.hadoop.hbase.regionserver.HRegion.internalFlushcache(HRegion.java:2416)
  at org.apache.hadoop.hbase.regionserver.HRegion.flushcache(HRegion.java:2306)
  at org.apache.hadoop.hbase.regionserver.HRegion.flush(HRegion.java:2209)
  at org.apache.hadoop.hbase.regionserver.snapshot.FlushSnapshotSubprocedure$RegionSnapshotTask.call(FlushSnapshotSubprocedure.java:115)
  at org.apache.hadoop.hbase.regionserver.snapshot.FlushSnapshotSubprocedure$RegionSnapshotTask.call(FlushSnapshotSubprocedure.java:77)

    MetricsTableSource prev = tableSources.putIfAbsent(table, source);

    if (prev != null) {
      return prev;
    } else {
      // register the new metrics now
      register(source);