Mulgara

During backups the following exception may be thrown:
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Caused by: java.lang.AssertionError
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    at org.mulgara.store.xa.Block.dispose(Block.java:524)
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    at org.mulgara.store.xa.Block.release(Block.java:512)
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    at org.mulgara.store.xa.AVLNode.release(AVLNode.java:1030)
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    at org.mulgara.store.stringpool.xa.XAStringPoolImpl$Phase$GNodeTuplesImpl.close(XAStringPoolImpl.java:2728)
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    at org.mulgara.resolver.BackupOperation.backupDatabase(BackupOperation.java:196)
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    at org.mulgara.resolver.BackupOperation.execute(BackupOperation.java:145)
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    at org.mulgara.resolver.MulgaraTransaction.execute(MulgaraTransaction.java:547)
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    ... 15 more
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This is happening as an AVLNode is being released twice.  The stack trace here shows the second release.
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The AVLNode that is being closed here is a member of XAStringPoolImpl called avlNode.  But this class appears to manage the avlNode member correctly.  The intermittent nature of the problem is unlikely to come from this class anyway.
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An alternative source of the problem could be in the [[ObjectPool]].  The XAStringPoolImpl class uses a member called writerObjectPool for managing all reusable objects for both reading and writing.  It may be that a simultaneous read/write could lead the object pool to passing out the same AVLNode to different phases.  Access to writerObjectPool is properly synchronized, so this is not a problem.  However, it is usually valid (and desirable) to reuse the same AVLNode when reading and writing on the same phase, and this optimization may be leading to re-use between phases.  If true, then this would also explain the intermittent nature of the problem.