[jbosscache-dev] Fixing MVCC(RC) + UnversionedNode for PCACHE-82

Hi all, Re: https://jira.jboss.org/jira/browse/PCACHE-82 Even though this is not urgent, it's a good exercise for me to get a better understanding of MVCC and hence why I decided to finally tackle it. I've been trying to figure out what exactly is the bug in this JIRA. I've created a test case that reproduces the bug (attached). The issue is clearly related to MVCC(ReadCommitted) and UnversionedNode instances but I'm failing to understand what the fix exactly is. Let me sum up what happens internally (note that a lot of the information below is from logging statements I added): 1. Create a Person, Galder, and attach it under /person/testDoubleAttachAndDetach 2. Create a Person, Asier, and attach it in the same fqn. 3. In the 2nd attachment, ReadCommittedNode.markForUpdate() backs up the NodeReference for /person/testDoubleAttachAndDetach creates a copy of the UnversionedNode representing underneath. 4. More importantly, because calling attach the 2nd time means detach needs to happen before to remove the old value, so MVCCNodeHelper.wrapNodesRecursivelyForRemoval() executes the addChild() below: // update child ref on parent to point to child as this is now a copy. if (parentLockNeeded && (needToCopyNode || needToCopyParent)) { if (parent == null) throw new NodeNotExistsException("Parent node " + parentFqn + " does not exist!"); parent.getDelegationTarget().addChild(node.getDelegationTarget()); } This leads to adding the UnversionedNode version of testDoubleAttachAndDetach to /person node's children. 5. At the end 2nd attachment, ReadCommittedNode.updateNode() method takes the backup (NodeReference) and the node (UnversionedNode) and swaps them again but the parent's children, /person, is still pointing to an UnversionedNode which leads to the ClassCastException that you see when you execute the test. I'm not sure what really should be happening here since I don't understand the rationaly of that addChild() call. Should it be happening at all in this case? Or maybe a corrective action of some sort needs to be done somewhere? Thanks for the help in advance! -- Galder Zamarreño Sr. Software Engineer Infinispan, JBoss Cache