CAY-2876 Memory leak in the ObjectStore

[stariy95]

Another approach to the CAY-2876

This PR introduces cleanup callback in the ReferenceMap keeping flattened paths as is in the ObjectStore and dropping them in sync with the main object map. This is a less reliable fix then before, but it guarantees that there will be no problems with the object store data consistency.

In 5.0 we could experiment a bit more and try something more robust.

[andrus]

+1 . Not pretty, but should work.

[Jugen]

+1. I was experimenting with changing trackedFlattenedPaths to a WeakHashMap but couldn't successfully write a test case for it, however I prefer this direct approach much more.

[Jugen]

I was wondering if trackedFlattenedPaths didn't need housekeeping in these two places as well ?

cayenne/cayenne-server/src/main/java/org/apache/cayenne/access/ObjectStore.java

Lines 421 to 429 in e1d6db2

	public void postprocessAfterCommit(GraphDiff parentChanges) {
	// scan through changed objects, set persistence state to committed
	for (Object id : changes.keySet()) {
	Persistent object = objectMap.get(id);
	switch (object.getPersistenceState()) {
	case PersistenceState.DELETED:
	objectMap.remove(id);

cayenne/cayenne-server/src/main/java/org/apache/cayenne/access/ObjectStore.java

Lines 630 to 652 in e1d6db2

	void processDeletedID(ObjectId nodeId) {
	// access object map directly - the method should be called in a synchronized context...
	Persistent object = objectMap.get(nodeId);
	if (object != null) {
	DataObject dataObject = (object instanceof DataObject)
	? (DataObject) object
	: null;
	DataContextDelegate delegate;
	switch (object.getPersistenceState()) {
	case PersistenceState.COMMITTED:
	case PersistenceState.HOLLOW:
	case PersistenceState.DELETED:
	// consult delegate
	delegate = context.nonNullDelegate();
	if (dataObject == null || delegate.shouldProcessDelete(dataObject)) {
	objectMap.remove(nodeId);

[stariy95]

@Jugen Yes, you are correct, we should clean tracked paths in those places too.