[Exist-development] FIXED XQuery Update deadlock issue

[Patrick B. <pat...@jo...>]

Hi everyone,

In my stress testing I've found a deadlock issue that in the XQuery Update
code, it's in Modification.selectAndLock(..) on line 147. The problem was
that a READ_LOCK was being acquired on the BrokerPool inside of the
try/finally that removed it. So, in the case the acquire failed and threw
an error, the release would pop off the wrong READ_LOCK.

If one of the core developers is good with my change, I'll commit it into
the stable/1.4.x branch

Was:

protected StoredNode[] selectAndLock(Txn transaction, Sequence nodes)
throws LockException, PermissionDeniedException,
    XPathException {
    Lock globalLock =
context.getBroker().getBrokerPool().getGlobalUpdateLock();
    try {
        globalLock.acquire(Lock.READ_LOCK);

        lockedDocuments = nodes.getDocumentSet();

        // acquire a lock on all documents
        // we have to avoid that node positions change
        // during the modification
        lockedDocuments.lock(context.getBroker(), true, false);

        StoredNode ql[] = new StoredNode[nodes.getItemCount()];
        for (int i = 0; i < ql.length; i++) {
            Item item = nodes.itemAt(i);
            if (!Type.subTypeOf(item.getType(), Type.NODE))
                throw new XPathException(this, "XQuery update expressions
can only be applied to nodes. Got: " +
                    item.getStringValue());
            NodeValue nv = (NodeValue)item;
            if (nv.getImplementationType() == NodeValue.IN_MEMORY_NODE)
                throw new XPathException(this, "XQuery update expressions
can not be applied to in-memory nodes.");
            Node n = nv.getNode();
            if (n == null)
                throw new XPathException(this, "Internal error: node to
update is null");
            if (n.getNodeType() == Node.DOCUMENT_NODE)
                throw new XPathException(this, "Updating the document
object is not allowed.");
            ql[i] = (StoredNode) n;
            DocumentImpl doc = (DocumentImpl)ql[i].getOwnerDocument();
            //prepare Trigger
            try {
                prepareTrigger(transaction, doc);
            } catch (TriggerException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        return ql;
    } finally {
        globalLock.release(Lock.READ_LOCK);
    }
}



Changed to: (moved lock out of try)

protected StoredNode[] selectAndLock(Txn transaction, Sequence nodes)
throws LockException, PermissionDeniedException,
    XPathException {
    Lock globalLock =
context.getBroker().getBrokerPool().getGlobalUpdateLock();
    globalLock.acquire(Lock.READ_LOCK);
    try {

        lockedDocuments = nodes.getDocumentSet();

        // acquire a lock on all documents
        // we have to avoid that node positions change
        // during the modification
        lockedDocuments.lock(context.getBroker(), true, false);

        StoredNode ql[] = new StoredNode[nodes.getItemCount()];
        for (int i = 0; i < ql.length; i++) {
            Item item = nodes.itemAt(i);
            if (!Type.subTypeOf(item.getType(), Type.NODE))
                throw new XPathException(this, "XQuery update expressions
can only be applied to nodes. Got: " +
                    item.getStringValue());
            NodeValue nv = (NodeValue)item;
            if (nv.getImplementationType() == NodeValue.IN_MEMORY_NODE)
                throw new XPathException(this, "XQuery update expressions
can not be applied to in-memory nodes.");
            Node n = nv.getNode();
            if (n == null)
                throw new XPathException(this, "Internal error: node to
update is null");
            if (n.getNodeType() == Node.DOCUMENT_NODE)
                throw new XPathException(this, "Updating the document
object is not allowed.");
            ql[i] = (StoredNode) n;
            DocumentImpl doc = (DocumentImpl)ql[i].getOwnerDocument();
            //prepare Trigger
            try {
                prepareTrigger(transaction, doc);
            } catch (TriggerException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        return ql;
    } finally {
        globalLock.release(Lock.READ_LOCK);
    }
}

-- 
Patrick Bosek
Jorsek Software
Cell (585) 820 9634
Office (877) 492 2960
Jorsek.com