localUpdate: move the DAG service Àdd call outside the lock

Since it's accessing a copy and not the original directory node.

dir.go

@@ -98,7 +98,6 @@ func (d *Directory) updateChildEntry(c child) error {
 // new node in the DAG layer.
 func (d *Directory) localUpdate(c child) (*dag.ProtoNode, error) {
 	d.lock.Lock()
-	defer d.lock.Unlock()
 
 	err := d.updateChild(c)
 	if err != nil {
@@ -117,13 +116,21 @@ func (d *Directory) localUpdate(c child) (*dag.ProtoNode, error) {
 		return nil, dag.ErrNotProtobuf
 	}
 
-	err = d.dagService.Add(d.ctx, nd)
+	nodeCopy := pbnd.Copy().(*dag.ProtoNode)
+	// We copy the underlying node to add it to the DAG service and
+	// propagate the update upwards (outside of the lock) because
+	// other callers might operate on this directory node later
+	// while *this* update is still in progress.
+
+	d.lock.Unlock()
+	// NOTE: The original `nd` can't be accessed below this point.
+
+	err = d.dagService.Add(d.ctx, nodeCopy)
 	if err != nil {
 		return nil, err
 	}
 
-	return pbnd.Copy().(*dag.ProtoNode), nil
-	// TODO: Why do we need a copy?
+	return nodeCopy, nil
 }
 
 // Update child entry in the underlying UnixFS directory.