merge closeChildUpdate with flushCurrentNode

Without the `sync` logic (now removed) the code is simplified for these tightly
coupled methods (the first one was the only caller of the second) to be merged
in a single method, correctly redistributing the use of the lock.

dir.go

@@ -104,10 +104,10 @@ func (d *Directory) updateChildEntry(c child) error {
 
 // This method implements the part of `updateChildEntry` that needs
 // to be locked around: in charge of updating the UnixFS layer and
-// generating the new node reflecting the update.
+// generating the new node reflecting the update. It also stores the
+// new node in the DAG layer.
 func (d *Directory) closeChildUpdate(c child) (*dag.ProtoNode, error) {
 	d.lock.Lock()
-	defer d.lock.Unlock()
 
 	err := d.updateChild(c)
 	if err != nil {
@@ -116,31 +116,24 @@ func (d *Directory) closeChildUpdate(c child) (*dag.ProtoNode, error) {
 	// TODO: Clearly define how are we propagating changes to lower layers
 	// like UnixFS.
 
-	return d.flushCurrentNode()
-}
-
-// Recreate the underlying UnixFS directory node and save it in the DAG layer.
-func (d *Directory) flushCurrentNode() (*dag.ProtoNode, error) {
 	nd, err := d.unixfsDir.GetNode()
 	if err != nil {
 		return nil, err
 	}
 
-	err = d.dagService.Add(d.ctx, nd)
-	if err != nil {
-		return nil, err
-	}
-	// TODO: This method is called in `closeChildUpdate` while the lock is
-	// taken, we need the lock while operating on `unixfsDir` to create the
-	// new node but do we also need to keep the lock while adding it to the
-	// DAG service? Evaluate refactoring these two methods together and better
-	// redistributing the node.
+	d.lock.Unlock()
+	// TODO: This part of the method locked around could be now extracted
+	// separately.
 
 	pbnd, ok := nd.(*dag.ProtoNode)
 	if !ok {
 		return nil, dag.ErrNotProtobuf
 	}
-	// TODO: Why do we check the node *after* adding it to the DAG service?
+
+	err = d.dagService.Add(d.ctx, nd)
+	if err != nil {
+		return nil, err
+	}
 
 	return pbnd.Copy().(*dag.ProtoNode), nil
 	// TODO: Why do we need a copy?