Sync: Handle empty (implicit) ParentId in GetPredecessorId and GetSuccessorId

sync/syncable/parent_child_index.cc

Index: sync/syncable/parent_child_index.cc
diff --git a/sync/syncable/parent_child_index.cc b/sync/syncable/parent_child_index.cc
index f45bcfe1bdde313a730ba2784f9e4f642bb408e5..a144e6c65503f80ab6e45f7d8a5373f305a55533 100644
--- a/sync/syncable/parent_child_index.cc
+++ b/sync/syncable/parent_child_index.cc
@@ -120,8 +120,10 @@ bool ParentChildIndex::Contains(EntryKernel *e) const {
   return children->count(e) > 0;
 }
 
-const OrderedChildSet* ParentChildIndex::GetChildren(const syncable::Id& id) {
-  ParentChildrenMap::iterator parent = parent_children_map_.find(id);
+const OrderedChildSet* ParentChildIndex::GetChildren(const Id& id) const {
+  DCHECK(!id.IsNull());
+
+  ParentChildrenMap::const_iterator parent = parent_children_map_.find(id);
   if (parent == parent_children_map_.end()) {
     return NULL;
   }
@@ -131,6 +133,17 @@ const OrderedChildSet* ParentChildIndex::GetChildren(const syncable::Id& id) {
   return parent->second;
 }
 
+const OrderedChildSet* ParentChildIndex::GetChildren(EntryKernel* e) const {
+  return GetChildren(e->ref(ID));
+}
+
+const OrderedChildSet* ParentChildIndex::GetSiblings(EntryKernel* e) const {
+  DCHECK(Contains(e));
+  const OrderedChildSet* siblings = GetChildren(GetParentId(e));
+  DCHECK(siblings && !siblings->empty());
+  return siblings;
+}
+
 const Id& ParentChildIndex::GetParentId(EntryKernel* e) const {
   const Id& parent_id = e->ref(PARENT_ID);
   if (!parent_id.IsNull()) {