[Sync] Add HasChildren() function and use it instead of GetFirstChildId()

chrome/browser/sync/syncable/syncable.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_SYNC_SYNCABLE_SYNCABLE_H_
 #define CHROME_BROWSER_SYNC_SYNCABLE_SYNCABLE_H_
 #pragma once
 
 #include <algorithm>
 #include <bitset>
@@ skipping 902 matching lines @@
   // Returns the child meta handles for given parent id.  Clears
   // |result| if there are no children.
   void GetChildHandlesById(BaseTransaction*, const Id& parent_id,
       ChildHandles* result);
 
   // Returns the child meta handles for given meta handle.  Clears
   // |result| if there are no children.
   void GetChildHandlesByHandle(BaseTransaction*, int64 handle,
       ChildHandles* result);
 
+  // Returns true iff |id| has children.

[rlarocque, 2011/10/26 22:43:18]

Should we mention that self-looped children are not counted as children?  

That's an honest question.  I have no idea what it means for a child to be
self-looped, or what a caller of this function might expect if there are only
self-looped children.

[akalin, 2011/10/27 00:19:21]

I don't think so, since deleted == self-looped.

However, the two functions above, which do return self-looped nodes, should be
documented as such.  Done.

+  bool HasChildren(BaseTransaction* trans, const Id& id);
+
   // Find the first or last child in the positional ordering under a parent,
   // and return its id.  Returns a root Id if parent has no children.
-  virtual Id GetFirstChildId(BaseTransaction* trans, const Id& parent_id);
+  Id GetFirstChildId(BaseTransaction* trans, const Id& parent_id);
   Id GetLastChildId(BaseTransaction* trans, const Id& parent_id);
 
   // Compute a local predecessor position for |update_item|.  The position
   // is determined by the SERVER_POSITION_IN_PARENT value of |update_item|,
   // as well as the SERVER_POSITION_IN_PARENT values of any up-to-date
   // children of |parent_id|.
   Id ComputePrevIdFromServerPosition(
       const EntryKernel* update_item,
       const syncable::Id& parent_id);
 
@@ skipping 206 matching lines @@
   // children of |parent_id| in the kernel's parent_id_child_index.
   ParentIdChildIndex::iterator GetParentChildIndexUpperBound(
       const ScopedKernelLock& lock,
       const Id& parent_id);
 
   // Append the handles of the children of |parent_id| to |result|.
   void AppendChildHandles(
       const ScopedKernelLock& lock,
       const Id& parent_id, Directory::ChildHandles* result);
 
+  // Returns a pointer to what is probably (but not certainly) the
+  // first child of |parent_id|, or NULL if |parent_id| definitely has
+  // no children.
+  EntryKernel* GetPossibleFirstChild(
+      const ScopedKernelLock& lock, const Id& parent_id);
+
   Kernel* kernel_;
 
   DirectoryBackingStore* store_;
 };
 
 class ScopedKernelLock {
  public:
   explicit ScopedKernelLock(const Directory*);
   ~ScopedKernelLock() {}
 
@@ skipping 85 matching lines @@
 bool IsLegalNewParent(BaseTransaction* trans, const Id& id, const Id& parentid);
 
 // This function sets only the flags needed to get this entry to sync.
 void MarkForSyncing(syncable::MutableEntry* e);
 
 }  // namespace syncable
 
 std::ostream& operator <<(std::ostream&, const syncable::Blob&);
 
 #endif  // CHROME_BROWSER_SYNC_SYNCABLE_SYNCABLE_H_