sync: Specialize functions that fetch type root

sync/syncable/entry.h

 // Copyright 2012 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 SYNC_SYNCABLE_ENTRY_H_
 #define SYNC_SYNCABLE_ENTRY_H_
 
 #include "sync/base/sync_export.h"
 #include "sync/syncable/entry_kernel.h"
 
@@ skipping 17 matching lines @@
 // constructor.
 enum GetById {
   GET_BY_ID
 };
 
 enum GetByClientTag {
   GET_BY_CLIENT_TAG
 };
 
 enum GetByServerTag {
-  GET_BY_SERVER_TAG
+  // Server tagged items are deprecated for all types but bookmarks.
+  GET_BY_SERVER_TAG_DEPRECATED

[tim (not reviewing), 2014/06/03 15:21:22]

The decision to deprecate this seems separate from wanting to specialize the
root fetching functions, which I agree makes sense.  Removing GetByServerTag
from the sync API ReadNode seems more than sufficient to prevent accidental use
by API clients, which is also a goal I agree makes sense.  I don't see as strong
of a case to rename this field to 'DEPRECATED' or even remove it from Entry, in
particular since we have no plan to remove it (at least for bookmarks) yet.  I
think it should just stay as-is at the lower levels here.

[rlarocque, 2014/06/03 17:29:40]

I added a suffix because I wanted the compiler to warn me if I missed a spot. 
_FOR_BOOKMARKS didn't seem quite right, since this layer is attempting to be
type-agnostic.

I suppose now that I've done the update, there's no harm in changing it back. 
That's uploaded in the latest patch.

As a separate note, though, are you sure server tags aren't deprecated?  I
thought there was a reason we didn't use them for experiment nodes.  If they
couldn't fit that use case, then I don't know what purpose they serve.

+};
+
+enum GetTypeRoot {
+  GET_TYPE_ROOT
 };
 
 enum GetByHandle {
   GET_BY_HANDLE
 };
 
 class SYNC_EXPORT Entry {
  public:
   // After constructing, you must check good() to test whether the Get
   // succeeded.
   Entry(BaseTransaction* trans, GetByHandle, int64 handle);
   Entry(BaseTransaction* trans, GetById, const Id& id);
+  Entry(BaseTransaction* trans, GetTypeRoot, ModelType type);
+  Entry(BaseTransaction* trans, GetByClientTag, const std::string& tag);
+
+  // This lookup function is deprecated.  All types except bookmarks can use
+  // the GetTypeRoot variant instead.
   Entry(BaseTransaction* trans, GetByServerTag, const std::string& tag);
-  Entry(BaseTransaction* trans, GetByClientTag, const std::string& tag);
 
   bool good() const { return 0 != kernel_; }
 
   BaseTransaction* trans() const { return basetrans_; }
 
   // Field accessors.
   int64 GetMetahandle() const {
     DCHECK(kernel_);
     return kernel_->ref(META_HANDLE);
   }
@@ skipping 202 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(Entry);
 };
 
 std::ostream& operator<<(std::ostream& os, const Entry& entry);
 
 }  // namespace syncable
 }  // namespace syncer
 
 #endif  // SYNC_SYNCABLE_ENTRY_H_