sync: Specialize functions that fetch type root

sync/syncable/entry.h

Index: sync/syncable/entry.h
diff --git a/sync/syncable/entry.h b/sync/syncable/entry.h
index 47b9c593301d0d96f88aefef8f4f9f7d5d80a6b1..8ac2735fe46513ad71ab3004de87fd0f9513cf00 100644
--- a/sync/syncable/entry.h
+++ b/sync/syncable/entry.h
@@ -35,7 +35,12 @@ enum GetByClientTag {
 };
 
 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 {
@@ -48,9 +53,13 @@ class SYNC_EXPORT Entry {
   // succeeded.
   Entry(BaseTransaction* trans, GetByHandle, int64 handle);
   Entry(BaseTransaction* trans, GetById, const Id& id);
-  Entry(BaseTransaction* trans, GetByServerTag, const std::string& tag);
+  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);
+
   bool good() const { return 0 != kernel_; }
 
   BaseTransaction* trans() const { return basetrans_; }