Remove dependency on server generated type root folders

sync/engine/syncer_util.cc

Index: sync/engine/syncer_util.cc
diff --git a/sync/engine/syncer_util.cc b/sync/engine/syncer_util.cc
index 7f63b37ddbdeeeb0a654b54a12ef6a597b67e34a..d97f34cc17039bcef680b6606ed185f905d95220 100644
--- a/sync/engine/syncer_util.cc
+++ b/sync/engine/syncer_util.cc
@@ -184,7 +184,23 @@ syncable::Id FindLocalIdToUpdate(
 
       return local_entry.GetId();
     }
+  } else if (update.has_server_defined_unique_tag() &&
+             !update.server_defined_unique_tag().empty()) {
+    // The client creates type root folders on demand with a local ID on
+    // demand when a progress marker for the given type is initially set.
+    // The server might also attempt to send a type root folder for the same
+    // type. It will does that during the transition period until support for
+    // root folders is removed on the server for most types. After that the
+    // server will still be creating type root folders for BOOKMARKS and
+    // NIGORI types.
+    syncable::Entry local_entry(trans, syncable::GET_BY_SERVER_TAG,
+                                update.server_defined_unique_tag());
+    if (local_entry.good() && !local_entry.GetId().ServerKnows()) {
+      DCHECK(local_entry.GetId() != update_id);
+      return local_entry.GetId();
+    }
   }
+
   // Fallback: target an entry having the server ID, creating one if needed.
   return update_id;
 }
@@ -227,16 +243,24 @@ UpdateAttemptResponse AttemptToUpdateEntry(
 
   if (!entry->GetServerIsDel()) {
     syncable::Id new_parent = entry->GetServerParentId();
-    Entry parent(trans, GET_BY_ID,  new_parent);
-    // A note on non-directory parents:
-    // We catch most unfixable tree invariant errors at update receipt time,
-    // however we deal with this case here because we may receive the child
-    // first then the illegal parent. Instead of dealing with it twice in
-    // different ways we deal with it once here to reduce the amount of code and
-    // potential errors.
-    if (!parent.good() || parent.GetIsDel() || !parent.GetIsDir()) {
-      DVLOG(1) <<  "Entry has bad parent, returning conflict_hierarchy.";
-      return CONFLICT_HIERARCHY;
+    if (!new_parent.IsNull()) {
+      // Perform this step only if the parent is specified.
+      // The unset parent means that the implicit type root would be used.
+      Entry parent(trans, GET_BY_ID, new_parent);
+      // A note on non-directory parents:
+      // We catch most unfixable tree invariant errors at update receipt time,
+      // however we deal with this case here because we may receive the child
+      // first then the illegal parent. Instead of dealing with it twice in
+      // different ways we deal with it once here to reduce the amount of code
+      // and
+      // potential errors.
+      if (!parent.good() || parent.GetIsDel() || !parent.GetIsDir()) {
+        DVLOG(1) << "Entry has bad parent, returning conflict_hierarchy.";
+        return CONFLICT_HIERARCHY;
+      }
+    } else {
+      DCHECK(
+          !IsTypeWithServerGeneratedRoot(GetModelTypeFromSpecifics(specifics)));
     }
     if (entry->GetParentId() != new_parent) {
       if (!entry->GetIsDel() && !IsLegalNewParent(trans, id, new_parent)) {