[Sync] Make SESSIONS an implicit type

sync/protocol/sync.proto

 // Copyright (c) 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.
 //
 // Sync protocol for communication between sync client and server.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 90 matching lines @@
   optional SessionSpecifics session = 50119;
   optional AutofillProfileSpecifics autofill_profile = 63951;
   optional SearchEngineSpecifics search_engine = 88610;
   optional ExtensionSettingSpecifics extension_setting = 96159;
   optional AppSettingSpecifics app_setting = 103656;
   optional HistoryDeleteDirectiveSpecifics history_delete_directive = 150251;
   optional SyncedNotificationSpecifics synced_notification = 153108;
   optional DeviceInfoSpecifics device_info = 154522;
   optional ExperimentsSpecifics experiments = 161496;
   optional PriorityPreferenceSpecifics priority_preference = 163425;
+  optional string tab = 174470;  // Placeholder for tabs, a virtual datatype.

[Raz Mathias, 2013/01/19 01:29:17]

i didn't realize that local/virtual types would have representation in
sync.proto.  i'm a little worried that server-side we key a lot of behavioral
mappings off the proto field number.  Of course, we could put in stubs,  and
asserts to prevent strange behavior but it really does feel like a very specific
client detail that will bleed into the server.  would it be possible to
implement local/virtual types client-side in a way that the server never needs
to see it?

 }
 
 message SyncEntity {
   // This item's identifier.  In a commit of a new item, this will be a
   // client-generated ID.  If the commit succeeds, the server will generate
   // a globally unique ID and return it to the committing client in the
   // CommitResponse.EntryResponse.  In the context of a GetUpdatesResponse,
   // |id_string| is always the server generated ID.  The original
   // client-generated ID is preserved in the |originator_client_id| field.
   // Present in both GetUpdatesResponse and CommitMessage.
@@ skipping 631 matching lines @@
     // is absent then the whole client (all datatypes) is throttled.
     repeated int32 error_data_type_ids = 5;
   }
   optional Error error = 13;
 
   // The new per-client state for this client. If set, should be persisted and
   // sent with any subsequent ClientToServerMessages.
   optional ChipBag new_bag_of_chips = 14;
 };