Implemented initial version of server-issued notification client.

chrome/browser/sync/notification_method.h

 // Copyright (c) 2010 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_NOTIFICATION_METHOD_H_
 #define CHROME_BROWSER_SYNC_NOTIFICATION_METHOD_H_
 
 #include <string>
 
 namespace browser_sync {
 
 // This is the matrix for the interaction between clients with
-// different notification methods:
+// different notification methods (except for NOTIFICATION_SERVER):
 //
 //          Listen
 //          L T N
 //        +-------+
 //      L | E E E |
 // Send T | Y Y Y |
 //      N | E Y Y |
 //        +-------+
 //
 // 'Y' means a client listening with the column notification method
 // will receive notifications from a client sending with the row
 // notification method. 'E' means means that the notification will be
 // an empty one, which may be dropped by the server in the future.
+//
+// As for NOTIFICATION_SERVER, server-issued notifications will also
+// simulate a peer-issued notification, so that any client with
+// NOTIFICATION_TRANSITIONAL or NOTIFICATION_NEW will be able to
+// receive those, too.  This support will be removed once everyone is
+// on NOTIFICATION_SERVER.
 
 enum NotificationMethod {
   // Old, broken notification method.  Works only if notification
   // servers don't drop empty notifications.
   NOTIFICATION_LEGACY,
   // Compatible with new notifications.  Also compatible with legacy
   // notifications if the notification servers don't drop empty
   // notifications.
   NOTIFICATION_TRANSITIONAL,
-  // New, ideal notification method.  Compatible only with
-  // transitional notifications.
+  // New notification method.  Compatible only with transitional
+  // notifications.
+  //
+  // NOTE: "New" is kind of a misnomer, as it refers only to
+  // peer-issued notifications; the plan is to migrate everyone to
+  // using NOTIFICATION_SERVER.
   NOTIFICATION_NEW,
+
+  // Server-issued notifications.  Compatible only with transitional
+  // notifications.
+  NOTIFICATION_SERVER,
 };
 
 extern const NotificationMethod kDefaultNotificationMethod;
 
 std::string NotificationMethodToString(
     NotificationMethod notification_method);
 
-// If the given string is not one of "legacy", "transitional", or
-// "new", returns kDefaultNotificationMethod.
+// If the given string is not one of "legacy", "transitional", "new",
+// or "server", returns kDefaultNotificationMethod.
 NotificationMethod StringToNotificationMethod(const std::string& str);
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_NOTIFICATION_METHOD_H_