Add an action for server to send down rollback command.

sync/protocol/sync_enums.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 95 matching lines @@
 
   enum Action {
     UPGRADE_CLIENT              = 0; // Upgrade the client to latest version.
     CLEAR_USER_DATA_AND_RESYNC  = 1; // Clear user data from dashboard and
                                      // setup sync again.
     ENABLE_SYNC_ON_ACCOUNT      = 2; // The administrator needs to enable sync
                                      // on the account.
     STOP_AND_RESTART_SYNC       = 3; // Stop sync and set up sync again.
     DISABLE_SYNC_ON_CLIENT      = 4; // Wipe the client of all sync data and
                                      // stop syncing.
-    UNKNOWN_ACTION              = 5; // This is the default.
+    DISABLE_SYNC_AND_ROLLBACK   = 5; // Wipe out sync data, stop syncing and
+                                     // roll back local data to pre-sync state.
+    UNKNOWN_ACTION              = 6; // This is the default.

[Nicolas Zea, 2014/04/15 23:08:31]

it's a bit concerning that we're modifying the value. I suppose that's okay if
we don't persist it anywhere.

[maniscalco, 2014/04/15 23:16:26]

I was wondering the same thing.  Is Action ever sent over the wire?

If old code (pre this CL) tries to decode a new Action field (post this CL),
won't it interpret the value as UPGRADE_CLIENT?  Check out this section, it says
the first value of the enum is the default:
https://developers.google.com/protocol-buffers/docs/proto#optional

[haitaol1, 2014/04/15 23:44:03]

I think it's by design that UNKNOWN_ACTION is put at the end so that new enums
won't break legacy client because they will be interpreted as unknown. See
ConvertClientActionPBToLocalClientAction()

On 2014/04/15 23:08:31, Nicolas Zea wrote:

[maniscalco, 2014/04/16 00:09:20]

To make things "obviously safe" I suggest leaving UNKNOWN_ACTION as 5, moving
UNKOWN_ACTION to the top of the enum (based on the doc I referenced above), and
adding the new DISABLE_SYNC_AND_ROLLBACK as 6.  This way, it's clear that
UNKNOWN_ACTION won't be misinterpreted as a DISABLE_SYNC_AND_ROLLBACK and it's
clear the default for an enum Action is UNKNOWN_ACTION.

On 2014/04/15 23:44:03, haitaol1 wrote:

   }
 
   enum DeviceType {
     TYPE_WIN = 1;
     TYPE_MAC = 2;
     TYPE_LINUX = 3;
     TYPE_CROS = 4;
     TYPE_OTHER = 5;
     TYPE_PHONE = 6;
     TYPE_TABLET = 7;
@@ skipping 24 matching lines @@
                            // confused with FIRST_UPDATE.
     RECONFIGURATION = 10;  // The client is in configuration mode because the
                            // user opted to sync a different set of datatypes.
     GU_TRIGGER = 12;  // The client is in 'normal' mode.  It may have several
                       // reasons for requesting an update.  See the per-type
                       // GetUpdateTriggers message for more details.
     RETRY = 13;  // A retry GU to pick up updates missed by last GU due to
                  // replication delay, missing hints, etc.
   }
 }