Introduce the plumbing necessary to report Unrecoverable error from model safe workers.

chrome/browser/sync/engine/model_changing_syncer_command.h

 // Copyright (c) 2006-2009 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_ENGINE_MODEL_CHANGING_SYNCER_COMMAND_H_
 #define CHROME_BROWSER_SYNC_ENGINE_MODEL_CHANGING_SYNCER_COMMAND_H_
 #pragma once
 
 #include "chrome/browser/sync/engine/syncer_command.h"
+#include "chrome/browser/sync/util/unrecoverable_error_info.h"
 
 namespace browser_sync {
 namespace sessions {
 class SyncSession;
 }
 
 // An abstract SyncerCommand which dispatches its Execute step to the
 // model-safe worker thread.  Classes derived from ModelChangingSyncerCommand
 // instead of SyncerCommand must implement ModelChangingExecuteImpl instead of
 // ExecuteImpl, but otherwise, the contract is the same.
 //
 // A command should derive from ModelChangingSyncerCommand instead of
 // SyncerCommand whenever the operation might change any client-visible
 // fields on any syncable::Entry.  If the operation involves creating a
 // WriteTransaction, this is a sign that ModelChangingSyncerCommand is likely
 // necessary.
 class ModelChangingSyncerCommand : public SyncerCommand {
  public:
   ModelChangingSyncerCommand() : work_session_(NULL) { }
   virtual ~ModelChangingSyncerCommand() { }
 
   // SyncerCommand implementation. Sets work_session to session.
   virtual void ExecuteImpl(sessions::SyncSession* session);
 
   // wrapper so implementations don't worry about storing work_session
-  void StartChangingModel() {
+  UnrecoverableErrorInfo StartChangingModel() {
+    // TODO(lipalani): |ModelChangingExecuteImpl| should return an
+    // UnrecoverableErrorInfo struct.
     ModelChangingExecuteImpl(work_session_);
+    return UnrecoverableErrorInfo();
   }
 
   // Sometimes, a command has work to do that needs to touch global state
   // belonging to multiple ModelSafeGroups, but in a way that is known to be
   // safe.  This will be called once, prior to ModelChangingExecuteImpl,
   // *without* a ModelSafeGroup restriction in place on the SyncSession.
   // Returns true on success, false on failure.
   // TODO(tim): Remove this (bug 36594).
   virtual bool ModelNeutralExecuteImpl(sessions::SyncSession* session);
 
   // Abstract method to be implemented by subclasses to handle logic that
   // operates on the model.  This is invoked with a SyncSession ModelSafeGroup
   // restriction in place so that bits of state belonging to data types
   // running on an unsafe thread are siloed away.
   virtual void ModelChangingExecuteImpl(sessions::SyncSession* session) = 0;
 
  private:
   // ExecuteImpl is expected to be run by SyncerCommand to set work_session.
   // StartChangingModel is called to start this command running.
   // Implementations will implement ModelChangingExecuteImpl and not
   // worry about storing the session or setting it. They are given work_session.
   sessions::SyncSession* work_session_;
 
   DISALLOW_COPY_AND_ASSIGN(ModelChangingSyncerCommand);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_ENGINE_MODEL_CHANGING_SYNCER_COMMAND_H_