sync: make scheduling logic and job ownership more obvious.

sync/internal_api/public/util/syncer_error.h

 // 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.
 
 #ifndef SYNC_INTERNAL_API_PUBLIC_UTIL_SYNCER_ERROR_H_
 #define SYNC_INTERNAL_API_PUBLIC_UTIL_SYNCER_ERROR_H_
 
 namespace syncer {
 
 // This enum describes all the ways a SyncerCommand can fail.
 //
 // SyncerCommands do many different things, but they share a common function
 // signature.  This enum, the return value for all SyncerCommands, must be able
 // to describe any possible failure for all SyncerComand.
 //
 // For convenience, functions which are invoked only by SyncerCommands may also
 // return a SyncerError.  It saves us having to write a conversion function, and
 // it makes refactoring easier.
 enum SyncerError {
   UNSET = 0,       // Default value.
   DIRECTORY_LOOKUP_FAILED, // Local directory lookup failure.
 
   NETWORK_CONNECTION_UNAVAILABLE,  // Connectivity failure.
   NETWORK_IO_ERROR,                // Response buffer read error.
   SYNC_SERVER_ERROR,               // Non auth HTTP error.
   SYNC_AUTH_ERROR,                 // HTTP auth error.
 
   // Based on values returned by server.  Most are defined in sync.proto.
   SERVER_RETURN_INVALID_CREDENTIAL,
+  FIRST_SERVER_RETURN_VALUE = SERVER_RETURN_INVALID_CREDENTIAL,
+
   SERVER_RETURN_UNKNOWN_ERROR,
   SERVER_RETURN_THROTTLED,
   SERVER_RETURN_TRANSIENT_ERROR,
   SERVER_RETURN_MIGRATION_DONE,
   SERVER_RETURN_CLEAR_PENDING,
   SERVER_RETURN_NOT_MY_BIRTHDAY,
   SERVER_RESPONSE_VALIDATION_FAILED,
 
   SYNCER_OK
 };
 
 const char * GetSyncerErrorString(SyncerError);
 
 // Helper to check that |error| is set to something (not UNSET) and is not
 // SYNCER_OK.
 bool SyncerErrorIsError(SyncerError error);
 
 }  // namespace syncer
 
 #endif  // SYNC_INTERNAL_API_PUBLIC_UTIL_SYNCER_ERROR_H_