[Sync] Add support for dynamically enabling/disabling types

sync/api/sync_error.h

 // Copyright 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_API_SYNC_ERROR_H_
 #define SYNC_API_SYNC_ERROR_H_
 
 #include <iosfwd>
 #include <string>
 
@@ skipping 11 matching lines @@
 // exposed through Chrome's "about:sync" internal page.
 // This class is copy-friendly and thread-safe.
 class SYNC_EXPORT SyncError {
  public:
   // Error types are used to distinguish general datatype errors (which result
   // in the datatype being disabled) from actionable sync errors (which might
   // have more complicated results).
   enum ErrorType {
     UNSET,                // No error.
     UNRECOVERABLE_ERROR,  // An unrecoverable runtime error was encountered, and
-                          // sync should be disabled completely.
+                          // sync should be disabled and purged completely.
     DATATYPE_ERROR,       // A datatype error was encountered, and the datatype
-                          // should be disabled.
+                          // should be disabled and purged completely. Note that
+                          // datatype errors may be reset, triggering a
+                          // re-enable.
     PERSISTENCE_ERROR,    // A persistence error was detected, and the
                           // datataype should be associated after a sync update.
     CRYPTO_ERROR,         // A cryptographer error was detected, and the
                           // datatype should be associated after it is resolved.
+    UNREADY_ERROR,        // The type is not ready to start yet, so should be
+                          // neither purged nor enabled until it is ready.
   };
 
   // Default constructor refers to "no error", and IsSet() will return false.
   SyncError();
 
   // Create a new Sync error of type |error_type| triggered by |model_type|
   // from the specified location. IsSet() will return true afterward. Will
   // create and print an error specific message to LOG(ERROR).
   SyncError(const tracked_objects::Location& location,
             ErrorType error_type,
@@ skipping 49 matching lines @@
   ModelType model_type_;
   ErrorType error_type_;
 };
 
 // gmock printer helper.
 SYNC_EXPORT void PrintTo(const SyncError& sync_error, std::ostream* os);
 
 }  // namespace syncer
 
 #endif  // SYNC_API_SYNC_ERROR_H_