Introduce a new framework for back-and-forth tracked/protected preferences migration.

base/prefs/persistent_pref_store.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 BASE_PREFS_PERSISTENT_PREF_STORE_H_
 #define BASE_PREFS_PERSISTENT_PREF_STORE_H_
 
 #include <string>
 
 #include "base/prefs/base_prefs_export.h"
 #include "base/prefs/writeable_pref_store.h"
 
 // This interface is complementary to the PrefStore interface, declaring
 // additional functionality that adds support for setting values and persisting
 // the data to some backing store.
 class BASE_PREFS_EXPORT PersistentPrefStore : public WriteablePrefStore {
  public:
   // Unique integer code for each type of error so we can report them
   // distinctly in a histogram.
-  // NOTE: Don't change the order here as it will change the server's meaning
-  // of the histogram.
+  // NOTE: Don't change the explicit values of the enums as it will change the
+  // server's meaning of the histogram.
   enum PrefReadError {
     PREF_READ_ERROR_NONE = 0,
-    PREF_READ_ERROR_JSON_PARSE,
-    PREF_READ_ERROR_JSON_TYPE,
-    PREF_READ_ERROR_ACCESS_DENIED,
-    PREF_READ_ERROR_FILE_OTHER,
-    PREF_READ_ERROR_FILE_LOCKED,
-    PREF_READ_ERROR_NO_FILE,
-    PREF_READ_ERROR_JSON_REPEAT,
-    PREF_READ_ERROR_OTHER,
-    PREF_READ_ERROR_FILE_NOT_SPECIFIED,
+    PREF_READ_ERROR_JSON_PARSE = 1,
+    PREF_READ_ERROR_JSON_TYPE = 2,
+    PREF_READ_ERROR_ACCESS_DENIED = 3,
+    PREF_READ_ERROR_FILE_OTHER = 4,
+    PREF_READ_ERROR_FILE_LOCKED = 5,
+    PREF_READ_ERROR_NO_FILE = 6,
+    PREF_READ_ERROR_JSON_REPEAT = 7,
+    // PREF_READ_ERROR_OTHER = 8,  // Deprecated.
+    PREF_READ_ERROR_FILE_NOT_SPECIFIED = 9,
+    // Indicates that ReadPrefs() couldn't complete synchronously and is waiting
+    // for an asynchronous task to complete first.
+    PREF_READ_ERROR_ASYNCHRONOUS_TASK_INCOMPLETE = 10,
     PREF_READ_ERROR_MAX_ENUM
   };
 
   class ReadErrorDelegate {
    public:
     virtual ~ReadErrorDelegate() {}
 
     virtual void OnError(PrefReadError error) = 0;
   };
 
@@ skipping 32 matching lines @@
   virtual void ReadPrefsAsync(ReadErrorDelegate* error_delegate) = 0;
 
   // Lands any pending writes to disk.
   virtual void CommitPendingWrite() = 0;
 
  protected:
   virtual ~PersistentPrefStore() {}
 };
 
 #endif  // BASE_PREFS_PERSISTENT_PREF_STORE_H_