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

base/prefs/json_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_JSON_PREF_STORE_H_
 #define BASE_PREFS_JSON_PREF_STORE_H_
 
 #include <set>
 #include <string>
 
 #include "base/basictypes.h"
+#include "base/callback_forward.h"
 #include "base/compiler_specific.h"
 #include "base/files/file_path.h"
 #include "base/files/important_file_writer.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
 #include "base/message_loop/message_loop_proxy.h"
 #include "base/observer_list.h"
 #include "base/prefs/base_prefs_export.h"
 #include "base/prefs/persistent_pref_store.h"
 
 class PrefFilter;
 
 namespace base {
 class DictionaryValue;
 class FilePath;
 class SequencedTaskRunner;
 class SequencedWorkerPool;
 class Value;
 }
 
 
 // A writable PrefStore implementation that is used for user preferences.
 class BASE_PREFS_EXPORT JsonPrefStore
     : public PersistentPrefStore,
-      public base::ImportantFileWriter::DataSerializer {
+      public base::ImportantFileWriter::DataSerializer,
+      public base::SupportsWeakPtr<JsonPrefStore> {
  public:
   // Returns instance of SequencedTaskRunner which guarantees that file
   // operations on the same file will be executed in sequenced order.
   static scoped_refptr<base::SequencedTaskRunner> GetTaskRunnerForFile(
       const base::FilePath& pref_filename,
       base::SequencedWorkerPool* worker_pool);
 
   // |sequenced_task_runner| is must be a shutdown-blocking task runner, ideally
   // created by GetTaskRunnerForFile() method above.
   JsonPrefStore(const base::FilePath& pref_filename,
@@ skipping 10 matching lines @@
 
   // PersistentPrefStore overrides:
   virtual bool GetMutableValue(const std::string& key,
                                base::Value** result) OVERRIDE;
   virtual void SetValue(const std::string& key, base::Value* value) OVERRIDE;
   virtual void SetValueSilently(const std::string& key,
                                 base::Value* value) OVERRIDE;
   virtual void RemoveValue(const std::string& key) OVERRIDE;
   virtual bool ReadOnly() const OVERRIDE;
   virtual PrefReadError GetReadError() const OVERRIDE;
+  // Note this method may be asynchronous if this instance has a |pref_filter_|.
+  // See details in pref_filter.h.

[Bernhard Bauer, 2014/05/06 12:47:00]

Hmm... could we add a new PrefReadError value for "initialization not finished
yet" and have this method return that? That way, if someone uses an asynchronous
PrefFilter but expects ReadPrefs() to finish synchronously, they would at least
get an error code.

[gab, 2014/05/06 15:57:43]

Done.

   virtual PrefReadError ReadPrefs() OVERRIDE;
   virtual void ReadPrefsAsync(ReadErrorDelegate* error_delegate) OVERRIDE;
   virtual void CommitPendingWrite() OVERRIDE;
   virtual void ReportValueChanged(const std::string& key) OVERRIDE;
 
-  // This method is called after JSON file has been read. Method takes
-  // ownership of the |value| pointer. Note, this method is used with
-  // asynchronous file reading, so class exposes it only for the internal needs.
-  // (read: do not call it manually).
-  void OnFileRead(base::Value* value_owned, PrefReadError error, bool no_dir);
+  // Just like RemoveValue(), but doesn't notify observers. Used when doing some
+  // cleanup that shouldn't otherwise alert observers.
+  void RemoveValueSilently(const std::string& key);
+
+  // Registers |on_next_successful_write| to be called once, on the next
+  // successful write event of |writer_|.
+  void RegisterOnNextSuccessfulWriteCallback(
+      const base::Closure& on_next_successful_write);
+
+  // This method is called after the JSON file has been read.  It then hands
+  // |value| (or an empty dictionary in some read error cases) to the
+  // |pref_filter| if one is set. It also gives a callback pointing at
+  // FinalizeFileRead() to that |pref_filter_| which is then responsible for
+  // invoking it when done. If there is no |pref_filter_|, FinalizeFileRead()
+  // is invoked directly.
+  // Note, this method is used with asynchronous file reading, so this class
+  // exposes it only for the internal needs (read: do not call it manually).
+  // TODO(gab): Move this method to the private section and hand a callback to
+  // it to FileThreadDeserializer rather than exposing this public method and
+  // giving a JsonPrefStore* to FileThreadDeserializer.
+  void OnFileRead(scoped_ptr<base::Value> value,
+                  PrefReadError error,
+                  bool no_dir);
 
  private:
   virtual ~JsonPrefStore();
 
   // ImportantFileWriter::DataSerializer overrides:
   virtual bool SerializeData(std::string* output) OVERRIDE;
 
+  // This method is called after the JSON file has been read and the result has
+  // potentially been intercepted and modified by |pref_filter_|.
+  // |initialization_successful| is pre-determined by OnFileRead() and should
+  // be used when reporting OnInitializationCompleted().
+  // |schedule_write| indicates whether a write should be immediately scheduled
+  // (typically because the |pref_filter_| has already altered the |prefs|) --
+  // this will be ignored if this store is read-only.
+  void FinalizeFileRead(bool initialization_successful,
+                        scoped_ptr<base::DictionaryValue> prefs,
+                        bool schedule_write);
+
   base::FilePath path_;
   const scoped_refptr<base::SequencedTaskRunner> sequenced_task_runner_;
 
   scoped_ptr<base::DictionaryValue> prefs_;
 
   bool read_only_;
 
   // Helper for safely writing pref data.
   base::ImportantFileWriter writer_;
 
   scoped_ptr<PrefFilter> pref_filter_;
   ObserverList<PrefStore::Observer, true> observers_;
 
   scoped_ptr<ReadErrorDelegate> error_delegate_;
 
   bool initialized_;
   PrefReadError read_error_;
 
   std::set<std::string> keys_need_empty_value_;
 
   DISALLOW_COPY_AND_ASSIGN(JsonPrefStore);
 };
 
 #endif  // BASE_PREFS_JSON_PREF_STORE_H_