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

base/prefs/json_pref_store.h

Index: base/prefs/json_pref_store.h
diff --git a/base/prefs/json_pref_store.h b/base/prefs/json_pref_store.h
index ad13feba3f6e1279c7c2bee4d597df35d1698842..1ccd8c1ba7b22a066b275b0e7b01e0c0a97d527e 100644
--- a/base/prefs/json_pref_store.h
+++ b/base/prefs/json_pref_store.h
@@ -9,6 +9,7 @@
 #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"
@@ -34,6 +35,29 @@ class BASE_PREFS_EXPORT JsonPrefStore
     : public PersistentPrefStore,
       public base::ImportantFileWriter::DataSerializer {
  public:
+  // A callback to be invoked when |prefs| have been read (and possibly
+  // pre-modified) and are now ready to be handed back to this callback's
+  // builder. |schedule_write| indicates whether a write should be scheduled
+  // (typically because the OnFileReadInterceptor has already altered the
+  // |prefs|).
+  typedef base::Callback<void(scoped_ptr<base::DictionaryValue> prefs,
+                              bool schedule_write)>
+      FinalizePrefsReadCallback;
+
+  // A callback to be invoked from OnFileRead, after the read errors have been
+  // handled, but before |prefs| is handed to this JsonPrefStore.
+  // |read_only| indicates whether this store was turned read-only because of
+  // an error (in which case it is still possible to alter the in-memory state
+  // of this store, but the result won't be flushed back to disk). The callee
+  // of this callback is responsible for invoking |finalize_prefs_read| to hand
+  // |prefs| back to this JsonPrefStore when it's done -- this JsonPrefStore
+  // will not consider its read complete nor invoke observers until it has been
+  // invoked.
+  typedef base::Callback<void(scoped_ptr<base::DictionaryValue> prefs,
+                              bool read_only,
+                              const FinalizePrefsReadCallback&
+                                  finalize_prefs_read)> OnFileReadInterceptor;
+
   // 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(
@@ -68,11 +92,20 @@ class BASE_PREFS_EXPORT JsonPrefStore
   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);
+  // Registers |on_file_read_interceptor| to intercept the next OnFileRead
+  // event.
+  void InterceptNextFileRead(
+      const OnFileReadInterceptor& on_file_read_interceptor);
+
+  // 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
+  // |on_file_read_interceptor_| if one is set. It also gives a callback
+  // pointing at FinalizeFileRead() to the |on_file_read_interceptor_| which is
+  // then responsible for invoking it when done. If there is no
+  // |on_file_read_interceptor_|, FinalizeFileRead() is invoked directly.
+  void OnFileRead(scoped_ptr<base::Value> value,
+                  PrefReadError error,
+                  bool no_dir);
 
  private:
   virtual ~JsonPrefStore();
@@ -80,6 +113,26 @@ class BASE_PREFS_EXPORT 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 on_file_read_interceptor_.

[Bernhard Bauer, 2014/04/29 15:13:04]

Nit: Put |on_file_read_interceptor_| between pipes?

[gab, 2014/04/29 15:46:44]

Done.

+  // |initialization_successful| is pre-determined by OnFileRead() and should
+  // be used when reporting OnInitializationCompleted().
+  // |schedule_write| indicates whether a write should be scheduled (typically
+  // because the OnFileReadInterceptor 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);
+
+  // Callback to be invoked only once (and subsequently deleted) on the next
+  // OnFileRead event. It will be allowed to modify the JSON value read from
+  // disk before this JsonPrefStore gets to declare itself initialized and
+  // begins using it. It is responsible for handing it back to this
+  // JsonPrefStore via the FinalizePrefReadCallback handed to it.
+  // Note: Setting an |on_file_read_interceptor_| may make ReadPrefs()
+  // asynchronous if the |on_file_read_interceptor_| is asynchronous.
+  scoped_ptr<OnFileReadInterceptor> on_file_read_interceptor_;

[Bernhard Bauer, 2014/04/29 15:13:04]

Why do you need a scoped_ptr to a Callback? Callbacks are (cheaply) copyable and
can be NULL.

[gab, 2014/04/29 15:46:44]

I wasn't sure how to best express that this Callback will only be called once
and then deleted.

scoped_ptr::reset() felt easy and intuitive.

I guess I could make it a class member and do 
on_file_read_callback_ = OnFileReadCallback();
instead of a reset(), but that didn't feel as clean from my POV.

WDYT?

[Bernhard Bauer, 2014/04/29 16:38:53]

Callback also has a Reset() (capital R) method.

[gab, 2014/04/29 20:12:14]

Oh awesome :)! (I'd looked for that in callback.h, but apparently it's in
callback_internal.h which is exposed from callback.h....?!)

+
   base::FilePath path_;
   const scoped_refptr<base::SequencedTaskRunner> sequenced_task_runner_;