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

chrome/browser/prefs/pref_hash_filter.h

Index: chrome/browser/prefs/pref_hash_filter.h
diff --git a/chrome/browser/prefs/pref_hash_filter.h b/chrome/browser/prefs/pref_hash_filter.h
index 7cb08313d0615faf26bb9bccdc124412af3fdcb0..4ebed33beb87b4f5192df1e31dc9b7e42c46cc82 100644
--- a/chrome/browser/prefs/pref_hash_filter.h
+++ b/chrome/browser/prefs/pref_hash_filter.h
@@ -11,14 +11,15 @@
 #include <vector>
 
 #include "base/basictypes.h"
+#include "base/callback.h"
 #include "base/compiler_specific.h"
 #include "base/containers/scoped_ptr_hash_map.h"
 #include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
 #include "base/prefs/pref_filter.h"
 #include "chrome/browser/prefs/pref_hash_store.h"
 #include "chrome/browser/prefs/tracked/tracked_preference.h"
 
-class PersistentPrefStore;
 class PrefService;
 class PrefStore;
 
@@ -35,8 +36,23 @@ class PrefRegistrySyncable;
 // Intercepts preference values as they are loaded from disk and verifies them
 // using a PrefHashStore. Keeps the PrefHashStore contents up to date as values
 // are changed.
-class PrefHashFilter : public PrefFilter {
+class PrefHashFilter : public PrefFilter,
+                       public base::SupportsWeakPtr<PrefHashFilter> {
  public:
+  // A callback to be invoked by a FilterOnLoadInterceptor when its ready to
+  // hand back the |prefs| it was handed for early filtering. |prefs_altered|
+  // indicates whether the |prefs| were actually altered by the
+  // FilterOnLoadInterceptor before being handed back.
+  typedef base::Callback<void(scoped_ptr<base::DictionaryValue> prefs,
+                              bool prefs_altered)> FinalizeFilterOnLoadCallback;
+
+  // A callback to be invoked from FilterOnLoad. It takes ownership of prefs
+  // and may modify them before handing them back to this PrefHashFilter via
+  // |finalize_filter_on_load|.
+  typedef base::Callback<
+      void(const FinalizeFilterOnLoadCallback& finalize_filter_on_load,
+           scoped_ptr<base::DictionaryValue> prefs)> FilterOnLoadInterceptor;
+
   enum EnforcementLevel {
     NO_ENFORCEMENT,
     ENFORCE_ON_LOAD
@@ -84,23 +100,38 @@ class PrefHashFilter : public PrefFilter {
   // |pref_store|.
   void Initialize(const PrefStore& pref_store);
 
-  // Migrates protected values from |source| to |destination|. Values are
-  // migrated if they are protected according to this filter's configuration,
-  // the corresponding key has no value in |destination|, and the value in
-  // |source| is trusted according to this filter's PrefHashStore. Regardless of
-  // the state of |destination| or the trust status, the protected values will
-  // be removed from |source|.
-  void MigrateValues(PersistentPrefStore* source,
-                     PersistentPrefStore* destination);
-
   // PrefFilter implementation.
-  virtual bool FilterOnLoad(base::DictionaryValue* pref_store_contents)
-      OVERRIDE;
+  virtual void FilterOnLoad(
+      const PostFilterOnLoadCallback& post_filter_on_load_callback,
+      scoped_ptr<base::DictionaryValue> pref_store_contents) OVERRIDE;
   virtual void FilterUpdate(const std::string& path) OVERRIDE;
   virtual void FilterSerializeData(
       const base::DictionaryValue* pref_store_contents) OVERRIDE;
 
+  // Registers |filter_on_load_interceptor| to intercept the next FilterOnLoad
+  // event. At most one FilterOnLoadInterceptor should be registered per
+  // PrefHashFilter.
+  void InterceptNextFilterOnLoad(
+      const FilterOnLoadInterceptor& filter_on_load_interceptor);
+
  private:
+  // Validates values in |pref_store_contents| according to stored hashes,
+  // reports validation results via UMA, and updates hashes in case of mismatch.
+  // |prefs_altered| is true if the prefs were modified already since they were
+  // read; in which case this method is guaranteed to report, via the
+  // |post_filter_on_load_callback|, that a write is required. Hands ownership
+  // of |pref_store_contents| back to the |post_filter_on_load_callback| when
+  // done.
+  void FinalizeFilterOnLoad(
+      const PostFilterOnLoadCallback& post_filter_on_load_callback,
+      scoped_ptr<base::DictionaryValue> pref_store_contents,
+      bool prefs_altered);
+
+  // Callback to be invoked only once (and subsequently reset) on the next
+  // FilterOnLoad event. It will be allowed to modify the |prefs| handed to
+  // FilterOnLoad before handing them back to this PrefHashFilter.
+  FilterOnLoadInterceptor filter_on_load_interceptor_;
+
   // A map of paths to TrackedPreferences; this map owns this individual
   // TrackedPreference objects.
   typedef base::ScopedPtrHashMap<std::string, TrackedPreference>