Introduce a temporary workaround to ensure tracked preferences set to empty list/dict are not confu…

chrome/browser/prefs/pref_metrics_service.cc

 // Copyright 2013 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.
 
 #include "chrome/browser/prefs/pref_metrics_service.h"
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/json/json_string_value_serializer.h"
 #include "base/metrics/histogram.h"
@@ skipping 74 matching lines @@
   pref_hash_seed_ = ResourceBundle::GetSharedInstance().GetRawDataResource(
       IDR_PREF_HASH_SEED_BIN).as_string();
 
   RecordLaunchPrefs();
 
   PrefServiceSyncable* prefs = PrefServiceSyncable::FromProfile(profile_);
   synced_pref_change_registrar_.reset(new SyncedPrefChangeRegistrar(prefs));
 
   RegisterSyncedPrefObservers();
 
+  MarkNeedsEmptyValueForTrackedPreferences();
+
   // The following code might cause callbacks into this instance before we exit
   // the constructor. This instance should be initialized at this point.
 #if defined(OS_WIN) || defined(OS_MACOSX)
   // We need the machine id to compute pref value hashes. Fetch that, and then
   // call CheckTrackedPreferences in the callback.
   extensions::api::DeviceId::GetDeviceId(
       "PrefMetricsService",  // non-empty string to obfuscate the device id.
       Bind(&PrefMetricsService::GetDeviceIdCallback,
            weak_factory_.GetWeakPtr()));
 #endif  // defined(OS_WIN) || defined(OS_MACOSX)
 }
 
 // For unit testing only.
 PrefMetricsService::PrefMetricsService(Profile* profile,
                                        PrefService* local_state,
                                        const std::string& device_id,
                                        const char** tracked_pref_paths,
                                        int tracked_pref_path_count)
     : profile_(profile),
       prefs_(profile->GetPrefs()),
       local_state_(local_state),
       profile_name_(profile_->GetPath().AsUTF8Unsafe()),
       pref_hash_seed_(kSHA256DigestSize, 0),
       device_id_(device_id),
       tracked_pref_paths_(tracked_pref_paths),
       tracked_pref_path_count_(tracked_pref_path_count),
       checked_tracked_prefs_(false),
       weak_factory_(this) {
+  MarkNeedsEmptyValueForTrackedPreferences();
   CheckTrackedPreferences();
 }
 
 PrefMetricsService::~PrefMetricsService() {
 }
 
 void PrefMetricsService::RecordLaunchPrefs() {
   bool show_home_button = prefs_->GetBoolean(prefs::kShowHomeButton);
   bool home_page_is_ntp = prefs_->GetBoolean(prefs::kHomePageIsNewTabPage);
   UMA_HISTOGRAM_BOOLEAN("Settings.ShowHomeButton", show_home_button);
@@ skipping 126 matching lines @@
   histogram->Add(integer_value);
 }
 
 void PrefMetricsService::GetDeviceIdCallback(const std::string& device_id) {
   device_id_ = device_id;
   // On Aura, this seems to be called twice.
   if (!checked_tracked_prefs_)
     CheckTrackedPreferences();
 }
 
+void PrefMetricsService::MarkNeedsEmptyValueForTrackedPreferences() {
+  for (int i = 0; i < tracked_pref_path_count_; ++i) {
+    // Skip prefs that haven't been registered.
+    if (!prefs_->FindPreference(tracked_pref_paths_[i]))
+      continue;
+
+    // Make sure tracked prefs are saved to disk even if empty.
+    // TODO(gab): Guarantee this for all prefs at a lower level and remove this
+    // hack.
+    prefs_->MarkUserStoreNeedsEmptyValue(tracked_pref_paths_[i]);
+  }
+}
+
 // To detect changes to Preferences that happen outside of Chrome, we hash
 // selected pref values and save them in local state. CheckTrackedPreferences
 // compares the saved values to the values observed in the profile's prefs. A
 // dictionary of dictionaries in local state holds the hashed values, grouped by
 // profile. To make the system more resistant to spoofing, pref values are
 // hashed with the pref path and the device id.
 void PrefMetricsService::CheckTrackedPreferences() {
   DCHECK(!checked_tracked_prefs_);
 
   const base::DictionaryValue* pref_hash_dicts =
@@ skipping 171 matching lines @@
 }
 
 bool PrefMetricsService::Factory::ServiceIsNULLWhileTesting() const {
   return false;
 }
 
 content::BrowserContext* PrefMetricsService::Factory::GetBrowserContextToUse(
     content::BrowserContext* context) const {
   return chrome::GetBrowserContextRedirectedInIncognito(context);
 }