Fix a race condition in preference metric reporting.

chrome/browser/prefs/pref_hash_store_impl.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_hash_store_impl.h"
+
+#include "base/logging.h"
+#include "base/prefs/pref_registry_simple.h"
+#include "base/prefs/pref_service.h"
+#include "base/prefs/scoped_user_pref_update.h"
+#include "base/values.h"
+#include "chrome/common/pref_names.h"
+
+PrefHashStoreImpl::PrefHashStoreImpl(const PrefHashCalculator& hash_calculator,
+                                     PrefService* local_state,
+                                     const std::string& hash_store_id)
+    : local_state_(local_state),
+      hash_store_id_(hash_store_id),
+      pref_hash_calculator_(hash_calculator) {}
+
+// static
+void PrefHashStoreImpl::RegisterPrefs(PrefRegistrySimple* registry) {
+  // Register the top level dictionary to map profile names to dictionaries of
+  // tracked preferences.
+  registry->RegisterDictionaryPref(prefs::kProfilePreferenceHashes);
+}
+
+PrefHashStore::InitializationResult PrefHashStoreImpl::InitializeTrackedValue(
+    const std::string& path, const base::Value* initial_value) {
+  tracked_paths_.insert(path);
+
+  const base::DictionaryValue* pref_hash_dicts =
+      local_state_->GetDictionary(prefs::kProfilePreferenceHashes);
+  const base::DictionaryValue* hashed_prefs = NULL;
+  pref_hash_dicts->GetDictionaryWithoutPathExpansion(hash_store_id_,
+                                                     &hashed_prefs);
+
+  std::string last_hash;
+  // First try to get the stored expected hash...
+  if (hashed_prefs && hashed_prefs->GetString(path, &last_hash)) {
+    PrefHashCalculator::ValidationResult validation_result =
+        pref_hash_calculator_.Validate(path, initial_value, last_hash);
+    if (validation_result == PrefHashCalculator::VALID)
+      return PrefHashStore::UNCHANGED;
+
+    // No matter what the reason for the mismatch, we will correct it now.
+    OnPrefValueChanged(path, initial_value);
+
+    switch (validation_result) {
+      case PrefHashCalculator::VALID_LEGACY:
+        return PrefHashStore::MIGRATED;
+      case PrefHashCalculator::INVALID:
+        return initial_value ? PrefHashStore::CHANGED : PrefHashStore::CLEARED;
+      default:
+        NOTREACHED() << "Unexpected PrefHashCalculator::ValidationResult: "
+                     << validation_result;
+        return PrefHashStore::INITIALIZED;
+    }
+  }
+
+  // We haven't tracked this preference yet, or the hash in local state was
+  // removed.
+  OnPrefValueChanged(path, initial_value);
+  return PrefHashStore::INITIALIZED;
+}
+
+void PrefHashStoreImpl::OnPrefValueChanged(const std::string& path,
+                                           const base::Value* new_value) {
+  if (tracked_paths_.find(path) == tracked_paths_.end())
+    return;
+
+  DictionaryPrefUpdate update(local_state_, prefs::kProfilePreferenceHashes);
+  DictionaryValue* child_dictionary = NULL;
+
+  // Get the dictionary corresponding to the profile name,
+  // which may have a '.'
+  if (!update->GetDictionaryWithoutPathExpansion(hash_store_id_,
+                                                 &child_dictionary)) {
+    child_dictionary = new DictionaryValue;
+    update->SetWithoutPathExpansion(hash_store_id_, child_dictionary);
+  }
+
+  child_dictionary->SetString(
+      path, pref_hash_calculator_.Calculate(path, new_value));
+}