Extension Storage API: expose storage quota information to extensions, via:

chrome/browser/extensions/settings/settings_storage_quota_enforcer.cc

 // 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.
 
 #include "chrome/browser/extensions/settings/settings_storage_quota_enforcer.h"
 
 #include "base/bind.h"
 #include "base/json/json_writer.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/message_loop.h"
 #include "base/metrics/histogram.h"
 
 namespace extensions {
 
 namespace {
 
 const char* kExceededQuotaErrorMessage = "Quota exceeded";
 
 // Resources there are a quota for.
 enum Resource {
-  TOTAL_BYTES,
-  BYTES_PER_SETTING,
-  KEY_COUNT
+  QUOTA_BYTES,
+  QUOTA_BYTES_PER_ITEM,
+  MAX_ITEMS
 };
 
 // Allocates a setting in a record of total and per-setting usage.
 void Allocate(
     const std::string& key,
     const Value& value,
     size_t* used_total,
     std::map<std::string, size_t>* used_per_setting) {
   // Calculate the setting size based on its JSON serialization size.
   // TODO(kalman): Does this work with different encodings?
@@ skipping 13 matching lines @@
     size_t* used_total,
     std::map<std::string, size_t>* used_per_setting,
     const std::string& key) {
   *used_total -= (*used_per_setting)[key];
   used_per_setting->erase(key);
 }
 
 // Returns an error result and logs the quota exceeded to UMA.
 SettingsStorage::WriteResult QuotaExceededFor(Resource resource) {
   switch (resource) {
-    case TOTAL_BYTES:
+    case QUOTA_BYTES:
       UMA_HISTOGRAM_COUNTS_100(
           "Extensions.SettingsQuotaExceeded.TotalBytes", 1);
       break;
-    case BYTES_PER_SETTING:
+    case QUOTA_BYTES_PER_ITEM:
       UMA_HISTOGRAM_COUNTS_100(
           "Extensions.SettingsQuotaExceeded.BytesPerSetting", 1);
       break;
-    case KEY_COUNT:
+    case MAX_ITEMS:
       UMA_HISTOGRAM_COUNTS_100(
           "Extensions.SettingsQuotaExceeded.KeyCount", 1);
       break;
     default:
       NOTREACHED();
   }
   return SettingsStorage::WriteResult(kExceededQuotaErrorMessage);
 }
 
 }  // namespace
@@ skipping 10 matching lines @@
 
   for (DictionaryValue::Iterator it(maybe_settings.settings()); it.HasNext();
       it.Advance()) {
     Allocate(
         it.key(), it.value(), &used_total_, &used_per_setting_);
   }
 }
 
 SettingsStorageQuotaEnforcer::~SettingsStorageQuotaEnforcer() {}
 
+size_t SettingsStorageQuotaEnforcer::GetBytesInUse() {
+  // All SettingsStorage implementations rely on GetBytesInUse being
+  // implemented here.
+  return used_total_;
+}
+
 SettingsStorage::ReadResult SettingsStorageQuotaEnforcer::Get(
     const std::string& key) {
   return delegate_->Get(key);
 }
 
 SettingsStorage::ReadResult SettingsStorageQuotaEnforcer::Get(
     const std::vector<std::string>& keys) {
   return delegate_->Get(keys);
 }
 
 SettingsStorage::ReadResult SettingsStorageQuotaEnforcer::Get() {
   return delegate_->Get();
 }
 
 SettingsStorage::WriteResult SettingsStorageQuotaEnforcer::Set(
     WriteOptions options, const std::string& key, const Value& value) {
   size_t new_used_total = used_total_;
   std::map<std::string, size_t> new_used_per_setting = used_per_setting_;
   Allocate(key, value, &new_used_total, &new_used_per_setting);
 
   if (options != IGNORE_QUOTA) {
     if (new_used_total > limits_.quota_bytes) {
-      return QuotaExceededFor(TOTAL_BYTES);
+      return QuotaExceededFor(QUOTA_BYTES);
     }
-    if (new_used_per_setting[key] > limits_.quota_bytes_per_setting) {
-      return QuotaExceededFor(BYTES_PER_SETTING);
+    if (new_used_per_setting[key] > limits_.quota_bytes_per_item) {
+      return QuotaExceededFor(QUOTA_BYTES_PER_ITEM);
     }
-    if (new_used_per_setting.size() > limits_.max_keys) {
-      return QuotaExceededFor(KEY_COUNT);
+    if (new_used_per_setting.size() > limits_.max_items) {
+      return QuotaExceededFor(MAX_ITEMS);
     }
   }
 
   WriteResult result = delegate_->Set(options, key, value);
   if (result.HasError()) {
     return result;
   }
 
   used_total_ = new_used_total;
   used_per_setting_.swap(new_used_per_setting);
   return result;
 }
 
 SettingsStorage::WriteResult SettingsStorageQuotaEnforcer::Set(
     WriteOptions options, const DictionaryValue& values) {
   size_t new_used_total = used_total_;
   std::map<std::string, size_t> new_used_per_setting = used_per_setting_;
   for (DictionaryValue::Iterator it(values); it.HasNext(); it.Advance()) {
     Allocate(it.key(), it.value(), &new_used_total, &new_used_per_setting);
 
     if (options != IGNORE_QUOTA &&
-        new_used_per_setting[it.key()] > limits_.quota_bytes_per_setting) {
-      return QuotaExceededFor(BYTES_PER_SETTING);
+        new_used_per_setting[it.key()] > limits_.quota_bytes_per_item) {
+      return QuotaExceededFor(QUOTA_BYTES_PER_ITEM);
     }
   }
 
   if (options != IGNORE_QUOTA) {
     if (new_used_total > limits_.quota_bytes) {
-      return QuotaExceededFor(TOTAL_BYTES);
+      return QuotaExceededFor(QUOTA_BYTES);
     }
-    if (new_used_per_setting.size() > limits_.max_keys) {
-      return QuotaExceededFor(KEY_COUNT);
+    if (new_used_per_setting.size() > limits_.max_items) {
+      return QuotaExceededFor(MAX_ITEMS);
     }
   }
 
   WriteResult result = delegate_->Set(options, values);
   if (result.HasError()) {
     return result;
   }
 
   used_total_ = new_used_total;
   used_per_setting_ = new_used_per_setting;
@@ skipping 30 matching lines @@
     return result;
   }
 
   while (!used_per_setting_.empty()) {
     Free(&used_total_, &used_per_setting_, used_per_setting_.begin()->first);
   }
   return result;
 }
 
 }  // namespace extensions