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diff --git a/dom/storage/LocalStorageCache.h b/dom/storage/LocalStorageCache.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_LocalStorageCache_h
+#define mozilla_dom_LocalStorageCache_h
+
+#include "nsIPrincipal.h"
+
+#include "nsString.h"
+#include "nsTHashMap.h"
+#include "nsHashKeys.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/Atomics.h"
+
+namespace mozilla::dom {
+
+class LocalStorage;
+class LocalStorageCacheChild;
+class LocalStorageManager;
+class StorageUsage;
+class StorageDBBridge;
+
+// Interface class on which only the database or IPC may call.
+// Used to populate the cache with DB data.
+class LocalStorageCacheBridge {
+ public:
+  NS_IMETHOD_(MozExternalRefCountType) AddRef(void);
+  NS_IMETHOD_(void) Release(void);
+
+  // The origin of the cache, result is concatenation of OriginNoSuffix() and
+  // OriginSuffix(), see below.
+  virtual const nsCString Origin() const = 0;
+
+  // The origin attributes suffix alone, this is usually passed as an
+  // |aOriginSuffix| argument to various methods
+  virtual const nsCString& OriginSuffix() const = 0;
+
+  // The origin in the database usage format (reversed) and without the suffix
+  virtual const nsCString& OriginNoSuffix() const = 0;
+
+  // Whether the cache is already fully loaded
+  virtual bool Loaded() = 0;
+
+  // How many items has so far been loaded into the cache, used
+  // for optimization purposes
+  virtual uint32_t LoadedCount() = 0;
+
+  // Called by the database to load a key and its value to the cache
+  virtual bool LoadItem(const nsAString& aKey, const nsAString& aValue) = 0;
+
+  // Called by the database after all keys and values has been loaded
+  // to this cache
+  virtual void LoadDone(nsresult aRv) = 0;
+
+  // Use to synchronously wait until the cache gets fully loaded with data,
+  // this method exits after LoadDone has been called
+  virtual void LoadWait() = 0;
+
+ protected:
+  virtual ~LocalStorageCacheBridge() = default;
+
+  ThreadSafeAutoRefCnt mRefCnt;
+  NS_DECL_OWNINGTHREAD
+};
+
+// Implementation of scope cache that is responsible for preloading data
+// for persistent storage (localStorage) and hold data for non-private,
+// private and session-only cookie modes.  It is also responsible for
+// persisting data changes using the database, works as a write-back cache.
+class LocalStorageCache : public LocalStorageCacheBridge {
+ public:
+  void AssertIsOnOwningThread() const { NS_ASSERT_OWNINGTHREAD(LocalStorage); }
+
+  void SetActor(LocalStorageCacheChild* aActor);
+
+  void ClearActor() {
+    AssertIsOnOwningThread();
+
+    mActor = nullptr;
+  }
+
+  NS_IMETHOD_(void) Release(void) override;
+
+  enum MutationSource {
+    // The mutation is a result of an explicit JS mutation in this process.
+    // The mutation should be sent to the sDatabase. Quota will be checked and
+    // QuotaExceededError may be returned without the mutation being applied.
+    ContentMutation,
+    // The mutation initially was triggered in a different process and is being
+    // propagated to this cache via LocalStorage::ApplyEvent.  The mutation
+    // should
+    // not be sent to the sDatabase because the originating process is already
+    // doing that.  (In addition to the redundant writes being wasteful, there
+    // is the potential for other processes to see inconsistent state from the
+    // database while preloading.)  Quota will be updated but not checked
+    // because it's assumed it was checked in another process and data-coherency
+    // is more important than slightly exceeding quota.
+    E10sPropagated
+  };
+
+  // Note: We pass aOriginNoSuffix through the ctor here, because
+  // LocalStorageCacheHashKey's ctor is creating this class and
+  // accepts reversed-origin-no-suffix as an argument - the hashing key.
+  explicit LocalStorageCache(const nsACString* aOriginNoSuffix);
+
+ protected:
+  virtual ~LocalStorageCache();
+
+ public:
+  void Init(LocalStorageManager* aManager, bool aPersistent,
+            nsIPrincipal* aPrincipal, const nsACString& aQuotaOriginScope);
+
+  // Get size of per-origin data.
+  int64_t GetOriginQuotaUsage(const LocalStorage* aStorage) const;
+
+  // Starts async preload of this cache if it persistent and not loaded.
+  void Preload();
+
+  // The set of methods that are invoked by DOM storage web API.
+  // We are passing the LocalStorage object just to let the cache
+  // read properties like mPrivate and mSessionOnly.
+  // Get* methods return error when load from the database has failed.
+  nsresult GetLength(const LocalStorage* aStorage, uint32_t* aRetval);
+  nsresult GetKey(const LocalStorage* aStorage, uint32_t index,
+                  nsAString& aRetval);
+  nsresult GetItem(const LocalStorage* aStorage, const nsAString& aKey,
+                   nsAString& aRetval);
+  nsresult SetItem(const LocalStorage* aStorage, const nsAString& aKey,
+                   const nsAString& aValue, nsString& aOld,
+                   const MutationSource aSource = ContentMutation);
+  nsresult RemoveItem(const LocalStorage* aStorage, const nsAString& aKey,
+                      nsString& aOld,
+                      const MutationSource aSource = ContentMutation);
+  nsresult Clear(const LocalStorage* aStorage,
+                 const MutationSource aSource = ContentMutation);
+
+  void GetKeys(const LocalStorage* aStorage, nsTArray<nsString>& aKeys);
+
+  // LocalStorageCacheBridge
+
+  const nsCString Origin() const override;
+  const nsCString& OriginNoSuffix() const override { return mOriginNoSuffix; }
+  const nsCString& OriginSuffix() const override { return mOriginSuffix; }
+  bool Loaded() override { return mLoaded; }
+  uint32_t LoadedCount() override;
+  bool LoadItem(const nsAString& aKey, const nsAString& aValue) override;
+  void LoadDone(nsresult aRv) override;
+  void LoadWait() override;
+
+  // Cache keeps 3 sets of data: regular, private and session-only.
+  // This class keeps keys and values for a set and also caches
+  // size of the data for quick per-origin quota checking.
+  class Data {
+   public:
+    Data() : mOriginQuotaUsage(0) {}
+    int64_t mOriginQuotaUsage;
+    nsTHashMap<nsStringHashKey, nsString> mKeys;
+  };
+
+ public:
+  // Number of data sets we keep: default, session
+  static const uint32_t kDataSetCount = 2;
+
+ private:
+  // API to clear the cache data, this is invoked by chrome operations
+  // like cookie deletion.
+  friend class LocalStorageManager;
+
+  static const uint32_t kUnloadDefault = 1 << 0;
+  static const uint32_t kUnloadSession = 1 << 1;
+  static const uint32_t kUnloadComplete = kUnloadDefault | kUnloadSession;
+
+#ifdef DOM_STORAGE_TESTS
+  static const uint32_t kTestReload = 1 << 15;
+#endif
+
+  void UnloadItems(uint32_t aUnloadFlags);
+
+ private:
+  // Synchronously blocks until the cache is fully loaded from the database
+  void WaitForPreload(mozilla::Telemetry::HistogramID aTelemetryID);
+
+  // Helper to get one of the 3 data sets (regular, private, session)
+  Data& DataSet(const LocalStorage* aStorage);
+
+  // Used for firing storage events and synchronization of caches in other
+  // content processes.
+  void NotifyObservers(const LocalStorage* aStorage, const nsAString& aKey,
+                       const nsAString& aOldValue, const nsAString& aNewValue);
+
+  // Whether the storage change is about to persist
+  bool Persist(const LocalStorage* aStorage) const;
+
+  // Changes the quota usage on the given data set if it fits the quota.
+  // If not, then false is returned and no change to the set must be done.
+  // A special case is if aSource==E10sPropagated, then we will return true even
+  // if the change would put us over quota.  This is done to ensure coherency of
+  // caches between processes in the face of races.  It does allow an attacker
+  // to potentially use N multiples of the quota storage limit if they can
+  // arrange for their origin to execute code in N processes.  However, this is
+  // not considered a particularly concerning threat model because it's already
+  // very possible for a rogue page to attempt to intentionally fill up the
+  // user's storage through the use of multiple domains.
+  bool ProcessUsageDelta(uint32_t aGetDataSetIndex, const int64_t aDelta,
+                         const MutationSource aSource = ContentMutation);
+  bool ProcessUsageDelta(const LocalStorage* aStorage, const int64_t aDelta,
+                         const MutationSource aSource = ContentMutation);
+
+ private:
+  // When a cache is reponsible for its life time (in case of localStorage data
+  // cache) we need to refer our manager since removal of the cache from the
+  // hash table is handled in the destructor by call to the manager.  Cache
+  // could potentially overlive the manager, hence the hard ref.
+  RefPtr<LocalStorageManager> mManager;
+
+  // Reference to the usage counter object we check on for eTLD+1 quota limit.
+  // Obtained from the manager during initialization (Init method).
+  RefPtr<StorageUsage> mUsage;
+
+  // The LocalStorageCacheChild is created at the same time of this class.
+  // In normal operation, the actor will be synchronously cleared in our
+  // destructor when we tell it to delete itself.  In a shutdown-related edge
+  // case in the parent process for JSM's, it is possible for the actor to be
+  // destroyed while this class remains alive, in which case it will be nulled
+  // out.
+  LocalStorageCacheChild* mActor;
+
+  // The origin this cache belongs to in the "DB format", i.e. reversed
+  nsCString mOriginNoSuffix;
+
+  // The origin attributes suffix
+  nsCString mOriginSuffix;
+
+  // The eTLD+1 scope used to count quota usage.  It is in the reversed format
+  // and contains the origin attributes suffix.
+  nsCString mQuotaOriginScope;
+
+  // Non-private Browsing, Private Browsing and Session Only sets.
+  Data mData[kDataSetCount];
+
+  // This monitor is used to wait for full load of data.
+  mozilla::Monitor mMonitor MOZ_UNANNOTATED;
+
+  // Flag that is initially false.  When the cache is about to work with
+  // the database (i.e. it is persistent) this flags is set to true after
+  // all keys and coresponding values are loaded from the database.
+  // This flag never goes from true back to false.  Since this flag is
+  // critical for mData hashtable synchronization, it's made atomic.
+  Atomic<bool, ReleaseAcquire> mLoaded;
+
+  // Result of load from the database.  Valid after mLoaded flag has been set.
+  nsresult mLoadResult;
+
+  // Expected to be only 0 or 1.
+  uint32_t mPrivateBrowsingId;
+
+  // Init() method has been called
+  bool mInitialized : 1;
+
+  // This cache is about to be bound with the database (i.e. it has
+  // to load from the DB first and has to persist when modifying the
+  // default data set.)
+  bool mPersistent : 1;
+
+  // Whether we have already captured state of the cache preload on our first
+  // access.
+  bool mPreloadTelemetryRecorded : 1;
+};
+
+// StorageUsage
+// Infrastructure to manage and check eTLD+1 quota
+class StorageUsageBridge {
+ public:
+  NS_INLINE_DECL_THREADSAFE_VIRTUAL_REFCOUNTING(StorageUsageBridge)
+
+  virtual const nsCString& OriginScope() = 0;
+  virtual void LoadUsage(const int64_t aUsage) = 0;
+
+ protected:
+  // Protected destructor, to discourage deletion outside of Release():
+  virtual ~StorageUsageBridge() = default;
+};
+
+class StorageUsage : public StorageUsageBridge {
+ public:
+  explicit StorageUsage(const nsACString& aOriginScope);
+
+  bool CheckAndSetETLD1UsageDelta(
+      uint32_t aDataSetIndex, int64_t aUsageDelta,
+      const LocalStorageCache::MutationSource aSource);
+
+ private:
+  const nsCString& OriginScope() override { return mOriginScope; }
+  void LoadUsage(const int64_t aUsage) override;
+
+  nsCString mOriginScope;
+  int64_t mUsage[LocalStorageCache::kDataSetCount];
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_LocalStorageCache_h