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diff --git a/toolkit/components/places/Shutdown.h b/toolkit/components/places/Shutdown.h
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+/* 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_places_Shutdown_h_
+#define mozilla_places_Shutdown_h_
+
+#include "mozIStorageCompletionCallback.h"
+#include "nsIAsyncShutdown.h"
+#include "nsProxyRelease.h"
+
+namespace mozilla {
+namespace places {
+
+class Database;
+
+/**
+ * This is most of the code responsible for Places shutdown.
+ *
+ * PHASE 1 (Legacy clients shutdown)
+ * The shutdown procedure begins when the Database singleton receives
+ * profile-change-teardown (note that tests will instead notify nsNavHistory,
+ * that forwards the notification to the Database instance).
+ * Database::Observe first of all checks if initialization was completed
+ * properly, to avoid race conditions, then it notifies "places-shutdown" to
+ * legacy clients. Legacy clients are supposed to start and complete any
+ * shutdown critical work in the same tick, since we won't wait for them.
+
+ * PHASE 2 (Modern clients shutdown)
+ * Modern clients should instead register as a blocker by passing a promise to
+ * nsINavHistoryService::shutdownClient (for example see Sanitizer.jsm), so they
+ * block Places shutdown until the promise is resolved.
+ * When profile-change-teardown is observed by async shutdown, it calls
+ * ClientsShutdownBlocker::BlockShutdown. This class is registered as a teardown
+ * phase blocker in Database::Init (see Database::mClientsShutdown).
+ * ClientsShutdownBlocker::BlockShudown waits for all the clients registered
+ * through nsINavHistoryService::shutdownClient. When all the clients are done,
+ * its `Done` method is invoked, and it stops blocking the shutdown phase, so
+ * that it can continue.
+ *
+ * PHASE 3 (Connection shutdown)
+ * ConnectionBlocker is registered as a profile-before-change blocker in
+ * Database::Init (see Database::mConnectionShutdown).
+ * When profile-before-change is observer by async shutdown, it calls
+ * ConnectionShutdownBlocker::BlockShutdown.
+ * Then the control is passed to Database::Shutdown, that executes some sanity
+ * checks, clears cached statements and proceeds with asyncClose.
+ * Once the connection is definitely closed, Database will call back
+ * ConnectionBlocker::Complete. At this point a final
+ * places-connection-closed notification is sent, for testing purposes.
+ */
+
+/**
+ * A base AsyncShutdown blocker in charge of shutting down Places.
+ */
+class PlacesShutdownBlocker : public nsIAsyncShutdownBlocker,
+                              public nsIAsyncShutdownCompletionCallback {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIASYNCSHUTDOWNBLOCKER
+  NS_DECL_NSIASYNCSHUTDOWNCOMPLETIONCALLBACK
+
+  explicit PlacesShutdownBlocker(const nsString& aName);
+
+  already_AddRefed<nsIAsyncShutdownClient> GetClient();
+
+  // The current state, used internally and for forensics/debugging purposes.
+  // Not all the states make sense for all the derived classes.
+  enum States {
+    NOT_STARTED,
+    // Execution of `BlockShutdown` in progress.
+    RECEIVED_BLOCK_SHUTDOWN,
+
+    // Values specific to ClientsShutdownBlocker
+    // a. Set while we are waiting for clients to do their job and unblock us.
+    CALLED_WAIT_CLIENTS,
+    // b. Set when all the clients are done.
+    RECEIVED_DONE,
+
+    // Values specific to ConnectionShutdownBlocker
+    // a. Set after we notified observers that Places is closing the connection.
+    NOTIFIED_OBSERVERS_PLACES_WILL_CLOSE_CONNECTION,
+    // b. Set after we pass control to Database::Shutdown, and wait for it to
+    // close the connection and call our `Complete` method when done.
+    CALLED_STORAGESHUTDOWN,
+    // c. Set when Database has closed the connection and passed control to
+    // us through `Complete`.
+    RECEIVED_STORAGESHUTDOWN_COMPLETE,
+    // d. We have notified observers that Places has closed the connection.
+    NOTIFIED_OBSERVERS_PLACES_CONNECTION_CLOSED,
+  };
+  States State() { return mState; }
+
+  static Atomic<bool> sIsStarted;
+
+ protected:
+  // The blocker name, also used as barrier name.
+  nsString mName;
+  // The current state, see States.
+  States mState;
+  // The barrier optionally used to wait for clients.
+  nsMainThreadPtrHandle<nsIAsyncShutdownBarrier> mBarrier;
+  // The parent object who registered this as a blocker.
+  nsMainThreadPtrHandle<nsIAsyncShutdownClient> mParentClient;
+
+  // As tests may resurrect a dead `Database`, we use a counter to
+  // give the instances of `PlacesShutdownBlocker` unique names.
+  uint16_t mCounter;
+  static uint16_t sCounter;
+
+  virtual ~PlacesShutdownBlocker() = default;
+};
+
+/**
+ * Blocker also used to wait for clients, through an owned barrier.
+ */
+class ClientsShutdownBlocker final : public PlacesShutdownBlocker {
+ public:
+  NS_INLINE_DECL_REFCOUNTING_INHERITED(ClientsShutdownBlocker,
+                                       PlacesShutdownBlocker)
+
+  explicit ClientsShutdownBlocker();
+
+  NS_IMETHOD Done() override;
+
+ private:
+  ~ClientsShutdownBlocker() = default;
+};
+
+/**
+ * Blocker used to wait when closing the database connection.
+ */
+class ConnectionShutdownBlocker final : public PlacesShutdownBlocker,
+                                        public mozIStorageCompletionCallback {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_MOZISTORAGECOMPLETIONCALLBACK
+
+  explicit ConnectionShutdownBlocker(mozilla::places::Database* aDatabase);
+
+  NS_IMETHOD Done() override;
+
+ private:
+  ~ConnectionShutdownBlocker() = default;
+
+  // The owning database.
+  // The cycle is broken in method Complete(), once the connection
+  // has been closed by mozStorage.
+  RefPtr<mozilla::places::Database> mDatabase;
+};
+
+}  // namespace places
+}  // namespace mozilla
+
+#endif  // mozilla_places_Shutdown_h_