Document that HistoryBackend is sequence-affine, not thread-affine.

components/history/core/browser/history_backend.h

 // 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.
 
 #ifndef COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_BACKEND_H_
 #define COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_BACKEND_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 71 matching lines @@
   std::unique_ptr<HistoryDBTask> task_;
   scoped_refptr<base::SingleThreadTaskRunner> origin_loop_;
   base::CancelableTaskTracker::IsCanceledCallback is_canceled_;
 
   DISALLOW_COPY_AND_ASSIGN(QueuedHistoryDBTask);
 };
 
 // *See the .cc file for more information on the design.*
 //
 // Internal history implementation which does most of the work of the history
-// system. This runs on a background thread (to not block the browser when we
-// do expensive operations) and is NOT threadsafe, so it must only be called
-// from message handlers on the background thread. Invoking on another thread
-// requires threadsafe refcounting.
+// system. This runs on a dedicated sequence (to not block the browser when we
+// do expensive operations). It is NOT threadsafe: unless otherwise noted, all
+// methods must be called from the same sequence.
 //
 // Most functions here are just the implementations of the corresponding
 // functions in the history service. These functions are not documented
 // here, see the history service for behavior.
 class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
                        public HistoryBackendNotifier {
  public:
   // Interface implemented by the owner of the HistoryBackend object. Normally,
   // the history service implements this to send stuff back to the main thread.
   // The unit tests can provide a different implementation if they don't have
@@ skipping 54 matching lines @@
 
     // Notify HistoryService that keyword search term has been deleted.
     // The event will be forwarded to the HistoryServiceObservers in the correct
     // thread.
     virtual void NotifyKeywordSearchTermDeleted(URLID url_id) = 0;
 
     // Invoked when the backend has finished loading the db.
     virtual void DBLoaded() = 0;
   };
 
-  // Init must be called to complete object creation. This object can be
-  // constructed on any thread, but all other functions including Init() must
-  // be called on the history thread.
+  // This constructor can be invoked on any sequence. Init() must then be called
+  // on |task_runner|'s sequence to complete object creation. Unless otherwise
+  // noted, all methods must be called on |task_runner|'s sequence.
   //
   // |history_dir| is the directory where the history files will be placed.
   // See the definition of BroadcastNotificationsCallback above. This function
   // takes ownership of the callback pointer.
   //
   // |history_client| is used to determine bookmarked URLs when deleting and
   // may be null.
   //
   // This constructor is fast and does no I/O, so can be called at any time.
   HistoryBackend(Delegate* delegate,
@@ skipping 168 matching lines @@
   void DeleteMatchingURLsForKeyword(KeywordID keyword_id,
                                     const base::string16& term);
 
   // Observers -----------------------------------------------------------------
 
   void AddObserver(HistoryBackendObserver* observer);
   void RemoveObserver(HistoryBackendObserver* observer);
 
   // Generic operations --------------------------------------------------------
 
+  // This method can be called from any sequence.
   void ProcessDBTask(
       std::unique_ptr<HistoryDBTask> task,
       scoped_refptr<base::SingleThreadTaskRunner> origin_loop,
       const base::CancelableTaskTracker::IsCanceledCallback& is_canceled);
 
   virtual bool GetAllTypedURLs(URLRows* urls);
 
   virtual bool GetVisitsForURL(URLID id, VisitVector* visits);
 
   // Fetches up to |max_visits| most recent visits for the passed URL.
@@ skipping 384 matching lines @@
   // http://www.google.com/favicon.ico) has changed.
   void SendFaviconChangedNotificationForPageAndRedirects(const GURL& page_url);
 
   // Send notification that the bitmap data for the favicon at |icon_url| has
   // changed. Sending this notification is important because the favicon at
   // |icon_url| may be mapped to hundreds of page URLs.
   void SendFaviconChangedNotificationForIconURL(const GURL& icon_url);
 
   // Generic stuff -------------------------------------------------------------
 
-  // Processes the next scheduled HistoryDBTask, scheduling this method
-  // to be invoked again if there are more tasks that need to run.
+  // Processes the next scheduled HistoryDBTask, scheduling this method to be
+  // invoked again if there are more tasks that need to run. This method can be
+  // called from any sequence.
   void ProcessDBTaskImpl();
 
   // HistoryBackendNotifier:
   void NotifyFaviconsChanged(const std::set<GURL>& page_urls,
                              const GURL& icon_url) override;
   void NotifyURLVisited(ui::PageTransition transition,
                         const URLRow& row,
                         const RedirectList& redirects,
                         base::Time visit_time) override;
   void NotifyURLsModified(const URLRows& rows) override;
@@ skipping 112 matching lines @@
 
   // List of observers
   base::ObserverList<HistoryBackendObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(HistoryBackend);
 };
 
 }  // namespace history
 
 #endif  // COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_BACKEND_H_