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diff --git a/chrome/browser/session_backend.h b/chrome/browser/session_backend.h
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+// Copyright 2008, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//    * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//    * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//    * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef CHROME_BROWSER_SESSION_BACKEND_H__
+#define CHROME_BROWSER_SESSION_BACKEND_H__
+
+#include <vector>
+
+#include "base/ref_counted.h"
+#include "base/scoped_handle.h"
+#include "chrome/browser/session_service.h"
+
+class Pickle;
+
+// SessionCommand -------------------------------------------------------------
+
+// SessionCommand contains a command id and arbitrary amount chunk of memory.
+//
+// SessionBackend reads and writes SessionCommands.
+//
+// A SessionCommand may be created directly from a Pickle, which is useful
+// for types of arbitrary length.
+class SessionCommand {
+ public:
+  // These get written to disk, so we define types for them.
+  // Type for the identifier.
+  typedef uint8 id_type;
+  // Type for writing the size.
+  typedef uint16 size_type;
+
+  // Creates a session command with the specified id. This allocates a buffer
+  // of size |size| that must be filled via contents().
+  SessionCommand(id_type id, size_type size);
+
+  // Convenience constructor that creates a session command with the specified
+  // id whose contents is populated from the contents of pickle.
+  SessionCommand(id_type id, const Pickle& pickle);
+
+  // The contents of the command.
+  char* contents() { return &(contents_[0]); }
+  const char* contents() const { return &(contents_[0]); }
+
+  // Identifier for the command.
+  id_type id() const { return id_; }
+
+  // Size of data.
+  size_type size() const { return static_cast<size_type>(contents_.size()); }
+
+  // Convenience for extracting the data to a target. Returns false if
+  // count is not equal to the size of data this command contains.
+  bool GetPayload(void* dest, size_t count) const;
+
+  // Returns the contents as a pickle. It is up to the caller to delete the
+  // returned Pickle. The returned Pickle references the underlying data of
+  // this SessionCommand. If you need it to outlive the command, copy the
+  // pickle.
+  Pickle* PayloadAsPickle() const;
+
+ private:
+  const id_type id_;
+  std::string contents_;
+
+  DISALLOW_EVIL_CONSTRUCTORS(SessionCommand);
+};
+
+// SessionBackend -------------------------------------------------------------
+
+// SessionBackend is the backend used by SessionService. It is responsible
+// for maintaining up to 3 files:
+// . The current file, which is the file commands passed to AppendCommands
+//   get written to.
+// . The last file. When created the current file is moved to the last
+//   file.
+// . A save file, which is created with arbitrary commands.
+//
+// Each file contains an arbitrary set of commands supplied from
+// SessionService.
+
+class SessionBackend : public base::RefCountedThreadSafe<SessionBackend> {
+ public:
+  typedef SessionCommand::id_type id_type;
+  typedef SessionCommand::size_type size_type;
+
+  // Initial size of the buffer used in reading the file. This is exposed
+  // for testing.
+  static const int kFileReadBufferSize;
+
+  // Creates a SessionBackend. This method is invoked on the MAIN thread,
+  // and does NO IO. The real work is done from Init, which is invoked on
+  // the file thread.
+  //
+  // The supplied path is the directory the files are writen to.
+  explicit SessionBackend(const std::wstring& path_to_dir);
+
+  // Moves the current file to the last file, and recreates the current file.
+  //
+  // NOTE: this is invoked before every command, and does nothing if we've
+  // already Init'ed.
+  void Init();
+
+  // Recreates the save file with the specified commands.
+  //
+  // This deletes the SessionCommands passed to it.
+  void SaveSession(const std::vector<SessionCommand*>& commands);
+
+  // Appends the specified commands to the current file. If reset_first is
+  // true the the current file is recreated.
+  //
+  // NOTE: this deletes SessionCommands in commands as well as the supplied
+  // vector.
+  void AppendCommands(std::vector<SessionCommand*>* commands,
+                      bool reset_first);
+
+  // Invoked from the service, invokes ReadSessionImpl to do the work.
+  void ReadSession(
+      scoped_refptr<SessionService::InternalSavedSessionRequest> request);
+
+  // Reads the commands from the last file, or save file if
+  // use_save_file is true.
+  //
+  // On success, the read commands are added to commands. It is up to the
+  // caller to delete the commands.
+  bool ReadSessionImpl(bool use_save_file,
+                       std::vector<SessionCommand*>* commands);
+
+  // If saved_session is true, deletes the saved session, otherwise deletes
+  // the last file.
+  void DeleteSession(bool saved_session);
+
+  // Copies the contents of the last session file to the saved session file.
+  void CopyLastSessionToSavedSession();
+
+  // Moves the current session to the last and resets the current. This is
+  // called during startup and if the user launchs the app and no tabbed
+  // browsers are running.
+  void MoveCurrentSessionToLastSession();
+
+ private:
+  // Recreates the current file such that it only contains the header and
+  // NO commands.
+  void ResetFile();
+
+  // Opens the current file and writes the header. On success a handle to
+  // the file is returned.
+  HANDLE OpenAndWriteHeader(const std::wstring& path);
+
+  // Appends the specified commands to the specified file.
+  bool AppendCommandsToFile(HANDLE handle,
+                            const std::vector<SessionCommand*>& commands);
+
+  // Returns the path to the last file.
+  std::wstring GetLastSessionPath();
+
+  // Returns the path to the save file.
+  std::wstring GetSavedSessionPath();
+
+  // Returns the path to the current file.
+  std::wstring GetCurrentSessionPath();
+
+  // Directory files are relative to.
+  const std::wstring path_to_dir_;
+
+  // Whether the previous target file is valid.
+  bool last_session_valid_;
+
+  // Handle to the target file.
+  ScopedHandle current_session_handle_;
+
+  // Whether we've inited. Remember, the constructor is run on the
+  // Main thread, all others on the IO thread, hence lazy initialization.
+  bool inited_;
+
+  // If true, the file is empty (no commands have been added to it).
+  bool empty_file_;
+
+  DISALLOW_EVIL_CONSTRUCTORS(SessionBackend);
+};
+
+#endif  // #define CHROME_BROWSER_SESSION_BACKEND_H__