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diff --git a/payload_generator/filesystem_interface.h b/payload_generator/filesystem_interface.h
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+// Copyright 2015 The Chromium OS 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 UPDATE_ENGINE_PAYLOAD_GENERATOR_FILESYSTEM_INTERFACE_H_
+#define UPDATE_ENGINE_PAYLOAD_GENERATOR_FILESYSTEM_INTERFACE_H_
+
+// This class is used to abstract a filesystem and iterate the blocks
+// associated with the files and filesystem structures.
+// For the purposes of the update payload generation, a filesystem is a formated
+// partition composed by fixed-size blocks, since that's the interface used in
+// the update payload.
+
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <memory>
+#include <string>
+#include <vector>
+
+#include <base/macros.h>
+#include <chromeos/key_value_store.h>
+
+#include "update_engine/update_metadata.pb.h"
+
+namespace chromeos_update_engine {
+
+class FilesystemInterface {
+ public:
+  // This represents a file or pseudo-file in the filesystem. It can include
+  // all sort of files, like symlinks, hardlinks, directories and even a file
+  // entry representing the metadata, free space, journaling data, etc.
+  struct File {
+    File() {
+      memset(&file_stat, 0, sizeof(file_stat));
+    }
+
+    // The stat struct for the file. This is invalid (inode 0) for some
+    // pseudo-files.
+    struct stat file_stat;
+
+    // The absolute path to the file inside the filesystem, for example,
+    // "/usr/bin/bash". For pseudo-files, like blocks associated to internal
+    // filesystem tables or free space, the path doesn't start with a /.
+    std::string name;
+
+    // The list of all physical blocks holding the data of this file in
+    // the same order as the logical data. All the block numbers shall be
+    // between 0 and GetBlockCount() - 1. The blocks are encoded in extents,
+    // indicating the starting block, and the number of consecutive blocks.
+    std::vector<Extent> extents;
+  };
+
+  virtual ~FilesystemInterface() = default;
+
+  // Returns the size of a block in the filesystem.
+  virtual size_t GetBlockSize() const = 0;
+
+  // Returns the number of blocks in the filesystem.
+  virtual size_t GetBlockCount() const = 0;
+
+  // Stores in |files| the list of files and pseudo-files in the filesystem. See
+  // FileInterface for details. The paths returned by this method shall not
+  // be repeated; but the same block could be present in more than one file as
+  // happens for example with hard-linked files, but not limited to those cases.
+  // Returns whether the function succeeded.
+  virtual bool GetFiles(std::vector<File>* files) const = 0;
+
+  // Load the image settings stored in the filesystem in the
+  // /etc/update_engine.conf file. Returns whether the settings were found.
+  virtual bool LoadSettings(chromeos::KeyValueStore* store) const = 0;
+
+ protected:
+  FilesystemInterface() = default;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(FilesystemInterface);
+};
+
+}  // namespace chromeos_update_engine
+
+#endif  // UPDATE_ENGINE_PAYLOAD_GENERATOR_FILESYSTEM_INTERFACE_H_