diff --git a/filesystem_iterator.h b/filesystem_iterator.h
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--- /dev/null
+++ b/filesystem_iterator.h
@@ -0,0 +1,126 @@
+// Copyright (c) 2009 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 CHROMEOS_PLATFORM_UPDATE_ENGINE_FILESYSTEM_ITERATOR_H__
+#define CHROMEOS_PLATFORM_UPDATE_ENGINE_FILESYSTEM_ITERATOR_H__
+
+// This class is used to walk a filesystem. It will iterate over every file
+// on the same device as the file passed in the ctor. Directories will be
+// visited before their children. Children will be visited in no particular
+// order.
+
+// The iterator is a forward iterator. It's not random access nor can it be
+// decremented.
+
+// Note: If the iterator comes across a mount point where another filesystem
+// is mounted, that mount point will be present, but none of its children
+// will be. Technically the mount point is on the other filesystem (and
+// the Stat() call will verify that), but we return it anyway since:
+// 1. Such a folder must exist in the first filesystem if it got used
+//    as a mount point.
+// 2. You probably want to copy if it you're using the iterator to do a
+//    filesystem copy
+// 3. If you don't want that, you can just check Stat().st_dev and skip
+//    foreign filesystems manually.
+
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <dirent.h>
+#include <unistd.h>
+#include <string>
+#include <set>
+#include <vector>
+
+namespace chromeos_update_engine {
+
+class FilesystemIterator {
+ public:
+  FilesystemIterator(const std::string& path,
+                     const std::set<std::string>& excl_prefixes);
+
+  ~FilesystemIterator();
+
+  // Returns stat struct for the current file.
+  struct stat GetStat() const {
+    return stbuf_;
+  }
+
+  // Returns full path for current file.
+  std::string GetFullPath() const;
+
+  // Returns the path that's part of the iterator. For example, if
+  // the object were constructed by passing in "/foo/bar" and Path()
+  // returns "/foo/bar/baz/bat.txt", IterPath would return
+  // "/baz/bat.txt". When this object is on root (ie, the very first
+  // path), IterPath will return "", otherwise the first character of
+  // IterPath will be "/".
+  std::string GetPartialPath() const;
+
+  // Returns name for current file.
+  std::string GetBasename() const {
+    return names_.back();
+  }
+
+  // Increments to the next file.
+  void Increment();
+
+  // If we're at the end. If at the end, do not call Stat(), Path(), etc.,
+  // since this iterator currently isn't pointing to any file at all.
+  bool IsEnd() const {
+    return is_end_;
+  }
+
+  // Returns true if the iterator is in an error state.
+  bool IsErr() const {
+    return is_err_;
+  }
+ private:
+  // Helper for Increment.
+  void IncrementInternal();
+
+  // Returns true if path exists and it's a directory.
+  bool DirectoryExists(const std::string& path);
+
+  // In general (i.e., not midway through a call to Increment()), there is a
+  // relationship between dirs_ and names_: dirs[i] == names_[i - 1].
+  // For example, say we are asked to iterate "/usr/local" and we're currently
+  // at /usr/local/share/dict/words. dirs_ contains DIR* variables for the
+  // dirs at: {"/usr/local", ".../share", ".../dict"} and names_ contains:
+  // {"share", "dict", "words"}. root_path_ contains "/usr/local".
+  // root_dev_ would be the dev for root_path_
+  // (and /usr/local/share/dict/words). stbuf_ would be the stbuf for
+  // /usr/local/share/dict/words.
+
+  // All opened directories. If this is empty, we're currently on the root,
+  // but not descended into the root.
+  // This will always contain the current directory and all it's ancestors
+  // in root-to-leaf order. For more details, see comment above.
+  std::vector<DIR*> dirs_;
+
+  // The list of all filenames for the current path that we've descended into.
+  std::vector<std::string> names_;
+
+  // The device of the root path we've been asked to iterate.
+  dev_t root_dev_;
+
+  // The root path we've been asked to iteratate.
+  std::string root_path_;
+
+  // Exclude items w/ this prefix.
+  std::set<std::string> excl_prefixes_;
+
+  // The struct stat of the current file we're at.
+  struct stat stbuf_;
+
+  // Generally false; set to true when we reach the end of files to iterate
+  // or error occurs.
+  bool is_end_;
+
+  // Generally false; set to true if an error occurrs.
+  bool is_err_;
+};
+
+}  // namespace chromeos_update_engine
+
+#endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_FILESYSTEM_ITERATOR_H__