diff --git a/file_descriptor.h b/file_descriptor.h
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+// Copyright (c) 2012 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 CHROMEOS_PLATFORM_UPDATE_ENGINE_FILE_DESCRIPTOR_H__
+#define CHROMEOS_PLATFORM_UPDATE_ENGINE_FILE_DESCRIPTOR_H__
+
+#include <errno.h>
+#include <sys/types.h>
+
+#include "base/logging.h"
+
+#include "update_engine/utils.h"
+
+// Abstraction for managing opening, reading, writing and closing of file
+// descriptors. This includes an abstract class and one standard implementation
+// based on POSIX system calls.
+//
+// TODO(garnold) this class is modeled after (and augments the functionality of)
+// the FileWriter class; ultimately, the latter should be replaced by the former
+// throughout the codebase.  A few deviations from the original FileWriter:
+//
+// * Providing two flavors of Open()
+//
+// * A FileDescriptor is reusable and can be used to read/write multiple files
+//   as long as open/close preconditions are respected.
+//
+// * Write() returns the number of bytes written: this appears to be more useful
+//   for clients, who may wish to retry or otherwise do something useful with
+//   the remaining data that was not written.
+
+namespace chromeos_update_engine {
+
+// An abstract class defining the file descriptor API.
+class FileDescriptor {
+ public:
+  FileDescriptor() {}
+  virtual ~FileDescriptor() {}
+
+  // Opens a file descriptor. The descriptor must be in the closed state prior
+  // to this call. Returns true on success, false otherwise. Specific
+  // implementations may set errno accordingly.
+  virtual bool Open(const char* path, int flags, mode_t mode) = 0;
+  virtual bool Open(const char* path, int flags) = 0;
+
+  // Reads from a file descriptor up to a given count. The descriptor must be
+  // open prior to this call. Returns the number of bytes read, or -1 on error.
+  // Specific implementations may set errno accordingly.
+  virtual ssize_t Read(void* buf, size_t count) = 0;
+
+  // Writes to a file descriptor. The descriptor must be open prior to this
+  // call. Returns the number of bytes written, or -1 if an error occurred and
+  // no bytes were written. Specific implementations may set errno accordingly.
+  virtual ssize_t Write(const void* buf, size_t count) = 0;
+
+  // Wrapper around close. The descriptor must be open prior to this call.
+  // Returns true on success, false otherwise. Specific implementations may set
+  // errno accordingly.
+  virtual bool Close() = 0;
+
+  // Indicates whether or not an implementation sets meaningful errno.
+  virtual bool IsSettingErrno() = 0;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(FileDescriptor);
+};
+
+// A simple EINTR-immune wrapper implementation around standard system calls.
+class EintrSafeFileDescriptor : public FileDescriptor {
+ public:
+  EintrSafeFileDescriptor() : fd_(-1) {}
+  virtual ~EintrSafeFileDescriptor() {};
+
+  // Interface methods.
+  virtual bool Open(const char* path, int flags, mode_t mode);
+  virtual bool Open(const char* path, int flags);
+  virtual ssize_t Read(void* buf, size_t count);
+  virtual ssize_t Write(const void* buf, size_t count);
+  virtual bool Close();
+  virtual bool IsSettingErrno() {
+    return true;
+  }
+
+ private:
+  int fd_;
+};
+
+// A scoped closer for a FileDescriptor object.
+class ScopedFileDescriptorCloser {
+ public:
+  explicit ScopedFileDescriptorCloser(FileDescriptor* descriptor)
+      : descriptor_(descriptor) {}
+  ~ScopedFileDescriptorCloser() {
+    if (descriptor_ && !descriptor_->Close())
+      LOG(ERROR) << "FileDescriptor::Close failed: "
+                 << (descriptor_->IsSettingErrno() ?
+                     utils::ErrnoNumberAsString(errno) :
+                     "(no error code)");
+  }
+ private:
+  FileDescriptor* descriptor_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedFileDescriptorCloser);
+};
+
+}  // namespace chromeos_update_engine
+
+#endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_FILE_DESCRIPTOR_H__