dbus: add support for passing file descriptors

dbus/file_descriptor.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 DBUS_FILE_DESCRIPTOR_H_
+#define DBUS_FILE_DESCRIPTOR_H_
+#pragma once
+
+namespace dbus {
+
+// FileDescriptor is a type used to encapsulate D-Bus file descriptors
+// and to follow the RAII idiom appropiate for use with message operations
+// where the descriptor might be easily leaked.  To guard against this the
+// descriptor is closed when an instance is destroyed if it is owned.
+// Ownership is asserted only when PutValue is used and TakeValue can be
+// used to take ownership.
+//
+// For example, in the following
+//  FileDescriptor fd;
+//  if (!reader->PopString(&name) ||
+//      !reader->PopFileDescriptor(&fd) ||
+//      !reader->PopUint32(&flags)) {
+// the descriptor in fd will be closed if the PopUint32 fails.  But
+//   writer.AppendFileDescriptor(dbus::FileDescriptor(1));
+// will not automatically close "1" because it is not owned.

[satorux1, 2012/03/28 20:49:37]

Oh I see. This comment explains why owner_ is needed. Thanks.

+class FileDescriptor {
+ public:
+  // Permits initialization without a value for passing to
+  // dbus::MessageReader::PopFileDescriptor to fill in and from int values.
+  FileDescriptor() : value_(-1), owner_(false) {}
+  explicit FileDescriptor(int value) : value_(value), owner_(false) {}
+
+  virtual ~FileDescriptor();
+
+  // Retrieves value as an int without affecting ownership.
+  int value() const { return value_; }
+
+  // Sets the value and assign ownership.
+  void PutValue(int value) {

[satorux1, 2012/03/28 20:49:37]

I found it confusing. For this, TakeValue() sounds more natural to me, as *this
object* takes the ownership. I'd go with

PutValue(int) -> Take(int)
TakeValue() -> Release().

[satorux1, 2012/03/29 02:46:48]

This question wasn't answered. I wanted it to be fixed...

+    value_ = value;
+    owner_ = true;
+  }
+
+  // Takes the value and ownership.
+  int TakeValue() {
+    owner_ = false;
+    return value_;
+  }
+
+ private:
+  int value_;
+  bool owner_;
+
+  DISALLOW_COPY_AND_ASSIGN(FileDescriptor);
+};
+
+}  // namespace dbus
+
+#endif  // DBUS_FILE_DESCRIPTOR_H_