Support sending/receiving file descriptors over sockets

[robert-ancell]

On Linux you are able to send and receive file descriptors using Unix domain sockets. This is done using sendmsg/recvmsg instead of sendto/recvfrom. This is desirable when interfacing with services that make use of this functionality, for example DBus.

A proposed API would be something like:

var socketAddress = InternetAddress('/var/run/dbus/system_bus_socket', type: InternetAddressType.unix);
var socket = await RawSocket.connect(socketAddress, 0);
var data = [0];
var fd = getFileDescriptorFromSomewhere();
var message = SocketMessage([data], controlMessages: [SocketFileDescriptorMessage([fd])]);
socket.writeMessage(message);

var message = socket.readMessage();
for (var data in message.vectors) {
  processData(data);
}
for (var controlMessage in message.controlMessages) {
  if (message is SocketFileDescriptorMessage) {
    processFileDescriptors(message.fileDescriptors);
  }
}

This API is similar to the one used in GLib.

This new API would throw an exception on platforms that didn't support it.

[robert-ancell]

See associated issue for making use of file descriptors in Dart.

[robert-ancell]

I'll give a real world example of how this API will be consumed in DBus. D-DBus is the IPC mechanism used in Linux, and allows messages to be sent between processes containing file descriptors as well as common data types (integers, strings, arrays etc).

The first step is to update dbus.dart with Unix file descriptor support. This draft PR currently used integers for the file descriptors, which would need to be updated with the objects that have been proposed as a solution to hide these integers from Dart code.

An app could then send and receive messages containing file descriptors. The following example shows installing a firmware file with the firmware update daemon. This is triggered by making an IPC method call with the file descriptor of the firmware file. This allows the server process to read the file without sending the contents over IPC (which would be inefficient).

import 'package:dbus/dbus.dart';

// Load a firmware file.
var file = await File('v42.firmware').open();

// Ask the firmware update daemon to install this firmware. The file is passed as a file descriptor and will be read from the server process.
var client = DBusClient.system();
var fwupd = DBusRemoteObject(client, destination: 'org.freedesktop.fwupd', path: DBusObjectPath('/'));
await fwupd.callMethod('org.freedesktop.fwupd', 'Install', [DBusString('92ed499703fad3db5e91533af659487e89ab840d'), DBusUnixFd(file.fd), DBusDict.stringVariant({})]); 

[aam]

@robert-ancell this has landed in dart and flutter. Can you let us know if what landed meets you needs please?

[robert-ancell]

@robert-ancell this has landed in dart and flutter. Can you let us know if what landed meets you needs please?

Thanks @aam! I've updated the dbus.dart branch and it's working great.