Re: GVariant support for Unix fds (Was Re: GDBus/GVariant plans for next GLib release)

[Ryan Lortie <desrt desrt ca>]

On Tue, 2009-10-20 at 11:17 -0400, David Zeuthen wrote:
> So how about something like 1. and 2. below? We'd put
> 
>  g_dbus_connection_get_handle_for_unix_fd()
>  g_dbus_connection_set_handle_for_unix_fd()

I was actually thinking that we could introduce a GDBusFDSet:

gint g_dbus_fd_set_add_fd (gint fd) {
  list_of_fds[fd_count] = dup (fd);
  return fd_count++;
}

gint g_dbus_fd_set_get_fd (gint index) {
  return dup (list_of_fds[index]);
}

g_dbus_fd_set_finalize () {
  foreach (fd in list_of_fds) close (fd);
}

You'd add fds to that and get ints back (starting at 0 per the set).
Then when sending the dbus message you'd give the GVariant and the
DBusFDSet to the appropriate function.

  var fd = open("file");
  var fdset = new FDSet ();
  var message = new GVariant ("(sh)",
                              "hello world",
                              fdset.add_fd (fd));

  g_dbus_proxy_invoke_with_fds (proxy, "Foo", message, fdset, ...);

When receiving messages, you get a DBusFDSet handed to you in the event
that there were file descriptors (or just %NULL in case not).

The fds held in the GDBusFDSet are closed when the set is finalized.
There is no doubt about who owns which fd in this case -- and in the
case you elect to ignore fds that were sent to you then everything is
still cleaned up properly.

We could also have _steal() and _give() API variants.  _steal would take
the fd out of the set (so the set no longer owns it, no longer closes
it) and _give would let you transfer ownership of an fd into the set --
we save a few syscalls that way in a few cases, I guess.

btw: I don't know if passing HANDLE on Windows is supported (and I would
guess not).  Passing a G[Input|Output|IO]Stream is certainly not
supported (in a portable way, anyway).

Cheers