Hello!  I see that there is some support for using dbus message fd lists in codegen/valagdbus*.vala.  But it is really limited right now.

It looks like the return type of the dbus method has to be exactly "h".  Some potentially more complicated but interesting types that dbus_method_uses_file_descriptor() will return false for:
"(hs)"
"a(hs)"
"v" that happens to have an "h" stuffed inside

The only way to get support for methods that return these seems to be to write the dbus server code manually, without any of the magic that vala can bring.

I understand that the last example (v with h inside) will be harder to detect automatically.  I'd be happy with just more extensive static analysis of the return type.  But dynamic analysis would also be neat.