Tidyups to 3722f04 and mirage#53 fix

* Add some comments about how the DHCP thread should background
* Do not `choose` between listen/configure, since that causes a
  cancellation when one succeeds.

lib/tcpip_stack_direct.ml

@@ -81,8 +81,11 @@ module Make
   let configure t config =
     match config with
     | `DHCP -> begin
+        (* TODO: spawn a background thread to reconfigure the interface
+           when future offers are received. *)
         let dhcp, offers = Dhcp.create t.c t.ipv4 t.udpv4 in
         listen_udpv4 t 68 (Dhcp.input dhcp);
+        (* TODO: stop listening to this port when done with DHCP. *)
         Lwt_stream.get offers
         >>= function
         | None -> fail (Failure "No DHCP offer received")
@@ -143,16 +146,22 @@ module Make
     let tcpv4_listeners = Hashtbl.create 7 in
     let t = { id; c; mode; netif; ethif; ipv4; tcpv4; udpv4;
       udpv4_listeners; tcpv4_listeners } in
-    ( Console.log_s t.c "Manager: configuring" >>
-      choose [ listen t;
-      configure t t.mode ] >> (* TODO: this is fine for now, because the 
-      DHCP state machine isn't fully implemented and its thread will terminate 
-      after one successful lease transaction.  For a DHCP thread that runs 
-      forever, `configure` will not terminate, and the client application will
-      never run. *)
-      Console.log_s t.c "Manager: configuration done") >> 
+    Console.log_s t.c "Manager: configuring"
+    >>= fun () ->
+    let _ = listen t in
+    configure t t.mode
+    >>= fun () ->
+    (* TODO: this is fine for now, because the DHCP state machine isn't fully 
+       implemented and its thread will terminate after one successful lease 
+       transaction.  For a DHCP thread that runs forever, `configure` will need
+       to spawn a background thread, but we need to consider how to inform the
+       application stack that the IP address has changed (perhaps via a control
+       Lwt_stream that the application can ignore if it doesn't care). *)
+    Console.log_s t.c "Manager: configuration done"
+    >>= fun () ->
     return (`Ok t)
 
   let disconnect t =
-    return ()
+    (* TODO: kill the listening thread *)
+    Console.log_s t.c "Manager: disconnect"
 end