hostnet: convert Uwt errors to Unix_errors, pretty-print "expected" c…

…ases

Previously we lacked a function to convert from Uwt.error to Unix.error.
Happily uwt.0.0.3 contains such a useful error converting function, so
we use it.

When a port forward is requested, use `Lwt.catch` to handle the exception
at the top-level and pretty-print the error message for the expected cases

- EADDRINUSE: the port was already 'allocated'
- EADDRNOTAVAIL: the interface IP was not found (probably it exists in a
  VM but not on the host)
- EPERM: insufficient privileges to bind a privileged port

The pretty-printing is intended to match the output of `docker` on
regular Linux.

This is related to [docker/compose#3277]

Signed-off-by: David Scott <dave.scott@docker.com>

src/hostnet/lib/forward.ml

@@ -305,30 +305,55 @@ let start_udp_proxy description vsock_path_var remote_port server =
 let start vsock_path_var t =
   match t.local with
   | `Tcp (local_ip, local_port)  ->
-    check_bind_allowed (Ipaddr.V4 local_ip)
-    >>= fun () ->
-    Socket.Stream.Tcp.bind (local_ip, local_port)
-    >>= fun server ->
-    t.server <- Some (`Tcp server);
-    (* Resolve the local port yet (the fds are already bound) *)
-    t.local <- ( match t.local with
-      | `Tcp (local_ip, 0) ->
-        let _, port = Socket.Stream.Tcp.getsockname server in
-        `Tcp (local_ip, port)
-      | _ ->
-        t.local );
-    start_tcp_proxy (to_string t) vsock_path_var t.remote_port server
-    >>= fun () ->
-    Lwt.return (Result.Ok t)
-  | `Udp (local_ip, local_port) ->
-    check_bind_allowed local_ip
-    >>= fun () ->
-    Socket.Datagram.Udp.bind (local_ip, local_port)
-    >>= fun server ->
-    t.server <- Some (`Udp server);
-    start_udp_proxy (to_string t) vsock_path_var t.remote_port server
-    >>= fun () ->
-    Lwt.return (Result.Ok t)
+    Lwt.catch
+      (fun () ->
+        check_bind_allowed (Ipaddr.V4 local_ip)
+        >>= fun () ->
+        Socket.Stream.Tcp.bind (local_ip, local_port)
+        >>= fun server ->
+        t.server <- Some (`Tcp server);
+        (* Resolve the local port yet (the fds are already bound) *)
+        t.local <- ( match t.local with
+          | `Tcp (local_ip, 0) ->
+            let _, port = Socket.Stream.Tcp.getsockname server in
+            `Tcp (local_ip, port)
+          | _ ->
+            t.local );
+        start_tcp_proxy (to_string t) vsock_path_var t.remote_port server
+        >>= fun () ->
+        Lwt.return (Result.Ok t)
+      ) (function
+        | Unix.Unix_error(Unix.EADDRINUSE, _, _) ->
+          Lwt.return (Result.Error (`Msg (Printf.sprintf "Bind for %s:%d failed: port is already allocated" (Ipaddr.V4.to_string local_ip) local_port)))
+        | Unix.Unix_error(Unix.EADDRNOTAVAIL, _, _) ->
+          Lwt.return (Result.Error (`Msg (Printf.sprintf "listen tcp %s:%d: bind: cannot assign requested address" (Ipaddr.V4.to_string local_ip) local_port)))
+        | Unix.Unix_error(Unix.EPERM, _, _) ->
+          Lwt.return (Result.Error (`Msg (Printf.sprintf "Bind for %s:%d failed: permission denied" (Ipaddr.V4.to_string local_ip) local_port)))
+        | e ->
+          Lwt.return (Result.Error (`Msg (Printf.sprintf "Bind for %s:%d: unexpected error %s" (Ipaddr.V4.to_string local_ip) local_port (Printexc.to_string e))))
+      )
+    | `Udp (local_ip, local_port) ->
+      Lwt.catch
+        (fun () ->
+          check_bind_allowed local_ip
+          >>= fun () ->
+          Socket.Datagram.Udp.bind (local_ip, local_port)
+          >>= fun server ->
+          t.server <- Some (`Udp server);
+          start_udp_proxy (to_string t) vsock_path_var t.remote_port server
+          >>= fun () ->
+          Lwt.return (Result.Ok t)
+      ) (function
+        | Unix.Unix_error(Unix.EADDRINUSE, _, _) ->
+          Lwt.return (Result.Error (`Msg (Printf.sprintf "Bind for %s:%d failed: port is already allocated" (Ipaddr.to_string local_ip) local_port)))
+        | Unix.Unix_error(Unix.EADDRNOTAVAIL, _, _) ->
+          Lwt.return (Result.Error (`Msg (Printf.sprintf "listen udp %s:%d: bind: cannot assign requested address" (Ipaddr.to_string local_ip) local_port)))
+        | Unix.Unix_error(Unix.EPERM, _, _) ->
+          Lwt.return (Result.Error (`Msg (Printf.sprintf "Bind for %s:%d failed: permission denied" (Ipaddr.to_string local_ip) local_port)))
+        | e ->
+          Lwt.return (Result.Error (`Msg (Printf.sprintf "Bind for %s:%d: unexpected error %s" (Ipaddr.to_string local_ip) local_port (Printexc.to_string e))))
+      )
+
 
 let stop t =
   Log.debug (fun f -> f "%s: closing listening socket" (to_string t));

src/hostnet/lib/host_uwt.ml

@@ -170,8 +170,9 @@ module Sockets = struct
         let fd = Uwt.Udp.init () in
         let result = Uwt.Udp.bind ~mode:[ Uwt.Udp.Reuse_addr ] fd ~addr:sockaddr () in
         if not(Uwt.Int_result.is_ok result) then begin
-          Log.err (fun f -> f "Socket.Datagram.bind returned %s" (Uwt.strerror (Uwt.Int_result.to_error result)));
-          Lwt.fail (Uwt.Int_result.to_exn result)
+          let error = Uwt.Int_result.to_error result in
+          Log.err (fun f -> f "Socket.Udp.bind(%s, %d): %s" (Ipaddr.to_string ip) port (Uwt.strerror error));
+          Lwt.fail (Unix.Unix_error(Uwt.to_unix_error error, "bind", ""))
         end else Lwt.return { fd; closed = false }
 
       let of_bound_fd fd =
@@ -351,21 +352,23 @@ module Sockets = struct
           let error = Uwt.Int_result.to_error result in
           let msg = Printf.sprintf "Socket.Tcp.bind(%s, %d): %s" (Ipaddr.to_string ip) port (Uwt.strerror error) in
           Log.err (fun f -> f "%s" msg);
-          if error = Uwt.EADDRINUSE
-          then Lwt.fail (Unix.Unix_error(Unix.EADDRINUSE, "bind", ""))
-          else Lwt.fail (Failure msg)
+          Lwt.fail (Unix.Unix_error(Uwt.to_unix_error error, "bind", ""))
         end else Lwt.return fd
 
       let bind (ip, port) =
         bind_one (Ipaddr.V4 ip, port)
         >>= fun fd ->
-        let local_port = match Uwt.Tcp.getsockname fd with
+        ( match Uwt.Tcp.getsockname fd with
           | Uwt.Ok sockaddr ->
             begin match ip_port_of_sockaddr sockaddr with
-              | Some (_, local_port) -> local_port
+              | Some (_, local_port) -> Lwt.return local_port
               | _ -> assert false
             end
-          | _ -> assert false in
+          | Uwt.Error error ->
+            let msg = Printf.sprintf "Socket.Tcp.bind(%s, %d): %s" (Ipaddr.V4.to_string ip) port (Uwt.strerror error) in
+            Log.err (fun f -> f "%s" msg);
+            Lwt.fail (Unix.Unix_error(Uwt.to_unix_error error, "bind", "")) )
+        >>= fun local_port ->
         (* On some systems localhost will resolve to ::1 first and this can
            cause performance problems (particularly on Windows). Perform a
            best-effort bind to the ::1 address. *)