I'm having trouble with OCaml Async. I wrote a small server with it, and right now I'm trying to unit test that server. Here's my code for the server:

let _main ()=
  print_endline "Server running";
  let handler = print_endline in
  let socket = Tcp.on_port 5554 in
  let server = Tcp.Server.create socket (fun addr r w ->
      (Reader.contents r) >>| handler; (Writer.write w "got it")) in
  server

In my unit test code I have:

let test_shutdown test_ctxt = Thread_safe.block_on_async_exn (fun () -> (
      print_endline "test_shutdown";
      let server = Server._main () in
      server >>= fun server ->
      let where = Tcp.to_host_and_port "127.0.0.1" 5554 in
      Tcp.connect where >>= fun s -> 
        let socket, r, w = s in
        ignore (Writer.write w "kill");
        ignore (Writer.flushed w);
        (Reader.recv r >>> function
          | `Ok result ->  print_endline ("writing shutdown to server" ^ result)
          | `Eof -> ());
        return ()
    )); ()

I see test_shutdown and Server running, but not sign of "writing shutdown to server" or even "got it"; why isn't my server or even any of the connection executing?