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(* The trick for generating a fresh port relies on a non-POSIX but
   standard behavior implemented by many operating systems, including
   Linux and macOS.

   [Unix.bind] binds the address (IP with port) given in parameters to
   the socket. If the port of the address given is [0], a fresh port
   is used, then the operating system actually replace the port with a
   fresh port which is known not used by the operating
   system. Consequently, to generate a fresh port, we open a socket,
   bind it to a fake address with port [0]. Ask for the address of the
   socket and return the port given by the operating system. Finally,
   we close the socket.
*)
let fresh () =
  let dummy_socket = Unix.(socket PF_INET SOCK_STREAM 0) in
  Fun.protect ~finally:(fun () -> Unix.close dummy_socket) @@ fun () ->
  Unix.bind dummy_socket Unix.(ADDR_INET (inet_addr_loopback, 0)) ;
  let addr = Unix.getsockname dummy_socket in
  match addr with ADDR_INET (_, port) -> port | _ -> assert false