package async_extra

For communicating a dynamically chosen TCP port from a child process to its parent.

This is used to fork+exec a child process that will create create a TCP server that listens to a dynamically chosen port, and to make the port number available in the parent process once the child process is listening on the port.

Here is the intended usage:

1. The parent creates a Dynamic_port_writer.t value together with a deferred that will eventually be determined with the port assigned to the child process by the OS.

2. The parent communicates the Dynamic_port_writer.t value to a child it has spawned. This can happen in a number of ways,

• via the child's command line using to_string and either arg or of_string.
• via a config file using "with sexp"
• over the wire using "with bin_io"

3. The child calls Tcp.Server.create with the value returned by where_to_listen.

Once the server created in step (3) is listening on its OS-assigned port, the parent's deferred obtained in step (1) will soon become determined with the value of the port.

Code for the parent process would look something like:

Dynamic_port_writer.create ()
>>= fun (dynamic_port_writer, port_d) ->
Unix.fork_exec ~prog
  ~argv:([ prog ]
         @ Dynamic_port_writer.flag_args dynamic_port_writer
         @ [ ... other args ... ])
  ()
>>= fun _child_pid ->
port_d
>>= fun r ->
let `Port port = ok_exn r in
Tcp.connect (Tcp.Where_to_connect.of_host_and_port {host = "localhost"; port})
>>= fun (_, reader, writer) ->
...

Code for the Command.t for the child process would look something like:

Command.basic
  ~summary:"child"
  (Command.Spec.(empty +> Dynamic_port_writer.flag))
  (fun dynamic_port_writer () ->
     ...
     Tcp.Server.create
         (Dynamic_port_writer.where_to_listen dynamic_port_writer)
         (fun _ reader writer -> ...))