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
        ~args:([ prog ]
               @ Dynamic_port_writer.flag_args dynamic_port_writer
               @ [ ... other args ... ])
      >>= fun _child_pid ->
      >>= fun r ->
      let `Port port = ok_exn r in
      Tcp.connect (Tcp.to_host_and_port "localhost" port)
      >>= fun (_, reader, writer) ->

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

        (Command.Spec.(empty +> Dynamic_port_writer.flag))
        (fun dynamic_port_writer () ->
               (Dynamic_port_writer.where_to_listen dynamic_port_writer)
               (fun _ reader writer -> ...))
type t
include Core.Std.Stringable with type t := t
val create : unit ->
(t * [ `Port of int ] Core.Std.Or_error.t Import.Deferred.t)
val where_to_listen : t -> (Import.Socket.Address.Inet.t, int) Tcp.Where_to_listen.t
val arg : t Core.Std.Command.Spec.Arg_type.t
For use with Command.
val flag : t Core.Std.Command.Spec.param
One can pass a t from parent to child by including flag_args t in the command-line arguments and using flag in the Command.t in the child.
val flag_args : t -> string list
val t_of_sexp : Sexplib.Sexp.t -> t
val sexp_of_t : t -> Sexplib.Sexp.t
val bin_t : t Core.Std.Bin_prot.Type_class.t
val bin_read_t : t Core.Std.Bin_prot.Read.reader
val __bin_read_t__ : (int -> t) Core.Std.Bin_prot.Read.reader
val bin_reader_t : t Core.Std.Bin_prot.Type_class.reader
val bin_size_t : t Core.Std.Bin_prot.Size.sizer
val bin_write_t : t Core.Std.Bin_prot.Write.writer
val bin_writer_t : t Core.Std.Bin_prot.Type_class.writer