Upload a file with ocaml, cohttp, and lwt without reading the entire file to memory.

upload.ml

open Lwt
open Cohttp
open Cohttp_lwt_unix

let read fd () =
  let buffer = Bytes.create 1024 in
  let%lwt len = Lwt_unix.read fd buffer 0 1024 in
  match len with
  | 1024 -> Lwt.return(Some (Bytes.to_string buffer))
  | 0 -> Lwt.return(None)
  | a -> Lwt.return(Some(Bytes.sub_string buffer 0 a))

let upload_file url path =
  let%lwt fd = Lwt_unix.(openfile path [O_RDONLY] 0) in
  let stream = Lwt_stream.from (read fd) in
  let body = Cohttp_lwt.Body.of_stream stream in
  let uri =  url |> Uri.of_string in
  let%lwt (resp, body) = Client.post ~body uri in
  let code = resp |> Response.status |> Code.code_of_status in
  match code with
  | _ ->
    let () = print_endline (string_of_int code) in
    body |> Cohttp_lwt.Body.to_string >|= fun body ->
    body