create HTTP redirect example

[avsm]

The trivial one used by realworldocaml.org to get HTTP->HTTPS fails every so often with exception leakage.

open Lwt
open Printf
open Cohttp
open Cohttp_lwt_unix

(* main callback function *)
let callback con_id ?body req =
  let path = Uri.path (Request.uri req) in
  Server.respond_redirect ~uri:(Uri.of_string ("https://realworldocaml.org" ^ path))  ()

let _ =
  Sys.set_signal Sys.sigpipe Sys.Signal_ignore;
  let conn_closed con_id () = () in
  let spec = { Cohttp_lwt_unix.Server.callback; conn_closed } in
  Lwt_main.run (Cohttp_lwt_unix.Server.create ~address:"0.0.0.0" ~port:80 spec)

It fails sometimes with

# ./redirect.native 
Fatal error: exception Unix.Unix_error(15, "accept", "")

[edwintorok]

On 1 August 2015 12:05:32 EEST, Anil Madhavapeddy [email protected] wrote:

The trivial one used by realworldocaml.org to get HTTP->HTTPS fails every so often with exception leakage.

open Lwt
open Printf
open Cohttp
open Cohttp_lwt_unix

(* main callback function *)
let callback con_id ?body req =
 let path = Uri.path (Request.uri req) in
Server.respond_redirect ~uri:(Uri.of_string
("https://realworldocaml.org" ^ path))  ()

let _ =
 Sys.set_signal Sys.sigpipe Sys.Signal_ignore;
 let conn_closed con_id () = () in
 let spec = { Cohttp_lwt_unix.Server.callback; conn_closed } in
Lwt_main.run (Cohttp_lwt_unix.Server.create ~address:"0.0.0.0" ~port:80
spec)

It fails sometimes with

# ./redirect.native 
Fatal error: exception Unix.Unix_error(15, "accept", "")

The error message is a little confusing, because errno number 15 doesn't make sense in this context, turns out it is EMFILE.

Perhaps conduit should provide a way to limit the number of active accepted connections, and when over limit wait until another conn is closed. Ocsigenserver limits it to about 350 active connections (configurable) IIRC, and then you have plenty file descriptors left to respond to the request. Different OSes have lower uimit defaults (BSD?), so the default should probably be proportional to RLIMIT_NOFILE.

Either way the server should not die due to EMFILE, if it still gets EMFILE from accept then it should wait until one active conn is closed and retry the accept.

Sent from my Android device with K-9 Mail. Please excuse my brevity.

[rathereasy]

I'm getting EMFILE on my server every so often and it makes the server unresponsive. How do I fix this?

[rgrinberg]

@edwintorok Doesn't your conduit patch help mitigate this?

[rgrinberg]

@rathereasy Could you give this a try with the latest version of conduit and see if it helps.

[edwintorok]

@rgrinberg the latest conduit fixes the leak in the OpenSSL-based server, which might help @rathereasy, however the server in this bug is an HTTP one, which can run out of file descriptors if there are too many clients connecting at the same time. The problem in this bug should be mitigated by https://github.com/mirage/ocaml-conduit/pull/116, I'll look into updating that PR.

[mseri]

Could this be related to https://github.com/mirage/ocaml-cohttp/issues/503 ?