Lwt 和 Cohttp："致命错误：异常Unix.Unix_error(Unix.ECONNRESET， "read" ， " " )"

Question

我有一个简单的HTTP服务器在Ocaml与Cohttp和Lwt。当我运行wrk时，一旦wrk完成，应用程序就会在大约50%的时间内崩溃。我想崩溃是由意外断开连接触发的。

我在控制台上看到以下错误:

Fatal error: exception Unix.Unix_error(Unix.ECONNRESET, "read", "")
Raised by primitive operation at file "src/unix/lwt_bytes.ml", line 130, characters 42-84
Called from file "src/unix/lwt_unix.ml", line 489, characters 13-24

有办法防止这种情况吗?

我的完整源代码是:

(* server_test.ml *)
open Unix
open Lwt
open Cohttp
open Cohttp_lwt_unix
open Yojson
open Yojson.Basic.Util
open Core.Std
type create = {
username: string;
email: string;
password: string;
} [@@deriving yojson]
let insert coll doc =
    let _id = Core.Std.Uuid.to_string (Uuid.create ()) in
    let uri = Uri.make ~scheme:"http" ~host:"127.0.0.1" ~port:5984 ~path:(coll ^ "/" ^ _id) () in
    Cohttp_lwt_unix.Client.put ~body:(Cohttp_lwt_body.of_string (Yojson.Safe.to_string doc)) uri
    >|= fun (r, _) -> Code.code_of_status @@ Response.status r
let callback _conn req body =
    body |> Cohttp_lwt_body.to_string 
    >>= (fun body -> 
        let mc = Yojson.Safe.from_string body |> create_of_yojson in
        match mc with
        | Ok c -> 
            insert "users" (create_to_yojson c)
            >>= fun status -> print_endline @@ string_of_int status; 
                Server.respond_string ~status:(`Code status) ~body:(string_of_int status) ()
        | _ -> Server.respond_string ~status:`OK ~body: "Not OK" ())
let timeit _conn req body =
    let start = Unix.gettimeofday () in
    callback _conn req body 
    >>= 
    fun result ->
        let finish = Unix.gettimeofday () in
        Lwt_io.printlf "Execution time took %fms" ((finish -. start) *. 1000.0)
        >|= fun _ -> result
let server =
    Server.create ~mode:(`TCP (`Port 8000)) (Server.make timeit ())
let () = ignore (Lwt_main.run server)

谢谢!

Answer 1

您看到的错误是客户端意外断开连接时引发的非手动异常。相关的异常被传递给Lwt的异步异常钩子(http://ocsigen.org/lwt/2.6.0/api/Lwt#VALasync_exception_hook)，在Lwt的默认情况下，该钩子打印一个反向跟踪并退出程序，退出代码为2。

在cohttp github问题跟踪器上有一个正在进行的讨论:https://github.com/mirage/ocaml-cohttp/issues/511

简而言之，如果你为Lwt的async/"background"线程定义一个自定义异常处理程序，那么你可以捕获和忽略/记录/处理客户端错误。在启动cohttp服务器之前添加以下内容:

Lwt.async_exception_hook := (function
  | Unix.Unix_error (error, func, arg) ->
    Logs.warn (fun m ->
      m  "Client connection error %s: %s(%S)"
        (Unix.error_message error) func arg
    )
  | exn -> Logs.err (fun m -> m "Unhandled exception: %a" Fmt.exn exn)
);

取自https://github.com/mirage/ocaml-cohttp/issues/511#issuecomment-258510531并使用logs库来记录事件:http://erratique.ch/software/logs