cohttp-eio: Allow streaming responses from handler

This changes the response type to `writer -> unit`. This allows
handlers to write the response inside the function, rather than
returning a request to cohttp to write it later. That's useful because
it allows e.g. streaming from an open file and then closing it
afterwards.

Partial application means that code using `respond_string` etc will
continue to work as before.

This also exposes a more polymorphic version of the `respond` function
that accepts sub-types of `Flow.source`, so that callers don't need to
cast the body.

CHANGES.md

@@ -1,5 +1,6 @@
 ## Unreleased
 
+- cohttp-eio: Make server response type abstract and allow streaming in cohttp-eio (talex5 #1024)
 - cohttp-eio: Improve error handling in example server (talex5 #1023)
 - cohttp-eio: Don't blow up `Server.callback` on client disconnections. (mefyl #1015)
 - http: Fix assertion in `Source.to_string_trim` when `pos <> 0` (mefyl #1017)

cohttp-eio/examples/dune

@@ -1,5 +1,5 @@
 (executables
- (names server1 client1 docker_client client_timeout client_tls)
+ (names server1 server2 client1 docker_client client_timeout client_tls)
  (libraries
   cohttp-eio
   eio_main

cohttp-eio/examples/server2.ml

@@ -0,0 +1,37 @@
+let () = Logs.set_reporter (Logs_fmt.reporter ())
+and () = Logs.Src.set_level Cohttp_eio.src (Some Debug)
+
+let ( / ) = Eio.Path.( / )
+
+(* To stream a file, we take the extra [writer] argument explicitly.
+   This means that we stream the response while the function is still
+   running and the file is still open. *)
+let handler dir _socket request _body writer =
+  let path =
+    Http.Request.resource request
+    |> String.split_on_char '/'
+    |> List.filter (( <> ) "")
+    |> String.concat "/"
+  in
+  let path = if path = "" then "index.html" else path in
+  Eio.Path.with_open_in (dir / path) @@ fun flow ->
+  Cohttp_eio.Server.respond () ~status:`OK
+    ~headers:(Http.Header.of_list [ ("content-type", "text/html") ])
+    ~body:flow writer
+
+let log_warning ex = Logs.warn (fun f -> f "%a" Eio.Exn.pp ex)
+
+let () =
+  let port = ref 8080 in
+  Arg.parse
+    [ ("-p", Arg.Set_int port, " Listening port number(8080 by default)") ]
+    ignore "An HTTP/1.1 server";
+  Eio_main.run @@ fun env ->
+  Eio.Switch.run @@ fun sw ->
+  (* Restrict to current directory: *)
+  let htdocs = Eio.Stdenv.cwd env in
+  let socket =
+    Eio.Net.listen env#net ~sw ~backlog:128 ~reuse_addr:true
+      (`Tcp (Eio.Net.Ipaddr.V4.loopback, !port))
+  and server = Cohttp_eio.Server.make ~callback:(handler htdocs) () in
+  Cohttp_eio.Server.run socket server ~on_error:log_warning

cohttp-eio/src/server.ml

@@ -3,26 +3,29 @@ module IO = Io.IO
 
 type body = Body.t
 type conn = IO.conn * Cohttp.Connection.t [@@warning "-3"]
-type response = Http.Response.t * Body.t
+type writer = IO.oc
+type response = writer -> unit
 
 type response_action =
   [ `Expert of Http.Response.t * (IO.ic -> IO.oc -> unit)
-  | `Response of Http.Response.t * body ]
-
-(* type handler =
- *   sw:Eio.Switch.t ->
- *   Eio.Net.Sockaddr.stream ->
- *   Http.Request.t ->
- *   Eio.Flow.source ->
- *   Http.Response.t * Eio.Flow.source *)
+  | `Response of response ]
 
 type t = {
   conn_closed : conn -> unit;
-  handler : conn -> Http.Request.t -> body -> response_action;
+  handler : conn -> Http.Request.t -> body -> IO.ic -> IO.oc -> unit;
 }
 
 let make_response_action ?(conn_closed = fun _ -> ()) ~callback () =
-  { conn_closed; handler = callback }
+  {
+    conn_closed;
+    handler =
+      (fun conn request body ic oc ->
+        match callback conn request body with
+        | `Expert (response, handler) ->
+            Io.Response.write_header response oc;
+            handler ic oc
+        | `Response fn -> fn oc);
+  }
 
 let make_expert ?conn_closed ~callback () =
   make_response_action ?conn_closed
@@ -31,12 +34,11 @@ let make_expert ?conn_closed ~callback () =
       `Expert expert)
     ()
 
-let make ?conn_closed ~callback () =
-  make_response_action ?conn_closed
-    ~callback:(fun conn request body ->
-      let response = callback conn request body in
-      `Response response)
-    ()
+let make ?(conn_closed = fun _ -> ()) ~callback () =
+  {
+    conn_closed;
+    handler = (fun conn request body _ic oc -> callback conn request body oc);
+  }
 
 let read input =
   match Io.Request.read input with
@@ -88,9 +90,9 @@ let write output (response : Cohttp.Response.t) body =
   in
   Eio.Buf_write.flush output
 
-let respond ?headers ?flush ~status ~body () =
+let respond ?headers ?flush ~status ~body () oc =
   let response = Cohttp.Response.make ?headers ?flush ~status () in
-  (response, body)
+  write oc response body
 
 let respond_string ?headers ?flush ~status ~body () =
   respond ?headers ?flush ~status ~body:(Body.of_string body) ()
@@ -117,12 +119,7 @@ let callback { conn_closed; handler } ((_, peer_address) as conn) input output =
           (Body.of_string e)
     | `Ok (request, body) ->
         let () =
-          try
-            match handler (conn, id) request body with
-            | `Response (response, body) -> write output response body
-            | `Expert (response, handler) ->
-                let () = Io.Response.write_header response output in
-                handler input output
+          try handler (conn, id) request body input output
           with Eio.Io (Eio.Net.E (Connection_reset _), _) ->
             Logs.info (fun m ->
                 m "%a: connection reset" Eio.Net.Sockaddr.pp peer_address)

cohttp-eio/src/server.mli

@@ -1,8 +1,18 @@
+type writer
+
 include
   Cohttp.Generic.Server.S
     with module IO = Io.IO
      and type body = Body.t
-     and type response = Http.Response.t * Body.t
+     and type response = writer -> unit
+
+val respond :
+  ?headers:Http.Header.t ->
+  ?flush:bool ->
+  status:Http.Status.t ->
+  body:_ Eio.Flow.source ->
+  unit ->
+  response IO.t
 
 val run :
   ?max_connections:int ->

cohttp-eio/tests/test.ml

@@ -1,5 +1,3 @@
-open Eio.Std
-
 let () =
   Logs.set_level ~all:true @@ Some Logs.Debug;
   Logs.set_reporter (Logs_fmt.reporter ())
@@ -13,7 +11,7 @@ let handler _conn request body =
         Eio_mock.Flow.on_read body
           [ `Return "Hello"; `Yield_then (`Return "World") ]
       in
-      Cohttp_eio.Server.respond ~status:`OK ~body:(body :> Eio.Flow.source_ty r) ()
+      Cohttp_eio.Server.respond ~status:`OK ~body ()
   | "/post" -> Cohttp_eio.Server.respond ~status:`OK ~body ()
   | _ -> Cohttp_eio.Server.respond_string ~status:`Not_found ~body:"" ()