Allow creating cohttp-async servers with existing reader/writers

CHANGES.md

@@ -17,6 +17,8 @@
     when deciding whether to use keep-alive. This allows a handler to decide to
     close a connection even if the client requested a keep-alive in the
     request.
+- async(server): allow creating a server without using conduit (anuragsoni, #839)
+  + Add `Cohttp_async.Server.Expert.create` and `Cohttp_async.Server.Expert.create_expert`that can be used to create a server without going through Conduit. This allows creating an async TCP server using the Tcp module from `Async_unix` and lets the user have more control over how the `Reader.t` and `Writer.t` are created.
 - *Breaking changes*
   + refactor: move opam metadata to dune-project (rgrinberg #811)
   + refactor: deprecate Cohttp_async.Io (rgrinberg #807)

cohttp-async/src/server.ml

@@ -149,3 +149,14 @@ let create ?max_connections ?backlog ?buffer_age_limit ?(mode = `TCP)
   in
   create_raw ?max_connections ?backlog ?buffer_age_limit ~on_handler_error ~mode
     where_to_listen handle_request
+
+module Expert = struct
+  let create handle_request addr reader writer =
+    let handle_request ~body addr request =
+      handle_request ~body addr request >>| fun r -> `Response r
+    in
+    handle_client handle_request addr reader writer
+
+  let create_expert handle_request addr reader writer =
+    handle_client handle_request addr reader writer
+end

cohttp-async/src/server.mli

@@ -104,3 +104,31 @@ val create :
   response Async_kernel.Deferred.t) ->
   ('address, 'listening_on) t Async_kernel.Deferred.t
 (** Build a HTTP server, based on the [Tcp.Server] interface *)
+
+module Expert : sig
+  val create :
+    (body:Body.t -> 'addr -> Http.Request.t -> response Async_kernel.Deferred.t) ->
+    'addr ->
+    Async_unix.Reader.t ->
+    Async_unix.Writer.t ->
+    unit Async_kernel.Deferred.t
+  (** [create] accepts a user provided cohttp handler, and creates a server
+      callback that works with user provided socket address,
+      [Async_unix.Reader.t] and [Async_unix.Writer.t]. This can be useful if
+      there is a need for more control over how the Reader and Writer get
+      created. *)
+
+  val create_expert :
+    (body:Body.t ->
+    'addr ->
+    Http.Request.t ->
+    response_action Async_kernel.Deferred.t) ->
+    'addr ->
+    Async_unix.Reader.t ->
+    Async_unix.Writer.t ->
+    unit Async_kernel.Deferred.t
+  (** [create_expert] is similar to [create] but the user provided handler can
+      use [Cohttp_async.Server.response_action], and has access to using the
+      Expert mode response that can access the underlying reader/writer pair
+      from within the http handler. *)
+end