feat: add ws endpoint detection function

2 files changed, 39 insertions, 5 deletions

diff --git a/lib/portal/dune b/lib/portal/dune
index 47b59ce..8ce0232 100644
--- a/lib/portal/dune
+++ b/lib/portal/dune
@@ -2,5 +2,5 @@
  (name portal_ws)
  (modules portal_ws)
  (public_name flesh_websockets)
- (libraries lwt js_of_ocaml)
+ (libraries lwt js_of_ocaml js_of_ocaml-lwt xmlm)
  (preprocess (pps js_of_ocaml-ppx)))
diff --git a/lib/portal/portal_ws.ml b/lib/portal/portal_ws.ml
index f2c0cf5..c6dbf81 100644
--- a/lib/portal/portal_ws.ml
+++ b/lib/portal/portal_ws.ml
@@ -1,13 +1,47 @@
 open Lwt.Syntax
-
 open Js_of_ocaml
 let jss = Js.string
 let sjs = Js.to_string
 
-let lwt_ws (url : string) =
-  (** [lwt_ws url] returns a stream and a push function for the websocket at [url].
+(* sic. XEP-0156: "host-meta files MUST be fetched only over HTTPS". I don't make the rules. *)
+let well_known_of (domain : string) = "https://" ^ domain ^ "/.well-known/host-meta"
+
+let ws_endpoint (domain : string) =
+  (** [ws_endpoint domain] is a promise containing the XMPP websocket endpoint associated with [domain], by using the
+      domain's Web-host Metadata.
+
+      This function uses XMLHttpRequest, so while it should work fine in the browser, in environments that don't provide
+      this constructor (Node.js), there should be some sort of polyfill.
+
+      Lastly, if [domain] doesn't provide a well-formed Web-host Metadata file, the function throws an exception. *)
+
+  let+ host_meta = Js_of_ocaml_lwt.XmlHttpRequest.perform_raw_url (well_known_of domain)
+  in let i = Xmlm.make_input (`String (0, host_meta.content))
+     (* This ugly function extracts the href element from a Link tag's attributes if it's a websocket. *)
+     and link_websocket attrs =
+       match attrs with
+       | ((_, "href"), href) :: ((_, "rel"), "urn:xmpp:alt-connections:websocket") :: _ -> Some href
+       | ((_, "rel"), "urn:xmpp:alt-connections:websocket") :: ((_, "href"), href) :: _ -> Some href
+       | _ -> None
+     in let parse_tree = (* Parse a single XRD tree. *)
+          Xmlm.input_tree
+            ~el:(fun tag children
+                 -> match tag with
+                    | ((_, "Link"), attributes) -> link_websocket attributes
+                    | ((_, "XRD"), _) -> List.find_map (fun x -> x) children
+                    | _ -> None)
+            ~data:(fun x -> Some x)
+        in ignore (Xmlm.input i); (* DTD *)
+           match parse_tree i with
+           | Some uri -> uri
+           | None -> failwith (domain ^ "doesn't advertise a WebSocket endpoint via Web-host Metadata.")
+
+let ws_stream (url : string) =
+  (** [ws_stream url] returns a stream (and its push function) that talk with the websocket located at [url].
+
       Pushing [None] closes the websocket.
-      If the websocket is closed server-side, it's up to the caller's to close the push stream. *)
+
+      If the websocket is closed server-side, it's still up to the caller to close the stream. *)
   let open Lwt_stream in
   let handle ws incoming () =
     let+ _ = iter (fun msg -> ws##send (jss msg)) incoming