Handle redirection

Since odoc 3, and the replacement of voodoo by odoc_driver as the odoc driver,
the layout for docs has changed. Since we do not want to break previously
existing links, we redirect the old layout to the new one.

Author: panglesd

src/ocamlorg_web/lib/handler.ml

@@ -1229,6 +1229,29 @@ let package_documentation t kind req =
   in
   let path = (Dream.path [@ocaml.warning "-3"]) req |> String.concat "/" in
   let hash = match kind with `Package -> None | `Universe u -> Some u in
+  let url =
+    Url.Package.documentation ?hash ~page:path ~version:version_from_url
+    @@ Dream.param req "name"
+  in
+  let* package_documentation_status =
+    Ocamlorg_package.documentation_status ~kind t package
+  in
+  let redirect =
+    match package_documentation_status with
+    | None -> None
+    | Some { redirections; _ } ->
+        List.find_map
+          (function
+            | { Ocamlorg_package.Documentation_status.old_path; new_path } -> (
+                match String.cut ~on:old_path url with
+                | Some (prefix, "") -> Some (prefix ^ new_path)
+                | _ -> None))
+          redirections
+  in
+  let handle_redirect redirect continue =
+    match redirect with Some r -> Dream.redirect req r | None -> continue ()
+  in
+  handle_redirect redirect @@ fun () ->
   let* docs = Ocamlorg_package.documentation_page ~kind package path in
   match docs with
   | None ->