Work around dream-encoding footgun

Dream-encoding adds a Transfer-Encoding header to a response. If that
response is an error code it will go through the error handler which may
change the response - but the new response *does not* go through the
dream-encoding middleware! This means we may rewrite the response with a
plaintext message but still have the Transfer-Encoding header! This
makes clients very unhappy.

lib/builder_web.ml

@@ -797,7 +797,19 @@ let error_template error _debug_info suggested_response =
     in
     Dream.set_header suggested_response "Content-Type" Dream.text_html;
     Dream.set_body suggested_response @@ string_of_html html;
-    Lwt.return suggested_response
+    (* NOTE: this does the same job as the dream-encoding middleware;
+       the middleware is not triggered in error templates *)
+    let preferred_algorithm =
+      Option.bind error.request
+        Dream_encoding.preferred_content_encoding
+    in
+    begin match preferred_algorithm with
+      | Some algorithm ->
+        let+ body = Dream.body suggested_response in
+        Dream_encoding.with_encoded_body body ~algorithm suggested_response
+      | None ->
+        Lwt.return suggested_response
+    end
   | _ ->
     Lwt.return suggested_response
 

lib/dune

@@ -2,4 +2,4 @@
  (name builder_web)
  (libraries builder builder_db dream tyxml bos duration ohex caqti-lwt
    opamdiff ptime.clock.os cmarkit tar tar.gz tar-unix owee solo5-elftool decompress.de
-   decompress.gz uri digestif))
+   decompress.gz uri digestif dream-encoding))