Return 429 for users exceeding 1 req/sec

Farside has been getting used by some to rapidly scrape sites, which
puts increased load and effort on maintainers of instances. Rather than
funneling traffic towards the last selected instance, farside will now
just return a 429 error when this behavior occurs.

Closes #147

lib/farside/router.ex

@@ -56,23 +56,21 @@ defmodule Farside.Router do
         Enum.join(glob, "/")
     end
 
-    instance = cond do
+    cond do
       conn.assigns[:throttle] != nil ->
-        Farside.get_service(service_name)
-        |> Farside.last_instance
-        |> Farside.amend_instance(service_name, path)
+        send_resp(conn, :too_many_requests, "Too many requests - max request rate is 1 per second")
       true ->
-        Farside.get_service(service_name)
-        |> Farside.pick_instance
-        |> Farside.amend_instance(service_name, path)
-    end
+        instance = Farside.get_service(service_name)
+            |> Farside.pick_instance
+            |> Farside.amend_instance(service_name, path)
 
-    # Redirect to the available instance
-    conn
-    |> Plug.Conn.resp(:found, "")
-    |> Plug.Conn.put_resp_header(
-      "location",
-      "#{instance}/#{path}#{get_query_params(conn)}"
-    )
+        # Redirect to the available instance
+        conn
+        |> Plug.Conn.resp(:found, "")
+        |> Plug.Conn.put_resp_header(
+          "location",
+          "#{instance}/#{path}#{get_query_params(conn)}"
+        )
+    end
   end
 end