Implement retrieving a single event over federation

Fix url encoding during homeserver signature check

lib/matrix_server_web/federation/controllers/event_controller.ex

@@ -2,9 +2,44 @@ defmodule MatrixServerWeb.Federation.EventController do
   use MatrixServerWeb, :controller
   use MatrixServerWeb.Federation.AuthenticateServer
 
-  def event(conn, %{"event_id" => _event_id}) do
-    conn
-    |> put_status(200)
-    |> json(%{})
+  import MatrixServerWeb.Error
+  import Ecto.Query
+
+  alias MatrixServer.{Repo, Event, RoomServer}
+  alias MatrixServerWeb.Federation.Transaction
+
+  def event(%Plug.Conn{assigns: %{origin: origin}} = conn, %{"event_id" => event_id}) do
+    query =
+      Event
+      |> where([e], e.event_id == ^event_id)
+      |> preload(:room)
+
+    case Repo.one(query) do
+      %Event{room: room} = event ->
+        case RoomServer.get_room_server(room) do
+          {:ok, pid} ->
+            if RoomServer.server_in_room(pid, origin) do
+              data = Transaction.new([event])
+
+              conn
+              |> put_status(200)
+              |> json(data)
+            else
+              put_error(
+                conn,
+                :unauthorized,
+                "Origin server is not allowed to see requested event."
+              )
+            end
+
+          _ ->
+            put_error(conn, :unknown)
+        end
+
+      nil ->
+        put_error(conn, :not_found, "Event or room not found.")
+    end
   end
+
+  def event(conn, _), do: put_error(conn, :bad_json)
 end