From f10977a99ac73ce5a702a12ed31e773a4b0f6961 Mon Sep 17 00:00:00 2001
From: Thomas Citharel <tcit@tcit.fr>
Date: Thu, 14 Dec 2023 16:31:58 +0100
Subject: [PATCH] feat(activitypub): implement FEP-2677 to identify the
 application actor used for federation

Instead of always assuming it will be @relay@host.tld

Closes #1367

Signed-off-by: Thomas Citharel <tcit@tcit.fr>
---
 lib/federation/activity_pub/relay.ex          | 10 +++---
 lib/federation/node_info.ex                   | 33 +++++++++++++++++++
 lib/web/controllers/node_info_controller.ex   | 10 ++++++
 .../controllers/nodeinfo_controller_test.exs  |  8 +++++
 4 files changed, 56 insertions(+), 5 deletions(-)
 create mode 100644 lib/federation/node_info.ex

diff --git a/lib/federation/activity_pub/relay.ex b/lib/federation/activity_pub/relay.ex
index b722eca62..3bc9e9212 100644
--- a/lib/federation/activity_pub/relay.ex
+++ b/lib/federation/activity_pub/relay.ex
@@ -13,7 +13,7 @@ defmodule Mobilizon.Federation.ActivityPub.Relay do
 
   alias Mobilizon.Federation.ActivityPub.{Actions, Activity, Transmogrifier}
   alias Mobilizon.Federation.ActivityPub.Actor, as: ActivityPubActor
-  alias Mobilizon.Federation.WebFinger
+  alias Mobilizon.Federation.{NodeInfo, WebFinger}
   alias Mobilizon.GraphQL.API.Follows
   alias Mobilizon.Service.Workers.Background
   import Mobilizon.Federation.ActivityPub.Utils, only: [create_full_domain_string: 1]
@@ -192,10 +192,10 @@ defmodule Mobilizon.Federation.ActivityPub.Relay do
         check_actor(address)
 
       !is_nil(host) ->
-        uri
-        |> create_full_domain_string()
-        |> then(&Kernel.<>("relay@", &1))
-        |> check_actor()
+        case NodeInfo.application_actor(host) do
+          nil -> check_actor("relay@#{host}")
+          actor_url when is_binary(actor_url) -> {:ok, actor_url}
+        end
 
       true ->
         {:error, :bad_url}
diff --git a/lib/federation/node_info.ex b/lib/federation/node_info.ex
new file mode 100644
index 000000000..688abdac8
--- /dev/null
+++ b/lib/federation/node_info.ex
@@ -0,0 +1,33 @@
+defmodule Mobilizon.Federation.NodeInfo do
+  @moduledoc """
+  Performs NodeInfo requests
+  """
+
+  alias Mobilizon.Service.HTTP.WebfingerClient
+  require Logger
+
+  @application_uri "https://www.w3.org/ns/activitystreams#Application"
+  @env Application.compile_env(:mobilizon, :env)
+
+  @spec application_actor(String.t()) :: String.t() | nil
+  def application_actor(host) do
+    prefix = if @env !== :dev, do: "https", else: "http"
+
+    case WebfingerClient.get("#{prefix}://#{host}/.well-known/nodeinfo") do
+      {:ok, %{body: body, status: code}} when code in 200..299 ->
+        extract_application_actor(body)
+
+      err ->
+        Logger.debug("Failed to fetch NodeInfo data #{inspect(err)}")
+        nil
+    end
+  end
+
+  defp extract_application_actor(body) do
+    body
+    |> Enum.find(%{rel: @application_uri, href: nil}, fn %{rel: rel, href: href} ->
+      rel == @application_uri and is_binary(href)
+    end)
+    |> Map.get(:href)
+  end
+end
diff --git a/lib/web/controllers/node_info_controller.ex b/lib/web/controllers/node_info_controller.ex
index f9a09b7f8..71e7b79aa 100644
--- a/lib/web/controllers/node_info_controller.ex
+++ b/lib/web/controllers/node_info_controller.ex
@@ -7,13 +7,17 @@ defmodule Mobilizon.Web.NodeInfoController do
   use Mobilizon.Web, :controller
 
   alias Mobilizon.Config
+  alias Mobilizon.Federation.ActivityPub.Relay
   alias Mobilizon.Service.Statistics
 
   @node_info_supported_versions ["2.0", "2.1"]
   @node_info_schema_uri "http://nodeinfo.diaspora.software/ns/schema/"
+  @application_uri "https://www.w3.org/ns/activitystreams#Application"
 
   @spec schemas(Plug.Conn.t(), any) :: Plug.Conn.t()
   def schemas(conn, _params) do
+    relay = Relay.get_actor()
+
     links =
       @node_info_supported_versions
       |> Enum.map(fn version ->
@@ -22,6 +26,12 @@ defmodule Mobilizon.Web.NodeInfoController do
           href: url(~p"/.well-known/nodeinfo/#{version}")
         }
       end)
+      |> Kernel.++([
+        %{
+          rel: @application_uri,
+          href: relay.url
+        }
+      ])
 
     json(conn, %{
       links: links
diff --git a/test/web/controllers/nodeinfo_controller_test.exs b/test/web/controllers/nodeinfo_controller_test.exs
index 08eca37a8..88bec3b2f 100644
--- a/test/web/controllers/nodeinfo_controller_test.exs
+++ b/test/web/controllers/nodeinfo_controller_test.exs
@@ -2,12 +2,16 @@ defmodule Mobilizon.Web.NodeInfoControllerTest do
   use Mobilizon.Web.ConnCase
 
   alias Mobilizon.Config
+  alias Mobilizon.Federation.ActivityPub.Relay
 
   use Mobilizon.Web, :verified_routes
 
   test "Get node info schemas", %{conn: conn} do
     conn = get(conn, url(~p"/.well-known/nodeinfo"))
 
+    relay = Relay.get_actor()
+    relay_url = relay.url
+
     assert json_response(conn, 200) == %{
              "links" => [
                %{
@@ -17,6 +21,10 @@ defmodule Mobilizon.Web.NodeInfoControllerTest do
                %{
                  "href" => url(~p"/.well-known/nodeinfo/2.1"),
                  "rel" => "http://nodeinfo.diaspora.software/ns/schema/2.1"
+               },
+               %{
+                 "href" => relay_url,
+                 "rel" => "https://www.w3.org/ns/activitystreams#Application"
                }
              ]
            }