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/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