fix(user): update the known user when fetched from a display URL

A fetch from a display URL (e.g. https://host/@user) returns the
actor with its canonical id. make_user_from_ap_id looked the user up
by the fetch URL, did not find it and tried to insert a duplicate.
Look the user up by the fetched id.

Based on Akkoma 48b3a3579.

changelog.d/user-fetch-display-url.fix

@@ -0,0 +1 @@
+Update the known user instead of failing when an actor is fetched from a display URL

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -2055,11 +2055,12 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
   def enqueue_pin_fetches(_), do: nil
 
   def make_user_from_ap_id(ap_id, additional \\ []) do
-    user = User.get_cached_by_ap_id(ap_id)
-
     with {:ok, data} <- fetch_and_prepare_user_from_ap_id(ap_id, additional) do
       enqueue_pin_fetches(data)
 
+      # the fetched actor's id differs from ap_id when it was fetched from a display URL
+      user = User.get_cached_by_ap_id(data.ap_id)
+
       if user do
         user
         |> User.remote_user_changeset(data)

test/pleroma/web/activity_pub/activity_pub_test.exs

@@ -269,6 +269,33 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubTest do
       refute User.get_by_ap_id(ap_id)
     end
 
+    test "updates the known user when fetched from a display URL" do
+      ap_id = "https://example.com/users/lain"
+      display_url = "https://example.com/@lain"
+      user = insert(:user, local: false, ap_id: ap_id, nickname: "lain@example.com")
+
+      user_data =
+        "test/fixtures/users_mock/user.json"
+        |> File.read!()
+        |> String.replace("{{nickname}}", "lain")
+        |> Jason.decode!()
+        |> Map.merge(%{"featured" => nil, "name" => "new name"})
+        |> Jason.encode!()
+
+      Tesla.Mock.mock(fn %{method: :get, url: url} when url in [ap_id, display_url] ->
+        %Tesla.Env{
+          status: 200,
+          body: user_data,
+          headers: [{"content-type", "application/activity+json"}]
+        }
+      end)
+
+      assert {:ok, %User{id: id, name: "new name"}} =
+               ActivityPub.make_user_from_ap_id(display_url)
+
+      assert id == user.id
+    end
+
     test "fetches user featured collection" do
       ap_id = "https://example.com/users/lain"