fix(activitypub): refetch a stale Move target once

A Move usually follows right after the alias was added, and the cached
remote target may not have it yet. Refetch the target when the alias is
missing instead of rejecting the Move. Same issue as Akkoma f18e2ba42.

changelog.d/move-refetch-target.fix

@@ -0,0 +1 @@
+Accept a Move when the cached remote target does not list the alias yet, by refetching the target once

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -799,7 +799,7 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
        ) do
     with %User{} = origin_user <- User.get_cached_by_ap_id(origin_actor),
          {:ok, %User{} = target_user} <- User.get_or_fetch_by_ap_id(target_actor),
-         true <- origin_actor in target_user.also_known_as do
+         {:ok, %User{} = target_user} <- refetch_unless_aliased(target_user, origin_actor) do
       ActivityPub.move(origin_user, target_user, false)
     else
       _e -> :error
@@ -1204,4 +1204,20 @@ defmodule Pleroma.Web.ActivityPub.Transmogrifier do
   def maybe_fix_user_url(data), do: data
 
   def maybe_fix_user_object(data), do: maybe_fix_user_url(data)
+
+  # A Move usually follows right after the alias was added, which the cached
+  # remote target may not have yet.
+  defp refetch_unless_aliased(%User{} = target, origin_ap_id, refetch? \\ true) do
+    cond do
+      origin_ap_id in target.also_known_as ->
+        {:ok, target}
+
+      refetch? and not target.local ->
+        with {:ok, %User{} = target} <- User.fetch_by_ap_id(target.ap_id),
+             do: refetch_unless_aliased(target, origin_ap_id, false)
+
+      true ->
+        :error
+    end
+  end
 end

test/pleroma/web/activity_pub/transmogrifier_test.exs

@@ -147,6 +147,53 @@ defmodule Pleroma.Web.ActivityPub.TransmogrifierTest do
       assert activity.data["type"] == "Move"
     end
 
+    test "it refetches a cached remote Move target that lacks the alias" do
+      old_user = insert(:user, local: false, ap_id: "https://old.example/users/old")
+
+      new_user =
+        insert(:user,
+          local: false,
+          ap_id: "https://new.example/users/new",
+          nickname: "new@new.example",
+          last_refreshed_at: NaiveDateTime.utc_now()
+        )
+
+      actor = %{
+        "@context" => "https://www.w3.org/ns/activitystreams",
+        "id" => new_user.ap_id,
+        "type" => "Person",
+        "preferredUsername" => "new",
+        "inbox" => new_user.ap_id <> "/inbox",
+        "outbox" => new_user.ap_id <> "/outbox",
+        "followers" => new_user.ap_id <> "/followers",
+        "following" => new_user.ap_id <> "/following",
+        "alsoKnownAs" => [old_user.ap_id]
+      }
+
+      Tesla.Mock.mock(fn
+        %{method: :get, url: "https://new.example/users/new"} ->
+          %Tesla.Env{
+            status: 200,
+            headers: [{"content-type", "application/activity+json"}],
+            body: Jason.encode!(actor)
+          }
+
+        _ ->
+          %Tesla.Env{status: 404}
+      end)
+
+      message = %{
+        "@context" => "https://www.w3.org/ns/activitystreams",
+        "type" => "Move",
+        "actor" => old_user.ap_id,
+        "object" => old_user.ap_id,
+        "target" => new_user.ap_id
+      }
+
+      assert {:ok, %Activity{data: %{"type" => "Move"}}} = Transmogrifier.handle_incoming(message)
+      assert old_user.ap_id in User.get_cached_by_ap_id(new_user.ap_id).also_known_as
+    end
+
     test "it fixes both the Create and object contexts in a reply" do
       insert(:user, ap_id: "https://mk.absturztau.be/users/8ozbzjs3o8")
       insert(:user, ap_id: "https://p.helene.moe/users/helene")