Add support for FEP-2c59 (#38239)

app/helpers/context_helper.rb

@@ -4,6 +4,7 @@ module ContextHelper
   NAMED_CONTEXT_MAP = {
     activitystreams: 'https://www.w3.org/ns/activitystreams',
     security: 'https://w3id.org/security/v1',
+    webfinger: 'https://purl.archive.org/socialweb/webfinger',
   }.freeze
 
   CONTEXT_EXTENSION_MAP = {

app/serializers/activitypub/actor_serializer.rb

@@ -4,7 +4,7 @@ class ActivityPub::ActorSerializer < ActivityPub::Serializer
   include RoutingHelper
   include FormattingHelper
 
-  context :security
+  context :security, :webfinger
 
   context_extensions :manually_approves_followers, :featured, :also_known_as,
                      :moved_to, :property_value, :discoverable, :suspended,
@@ -55,6 +55,10 @@ class ActivityPub::ActorSerializer < ActivityPub::Serializer
     ActivityPub::TagManager.instance.uri_for(object)
   end
 
+  def webfinger
+    object.local_username_and_domain
+  end
+
   def type
     if object.instance_actor?
       'Application'

app/services/activitypub/fetch_remote_actor_service.rb

@@ -27,11 +27,23 @@ class ActivityPub::FetchRemoteActorService < BaseService
     raise Error, "Unsupported JSON-LD context for document #{uri}" unless supported_context?
     raise Error, "Unexpected object type for actor #{uri} (expected any of: #{SUPPORTED_TYPES})" unless expected_type?
     raise Error, "Actor #{uri} has moved to #{@json['movedTo']}" if break_on_redirect && @json['movedTo'].present?
-    raise Error, "Actor #{uri} has no 'preferredUsername', which is a requirement for Mastodon compatibility" if @json['preferredUsername'].blank?
+    raise Error, "Actor #{uri} has neither 'preferredUsername' nor `webfinger`, which is a requirement for Mastodon compatibility" if @json['preferredUsername'].blank? && @json['webfinger'].blank?
 
-    @uri      = @json['id']
-    @username = @json['preferredUsername']
-    @domain   = Addressable::URI.parse(@uri).normalized_host
+    @uri = @json['id']
+
+    # FEP-2c59 defines a `webfinger` attribute that makes things more explicit and spares an extra request in some cases.
+    # It supersedes `preferredUsername`.
+    if @json['webfinger'].present? && @json['webfinger'].is_a?(String)
+      @username, @domain = split_acct(@json['webfinger'])
+      Rails.logger.debug { "Actor #{uri} has an invalid `webfinger` value, falling back to `preferredUsername`" }
+    end
+
+    if @username.blank? || @domain.blank?
+      raise "Actor #{uri} has no `preferredUsername`, and either a bogus or missing `webfinger`, which is a requirement for Mastodon compatibility" if @json['preferredUsername'].blank?
+
+      @username = @json['preferredUsername']
+      @domain   = Addressable::URI.parse(@uri).normalized_host
+    end
 
     check_webfinger! unless only_key