Fix situations in which instance actor can be set to a Mastodon-incompatible name (#22307)

* Validate internal actor

* Use “internal.actor” by default for the server actor username

* Fix instance actor username on the fly if it includes ':'

* Change actor name from internal.actor to mastodon.internal

app/models/account.rb

@@ -84,8 +84,8 @@ class Account < ApplicationRecord
   validates :username, presence: true
   validates_with UniqueUsernameValidator, if: -> { will_save_change_to_username? }
 
-  # Remote user validations
-  validates :username, format: { with: USERNAME_ONLY_RE }, if: -> { !local? && will_save_change_to_username? }
+  # Remote user validations, also applies to internal actors
+  validates :username, format: { with: USERNAME_ONLY_RE }, if: -> { (!local? || actor_type == 'Application') && will_save_change_to_username? }
 
   # Local user validations
   validates :username, format: { with: /\A[a-z0-9_]+\z/i }, length: { maximum: 30 }, if: -> { local? && will_save_change_to_username? && actor_type != 'Application' }

app/models/concerns/account_finder_concern.rb

@@ -13,9 +13,11 @@ module AccountFinderConcern
     end
 
     def representative
-      Account.find(-99).tap(&:ensure_keys!)
+      actor = Account.find(-99).tap(&:ensure_keys!)
+      actor.update!(username: 'mastodon.internal') if actor.username.include?(':')
+      actor
     rescue ActiveRecord::RecordNotFound
-      Account.create!(id: -99, actor_type: 'Application', locked: true, username: Rails.configuration.x.local_domain)
+      Account.create!(id: -99, actor_type: 'Application', locked: true, username: 'mastodon.internal')
     end
 
     def find_local(username)

db/seeds/02_instance_actor.rb

@@ -1 +1 @@
-Account.create_with(actor_type: 'Application', locked: true, username: ENV['LOCAL_DOMAIN'] || Rails.configuration.x.local_domain).find_or_create_by(id: -99)
+Account.create_with(actor_type: 'Application', locked: true, username: 'mastodon.internal').find_or_create_by(id: -99)