Fix instance actor being incorrectly created when running migrations (#18109)

* Add migration test about instance actor key

* Fix old migration

* Work around incorrect database state

app/models/account.rb

@@ -559,6 +559,12 @@ class Account < ApplicationRecord
   before_validation :prepare_username, on: :create
   before_destroy :clean_feed_manager
 
+  def ensure_keys!
+    return unless local? && private_key.blank? && public_key.blank?
+    generate_keys
+    save!
+  end
+
   private
 
   def prepare_contents

app/models/concerns/account_finder_concern.rb

@@ -13,7 +13,7 @@ module AccountFinderConcern
     end
 
     def representative
-      Account.find(-99)
+      Account.find(-99).tap(&:ensure_keys!)
     rescue ActiveRecord::RecordNotFound
       Account.create!(id: -99, actor_type: 'Application', locked: true, username: Rails.configuration.x.local_domain)
     end

db/migrate/20190715164535_add_instance_actor.rb

@@ -2,6 +2,14 @@ class AddInstanceActor < ActiveRecord::Migration[5.2]
   class Account < ApplicationRecord
     # Dummy class, to make migration possible across version changes
     validates :username, uniqueness: { scope: :domain, case_sensitive: false }
+
+    before_create :generate_keys
+
+    def generate_keys
+      keypair = OpenSSL::PKey::RSA.new(2048)
+      self.private_key = keypair.to_pem
+      self.public_key  = keypair.public_key.to_pem
+    end
   end
 
   def up

lib/tasks/tests.rake

@@ -33,6 +33,11 @@ namespace :tests do
         puts 'AccountConversation records not created as expected'
         exit(1)
       end
+
+      if Account.find(-99).private_key.blank?
+        puts 'Instance actor does not have a private key'
+        exit(1)
+      end
     end
 
     desc 'Populate the database with test data for 2.4.0'