move shared function user code into module

app/models/anonymous_user.rb

@@ -27,31 +27,25 @@
 #++
 
 class AnonymousUser < User
-  validate :validate_unique_anonymous_user, on: :create
-
-  # There should be only one AnonymousUser in the database
-  def validate_unique_anonymous_user
-    errors.add :base, "An anonymous user already exists." if AnonymousUser.any?
-  end
-
-  def available_custom_fields
-    []
-  end
-
-  # Overrides a few properties
-  def logged?; false end
-
-  def builtin?; true end
-
-  def admin; false end
+  include Users::FunctionUser
 
   def name(*_args); I18n.t(:label_user_anonymous) end
 
-  def mail; nil end
+  def self.first
+    anonymous_user = super
 
-  def time_zone; ActiveSupport::TimeZone[Setting.user_default_timezone.presence || "Etc/UTC"] end
+    if anonymous_user.nil?
+      (anonymous_user = new.tap do |u|
+        u.lastname = "Anonymous"
+        u.login = ""
+        u.firstname = ""
+        u.mail = ""
+        u.status = User.statuses[:active]
+      end).save
 
-  def rss_key; nil end
+      raise "Unable to create the anonymous user." if anonymous_user.new_record?
+    end
 
-  def destroy; false end
+    anonymous_user
+  end
 end

app/models/deleted_user.rb

@@ -1,23 +1,9 @@
 class DeletedUser < User
-  validate :validate_unique_deleted_user, on: :create
-
-  # There should be only one DeletedUser in the database
-  def validate_unique_deleted_user
-    errors.add :base, "A DeletedUser already exists." if DeletedUser.any?
-  end
-
   def self.first
     super || create(type: to_s, status: statuses[:locked])
   end
 
-  # Overrides a few properties
-  def available_custom_fields = []
-  def logged? = false
-  def builtin? = true
-  def admin = false
+  include Users::FunctionUser
+
   def name(*_args) = I18n.t("user.deleted")
-  def mail = nil
-  def time_zone; ActiveSupport::TimeZone[Setting.user_default_timezone.presence || "Etc/UTC"] end
-  def rss_key = nil
-  def destroy = false
 end

app/models/system_user.rb

@@ -31,31 +31,35 @@
 #
 
 class SystemUser < User
-  validate :validate_unique_system_user, on: :create
-
-  # There should be only one SystemUser in the database
-  def validate_unique_system_user
-    errors.add :base, "A SystemUser already exists." if SystemUser.any?
-  end
-
-  # Overrides a few properties
-  def logged?; false end
-
-  def builtin?; true end
+  include Users::FunctionUser
 
   def name(*_args); "System" end
 
-  def mail; nil end
+  def run_given
+    User.execute_as(self) do
+      yield self
+    end
+  end
 
-  def time_zone; ActiveSupport::TimeZone[Setting.user_default_timezone.presence || "Etc/UTC"] end
+  def self.first
+    system_user = super
 
-  def rss_key; nil end
+    if system_user.nil?
+      system_user = new(
+        firstname: "",
+        lastname: "System",
+        login: "",
+        mail: "",
+        admin: true,
+        status: User.statuses[:active],
+        first_login: false
+      )
 
-  def destroy; false end
+      system_user.save
 
-  def run_given
-    User.execute_as(self) do
-      yield self
+      raise "Unable to create the system user." unless system_user.persisted?
     end
+
+    system_user
   end
 end

app/models/user.rb

@@ -543,46 +543,12 @@ def missing_authentication_method?
 
   # Returns the anonymous user.  If the anonymous user does not exist, it is created.  There can be only
   # one anonymous user per database.
-  def self.anonymous # rubocop:disable Metrics/AbcSize
-    RequestStore[:anonymous_user] ||=
-      begin
-        anonymous_user = AnonymousUser.first
-
-        if anonymous_user.nil?
-          (anonymous_user = AnonymousUser.new.tap do |u|
-            u.lastname = "Anonymous"
-            u.login = ""
-            u.firstname = ""
-            u.mail = ""
-            u.status = User.statuses[:active]
-          end).save
-
-          raise "Unable to create the anonymous user." if anonymous_user.new_record?
-        end
-        anonymous_user
-      end
+  def self.anonymous
+    RequestStore[:anonymous_user] ||= AnonymousUser.first
   end
 
   def self.system
-    system_user = SystemUser.first
-
-    if system_user.nil?
-      system_user = SystemUser.new(
-        firstname: "",
-        lastname: "System",
-        login: "",
-        mail: "",
-        admin: true,
-        status: User.statuses[:active],
-        first_login: false
-      )
-
-      system_user.save(validate: false)
-
-      raise "Unable to create the automatic migration user." unless system_user.persisted?
-    end
-
-    system_user
+    SystemUser.first
   end
 
   protected

app/models/users/function_user.rb

@@ -0,0 +1,56 @@
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module Users::FunctionUser
+  extend ActiveSupport::Concern
+
+  included do
+    validate :validate_unique_function_user, on: :create
+
+    # There should be only one such user in the database
+    def validate_unique_function_user
+      errors.add :base, "A #{self.class.name} already exists." if self.class.any?
+    end
+
+    def available_custom_fields = []
+
+    def logged? = false
+
+    def builtin? = true
+
+    def name(*_args); raise NotImplementedError end
+
+    def mail = nil
+
+    def time_zone; ActiveSupport::TimeZone[Setting.user_default_timezone.presence || "Etc/UTC"] end
+
+    def rss_key = nil
+
+    def destroy = false
+  end
+end