Add a pre-upgrade check for katello-agent

definitions/checks/katello_agent.rb

@@ -0,0 +1,38 @@
+require 'yaml'
+
+class Checks::CheckKatelloAgentEnabled < ForemanMaintain::Check
+  metadata do
+    label :check_katello_agent_enabled
+    description 'Check whether the katello-agent feature is enabled before upgrading'
+    tags :pre_upgrade
+  end
+
+  confine do
+    feature(:instance).downstream
+  end
+
+  INSTALLER_ANSWERS_FILE = '/etc/foreman-installer/scenarios.d/satellite-answers.yaml'.freeze
+
+  def run
+    assert(
+      !katello_agent_enabled?,
+      "The katello-agent feature is enabled on this system. As of Satellite 6.15.0, katello-agent"\
+      " is removed and will no longer function. Before proceeding with the upgrade, you should ensure"\
+      " that you have deployed and configured an alternative tool for remote package management and"\
+      " patching for content hosts, such as the Remote Execution (REX) Pull Provider. See the"\
+      " Managing Hosts guide in the Satellite documentation. Disable katello-agent with the command"\
+      " `satellite-installer --foreman-proxy-content-enable-katello-agent false`"\
+      " before proceeding with the upgrade."
+    )
+  end
+
+  private
+
+  def katello_agent_enabled?
+    return false unless File.exist?(INSTALLER_ANSWERS_FILE)
+
+    answers = YAML.load_file(INSTALLER_ANSWERS_FILE)
+
+    answers['foreman_proxy_content']['enable_katello_agent'] == true
+  end
+end