Debugging detached? method

Author: neddp

src/bosh-director/lib/bosh/director/deployment_plan/instance_plan.rb

@@ -398,7 +398,21 @@ def packages_changed?
         end
 
         def already_detached?
-          new? ? false : @existing_instance.detached?
+          return false if new?
+          
+          msg = "already_detached? checking: existing_instance.class=#{existing_instance.class}, value=#{existing_instance.inspect}"
+          @logger.debug(msg)
+          $stderr.puts "[DEBUG] #{msg}"
+          
+          unless existing_instance.respond_to?(:detached?)
+            error_msg = "BUG: existing_instance does not respond to detached?! Class: #{existing_instance.class}, Value: #{existing_instance.inspect}"
+            @logger.error(error_msg)
+            $stderr.puts "[ERROR] #{error_msg}"
+            # Fallback to string comparison for safety
+            return existing_instance.is_a?(String) ? existing_instance == 'detached' : existing_instance.state == 'detached'
+          end
+          
+          existing_instance.detached?
         end
 
         def needs_disk?