MDL-55617 backup: Possible fatal errors in backup and restore

The unserialize() function will not always return an object
especially when as can happen the controller field of a
record in the backup_controllers table is empty.

This change ensures that in this case the script will
not cause a fatal error, but instead throw an error.

backup/util/dbops/backup_controller_dbops.class.php

@@ -105,6 +105,12 @@ abstract class backup_controller_dbops extends backup_dbops {
             throw new backup_dbops_exception('backup_controller_dbops_nonexisting');
         }
         $controller = unserialize(base64_decode($controllerrec->controller));
+        if (!is_object($controller)) {
+            // The controller field of the table did not contain a serialized object.
+            // It is made empty after it has been used successfully, it is likely that
+            // the user has pressed the browser back button at some point.
+            throw new backup_dbops_exception('backup_controller_dbops_loading_invalid_controller');
+        }
         // Check checksum is ok. Sounds silly but it isn't ;-)
         if (!$controller->is_checksum_correct($controllerrec->checksum)) {
             throw new backup_dbops_exception('backup_controller_dbops_loading_checksum_mismatch');

backup/util/dbops/restore_controller_dbops.class.php

@@ -95,6 +95,12 @@ abstract class restore_controller_dbops extends restore_dbops {
             throw new backup_dbops_exception('restore_controller_dbops_nonexisting');
         }
         $controller = unserialize(base64_decode($controllerrec->controller));
+        if (!is_object($controller)) {
+            // The controller field of the table did not contain a serialized object.
+            // It is made empty after it has been used successfully, it is likely that
+            // the user has pressed the browser back button at some point.
+            throw new backup_dbops_exception('restore_controller_dbops_loading_invalid_controller');
+        }
         // Check checksum is ok. Sounds silly but it isn't ;-)
         if (!$controller->is_checksum_correct($controllerrec->checksum)) {
             throw new backup_dbops_exception('restore_controller_dbops_loading_checksum_mismatch');