update.php no longer sets MAINTENANCE_MODE = 'update'

Problem/Motivation

Discovered in the CDN contrib module: #2969048: Core's DbUpdateController does not define MAINTENANCE_MODE, causes CDN module to run even for update.php → .

We still have this in the code base:

define('MAINTENANCE_MODE', 'install');

but not:

define('MAINTENANCE_MODE', 'update');

Yet much code still assumes this:

  if (defined('MAINTENANCE_MODE') && (MAINTENANCE_MODE == 'install' || MAINTENANCE_MODE == 'update')) {

in _drupal_maintenance_theme(). Other examples in:
error_displayable()
user_role_permissions()

#2288665: Remove _maintenance request attribute and replace it with a maintenance mode service → did not actually address this use case, nor does the https://www.drupal.org/node/2020005 → CR cover it.

Only authorize.php still sets it.

Proposed resolution

Fix it.

Remaining tasks

TBD

User interface changes

None.

API changes

None.

Data model changes