[Meta] Running Drupal with read only DB user (working title)

Problem/Motivation

When switching to a read only DB user on an environment with memcache or redis, there's various fatal errors due to code. Detailed description follows.

Steps to reproduce

On a vanilla Drupal and ddev local environment:

Mysql commands

• REVOKE ALL, GRANT OPTION FROM db@localhost;
• REVOKE ALL, GRANT OPTION FROM db@'%';
• SHOW GRANTS FOR db@localhost;
• Show grants for db@'%';
• GRANT SELECT, SHOW VIEW ON db.* TO 'db'@'localhost';
• GRANT SELECT, SHOW VIEW ON db.* TO 'db'@'%';

Steps to replicate:

• Execute mysql commands above after connecting to the DB using ddev mysql -u root -p (pw root)
• Make sure the DB user and PW in settings.local.php/settings.ddev.php is root/root
• Comment the cache bin definition in settings.local.php which makes bins to store in memcache

//$settings['cache']['bins']['config'] = 'cache.backend.memcache';
//$settings['cache']['bins']['bootstrap'] = 'cache.backend.memcache';
//$settings['cache']['bins']['data'] = 'cache.backend.memcache';
//$settings['cache']['bins']['discovery'] = 'cache.backend.memcache';
//$settings['cache']['bins']['default'] = 'cache.backend.memcache';
//$settings['cache']['bins']['render'] = 'cache.backend.memcache';
//$settings['cache']['bins']['entity'] = 'cache.backend.memcache';
//$settings['cache']['bins']['dynamic_page_cache'] = 'cache.backend.memcache';
//$settings['cache']['bins']['page'] = 'cache.backend.memcache';
//$settings['cache']['bins']['menu'] = 'cache.backend.memcache';
//$settings['cache']['bins']['toolbar'] = 'cache.backend.memcache';

• ddev drush si -y
• ddev drush en -y memcache
• In settings.php add

$settings['memcache']['servers'] = ['memcache:11211' => 'default'];
$settings['cache']['bins']['config'] = 'cache.backend.memcache';
$settings['cache']['bins']['bootstrap'] = 'cache.backend.memcache';
$settings['cache']['bins']['data'] = 'cache.backend.memcache';
$settings['cache']['bins']['discovery'] = 'cache.backend.memcache';
$settings['cache']['bins']['default'] = 'cache.backend.memcache';
$settings['cache']['bins']['render'] = 'cache.backend.memcache';
$settings['cache']['bins']['entity'] = 'cache.backend.memcache';
$settings['cache']['bins']['dynamic_page_cache'] = 'cache.backend.memcache';
$settings['cache']['bins']['page'] = 'cache.backend.memcache';
$settings['cache']['bins']['menu'] = 'cache.backend.memcache';
$settings['cache']['bins']['toolbar'] = 'cache.backend.memcache';

• ddev drush pmu -y dblog
• ddev drush uli and login with the login link
• ddev drush cr
• Change the DB user and PW in settings.local.php/settings.ddev.php is db/db
• Navigate to home or /node/add/article and observe errors as described in child issues

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet