Flushing caches inside an update hook is dangerous and can lead to data integrity problems (as well as poor update performance)

Problem/Motivation

admin_toolbar_tools_update_8203() has this:

/**
 * Module CSS file refactoring requires cache rebuild.
 */
function admin_toolbar_tools_update_8203() {
  drupal_flush_all_caches();
}

This should never be done. Simply declaring an update hook is sufficient to ensure a cache rebuild on update; doing a cache rebuild inside it will cause caches to be cleared twice and can lead to data integrity problems.

Proposed resolution

Remove the cache clear from the update hook, leaving the hook itself in place. A cache clear will still be triggered simply by its presence for any sites that have not yet applied the update. (Keeping the existing hook is safe; just remove the cache clear from inside it.)

Core does this all the time with code like this:

/**
 * Disable blocks that are placed into the "disabled" region.
 */
function block_post_update_disabled_region_update() {
  // An empty update will flush caches, forcing block_rebuild() to run.
}

Remaining tasks

In the future, to clear the cache on update, write an empty post-update hook. See this change record for more information : https://www.drupal.org/node/2960601

User interface changes

API changes

Data model changes