Lack of actions when wp_cache_flush is called

[k1sul1]

The function definition of wp_cache_flush doesn't have a do_action call, and neither does WP_Object_Cache->flush.

https://core.trac.wordpress.org/browser/tags/4.9.8/src/wp-includes/cache.php#L97
https://core.trac.wordpress.org/browser/tags/4.9.8/src/wp-includes/cache.php#L498

I want to be able to run additional code whenever the object cache is flushed but currently I can't.

Fixing this should be as trivial as adding

do_action('wp_cache_flush');

to the function definition of wp_cache_flush.

I don't care about the name of the action or if it runs before or after the flush, I'd just like to have this functionality in the core. Preferably before WordPress 6.0.

[jorbin]

A challenge with this is that if an object-cache dropin is used, then the core wp-includes/cache.php file won't be loaded. Any hooks we decide to add would thus become impossible to rely upon.