Warn when attempting to flush cache with `--url` on multisite

[MannyAdumbire]

Feature Request

• [ x ] Yes, I reviewed the contribution guidelines.

If I'm understanding right the --url flag is ignored when doing a wp cache flush. but no notice is given saying the flag is being ignored.

Is it feasible to update cache-command to clear caches for only subsite
e.g. when below command is run
wp cache flush --url=www.multisite.com/subsite
only flush www.multisite.com/subsite but don't affect caches on www.multisite.com/ or www.multisite.com/other-subsite

If not, maybe a notice that the flag is being ignored would be helpful.

[danielbachhuber]

Thanks for the suggestion, @MannyAdumbire !

Your assessment is correct: wp cache flush affects the entire cache, not just a specific site.

If not, maybe a notice that the flag is being ignored would be helpful.

This would be great! Want to submit a pull request for this? Here is some guidance on our pull request best practices, if it's helpful. You can also stop by the #cli channel on WordPress.org Slack if you'd like to chat.

[WPprodigy]

I'm not sure that Ignoring is the right terminology here? WP CLI is still switching the context to subsite in question (as it should be). But the new warning label makes it seem otherwise, as if WP CLI isn't loading up the requested site.

Depending on the object cache implementation, it's possible that not all subsites have their cache flushed. For example: https://github.com/Automattic/wp-cache-memcached/blob/848365047f2edacec76e0b345809bb3f5ce5c2fb/includes/wp-object-cache.php#L630-L651

Could we maybe update this text a bit, perhaps something like:

Warning: Flushing the cache may affect all sites in a multisite installation, depending on the object cache implementation.

Alternatively, a way to filter/remove this warning?

[danielbachhuber]

@WPprodigy I like your suggested changes to the text. Want to submit a PR?