When/how often is the ACL cache rebuilt?

[DaveFF]

In what circumstances and how often are ACL cache tables normally rebuilt? Does this always coincide with hook_civicrm_aclWhereClause being run? For example, does it happen when contacts or relationships are created?

Thanks,
Dave

[DaveFF]

I notice in CRM_Contact_BAO_Contact_Permission::cache, that new entries are inserted into the ACL contact cache table (if there aren't any for that user already, or the call has $force set), and then entries for is_deleted contacts are purged.

If the call is forced, there may be entries in civicrm_acl_contact_cache for that user already. These are not being deleted first, before inserting new/updated ones- should they be? As I am running into an issue (although in custom code) that suggests they should be.

(Also, while entries targeting contacts with is_deleted are purged, entries targeting permanently deleted contacts are not. Again, should they be?)

I'm happy to do a patch if this is an actual issue.