Getting a List of Currently Available Roles on a WordPress Site?

When writing WordPress plugins there is often a need to set up options for which roles on the site have access to certain functionality or content. To do this a plugin dev needs to fetch the list of roles that exist on the site to use in the option. Because custom roles can be created we cannot assume the default roles are the only ones available.

add_role() run only once?

I was surprised to discover that add_role() modifies the database and fails if the role already exists. There are two implications here, one first more serious than the other: 1) if you’re in development and update your add_role code, you must first remove_role() 2) once you have it right, you should never have to run that code again.