Enclosed is a patch that implements CREATE ROLE IF NOT EXISTS (along with the same support for USER/GROUP). This is a fairly straightforward approach in that we do no validation of anything other than existence, with the user needing to ensure that permissions/grants are set up in the proper way.
One little tricky aspect that occurs to me is the ALTER ROLE to set the role flag options: it really needs to mention *all* the available options if it is to leave the role in a specific state regardless of how it started out. For example, if the existing role has BYPASSRLS but you want the default NOBYPASSRLS you have to say so explicitly.
Because of this, I think my preference, based just on thinking about setting the flag options, would be for CREATE OR REPLACE.
However, I'm wondering about the role name options: IN ROLE, ROLE, ADMIN. With OR REPLACE should they replace the set of memberships or augment it? Either seems potentially problematic to me. By contrast it’s absolutely clear what IF NOT EXISTS should do with these.