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Fix for MDEV-38218 changed MDL conflict handling to allow high priority cluster threads to BF abort any conflicting MDL lock holder. With this, the InnoDB purge coordinator became also potential target for BF aborts. However, InnoDB purge thread has no statement boundary and could never reset THD::killed state, after the BF abort. Purge THDs are taken from a pool of innodb_purge_threads_MAX THDs, so about 32 purge runs later the killed THD coordinates purge again. From then on every open_purge_table() for a table with an indexed virtual column fails, and purge_sys_t::close_and_reopen() retries forever. Purge stops and server shutdown hangs. Separately, open_purge_table() allocates a TABLE_LIST on thd->mem_root and the coordinator THD never freed its mem_root, so it grew with every purge batch and every close_and_reopen() retry. Fix: - wsrep_handle_mdl_conflict(): when the granted thread is a SYSTEM_THREAD_GENERIC THD holding explicit MDL, make the BF requestor wait instead of aborting it. In practice this covers the InnoDB background threads (purge, statistics, FTS optimizer), which release their locks on their own. This restores the pre-MDEV-38218 behaviour for these threads only. - open_purge_table(): open with MYSQL_OPEN_IGNORE_KILLED so that a stale kill flag cannot block purge. - trx_purge_close_tables(): call reset_thd() instead of close_thread_tables() to free the coordinator's mem_root per batch. This commit has a new mtr test galera.MDEV-41450 This work is based on pull request https://github.com/MariaDB/server/pull/5854 by David Schilling, which is now rebased to MariaDB 10.11 and extended to have a mtr test. Assisted-by: https://mariadb.org/governance/governance-ai-policy/