While the bug was reported as a regression of
MDEV-11025 Make number of page cleaner threads variable dynamic
in MariaDB Server 10.3, the code that MariaDB Server 10.2
inherited from MySQL 5.7.4 (WL#6642) looks prone to similar errors.

pc_flush_slot(): If there is no work to do, reset the is_requested
signal, to avoid potential busy-waiting in
buf_flush_page_cleaner_worker(). If the coordinator thread has shut
down, avoid resetting the is_requested event, to avoid a potential
hang at shutdown if there are multiple worker threads.