Basically, $subject says it all. It's pretty easy to reproduce:
delete all the records from a large table and execute any sequentially
scanning query before autocvacuum comes around and cleans the table
up; the query will be uncancellable. This can result in fairly
pathological behavior in i/o constrained systems because the query
will bog itself down writing out hint bits for minutes or hours
without any way to cancel or effective i/o throttling (unlike vacuum).
IMO, this should be backpatched, and is likely fixed by injecting an
interrupts check at a strategic location. But where? I was thinking
in heapgetpage() but here are no checks elsehwere in heapam.c which is
a red flag.
merlin