Parallel vacuum: check XID wraparound failsafe between indexes

Hi,

I propose for PG20 that parallel vacuum check the XID wraparound
failsafe after each index, as non-parallel vacuum does. Currently the
check happens only after the parallel workers have vacuumed all the
indexes, and cost-based delays are applied until then (they are
otherwise ignored once the failsafe has triggered, so that the vacuum
finishes faster). With large indexes, this can delay advancing
relfrozenxid and relminmxid, bringing the database closer to XID
wraparound and eventually to downtime for writes. With parallel
autovacuum in PG19 and its default cost limit and delay, the
advancement can be delayed even further.

Please find the attached patch implementing the following idea. The
table's cutoffs are shared with the parallel workers, and each
parallel worker and the leader check the wraparound failsafe after
each index they vacuum and set a shared flag if the failsafe has
triggered. They check this shared flag at every vacuum delay point
and, once it is set, ignore the cost-based delays, so that the index
being vacuumed finishes sooner, and they skip the remaining indexes,
so that relfrozenxid and relminmxid can be advanced sooner to avoid
XID wraparound.

I chose to set the shared flag in the parallel worker or leader that
detects the failsafe, instead of having each one check the failsafe on
its own, to avoid taking XidGenLock and MultiXactGenLock so often.

Thoughts?

--
Bharath Rupireddy
Amazon Web Services: https://aws.amazon.com