Restore lock level to set vacuum flags
Commit
27838981be9d mistakenly reduced the lock level from exclusive to
shared that is acquired to set PGPROC->statusFlags; this was reverted
by
dcfff74fb166, but failed to do so in one spot. Fix it.
Backpatch to 14.
Noted by Andres Freund.
Discussion: https://postgr.es/m/
20211111020724.ggsfhcq3krq5r4hb@alap3.anarazel.de