Parallel vacuum workers prevent the oldest xmin from advancing

Hi all,

A customer reported that during parallel index vacuum, the oldest xmin
doesn't advance. Normally, the calculation of oldest xmin
(ComputeXidHorizons()) ignores xmin/xid of processes having
PROC_IN_VACUUM flag in MyProc->statusFlags. But since parallel vacuum
workers don’t set their statusFlags, the xmin of the parallel vacuum
worker is considered to calculate the oldest xmin. This issue happens
from PG13 where the parallel vacuum was introduced. I think it's a
bug.

Moreover, the same problem happens also in CREATE/REINDEX CONCURRENTLY
case in PG14 or later for the same reason (due to lack of
PROC_IN_SAFE_IC flag).

To fix it, I thought that we change the create index code and the
vacuum code so that the individual parallel worker sets its status
flags according to the leader’s one. But ISTM it’d be better to copy
the leader’s status flags to workers in ParallelWorkerMain(). I've
attached a patch for HEAD.

Regards,

-- 
Masahiko Sawada
EDB:  https://www.enterprisedb.com/