out-of-order XID insertion in KnownAssignedXids

Hi All,

we are running many postgresql master/slave setups. The slaves are initialised from a pg_basebackup from the master and are sync streaming from the master. When we determine the master has failed, the slave is promoted. Some time after that, the old master is again initialised with a pg_basebackup and starts streaming from the new master.

Our setup seems pretty "stock" and has been running for us for some years (with different versions of postgresql but also different OSes).

Recently, we have gotten this error a fair amount of times: "out-of-order XID insertion in KnownAssignedXids" when postgresql attempts to start after being initialised with a pg_basebackup from the current master.

The only reference I can find on that particular error, is from 2012 and the resulting issue is long fixed in our version of postgresql (9.5.3) ... (<https://www.postgresql.org/message-id/201205230849.59825.andres@anarazel.de>)

Once the issue has occurred, a subsequent re-initialisation (with a completely new pg_basebackup) does not resolve the issue.

I have a setup in the failing state, so I can produce any kind of log mesages / details that would be helpful.

Thank you for your support,
Fredrik