On Fri, Jul 21, 2017 at 4:16 PM, Neha Sharma
<neha.sha...@enterprisedb.com> wrote:
> Attached is the core dump file received on PG 10beta2 version.

Thanks Neha.  It's be best to post the back trace and if possible
print oldestXact and ShmemVariableCache->oldestXid from the stack
frame for TruncateCLOG.

The failing assertion in TruncateCLOG() has a comment that says
"vac_truncate_clog already advanced oldestXid", but vac_truncate_clog
calls SetTransactionIdLimit() to write ShmemVariableCache->oldestXid
*after* it calls TruncateCLOG().  What am I missing here?

What actually prevents ShmemVariableCache->oldestXid from going
backwards anyway?  Suppose there are two or more autovacuum processes
that reach vac_truncate_clog() concurrently.  They do a scan of
pg_database whose tuples they access without locking through a
pointer-to-volatile because they expect concurrent in-place writers,
come up with a value for frozenXID, and then arrive at
SetTransactionIdLimit() in whatever order and clobber
ShmemVariableCache->oldestXid.  What am I missing here?

Thomas Munro

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