Re: [TRAP: FailedAssertion] causing server to crash

On Fri, Jul 21, 2017 at 4:16 PM, Neha Sharma
<neha(dot)sharma(at)enterprisedb(dot)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
http://www.enterprisedb.com