Re: [HACKERS] logical replication and PANIC during shutdowncheckpoint in publisher

On 2017-04-15 02:33:59 +0900, Fujii Masao wrote:
> On Fri, Apr 14, 2017 at 10:33 PM, Petr Jelinek
>  wrote:
> > On 12/04/17 15:55, Fujii Masao wrote:
> >> Hi,
> >>
> >> When I shut down the publisher while I repeated creating and dropping
> >> the subscription in the subscriber, the publisher emitted the following
> >> PANIC error during shutdown checkpoint.
> >>
> >> PANIC:  concurrent transaction log activity while database system is
> >> shutting down
> >>
> >> The cause of this problem is that walsender for logical replication can
> >> generate WAL records even during shutdown checkpoint.
> >>
> >> Firstly walsender keeps running until shutdown checkpoint finishes
> >> so that all the WAL including shutdown checkpoint record can be
> >> replicated to the standby. This was safe because previously walsender
> >> could not generate WAL records. However this assumption became
> >> invalid because of logical replication. That is, currenty walsender for
> >> logical replication can generate WAL records, for example, by executing
> >> CREATE_REPLICATION_SLOT command. This is an oversight in
> >> logical replication patch, I think.
> >
> > Hmm, but CREATE_REPLICATION_SLOT should not generate WAL afaik. I agree
> > that the issue with walsender still exist (since we now allow normal SQL
> > to run there) but I think it's important to identify what exactly causes
> > the WAL activity in your case
> 
> At least in my case, the following CREATE_REPLICATION_SLOT command
> generated WAL record.
> 
>     BEGIN READ ONLY ISOLATION LEVEL REPEATABLE READ;
>     CREATE_REPLICATION_SLOT testslot TEMPORARY LOGICAL pgoutput USE_SNAPSHOT;
> 
> Here is the pg_waldump output of the WAL record that CREATE_REPLICATION_SLOT
> generated.
> 
>     rmgr: Standby     len (rec/tot):     24/    50, tx:          0,
> lsn: 0/01601438, prev 0/01601400, desc: RUNNING_XACTS nextXid 692
> latestCompletedXid 691 oldestRunningXid 692
> 
> So I guess that CREATE_REPLICATION_SLOT code calls LogStandbySnapshot()
> and which generates WAL record about snapshot of running transactions.

Erroring out in these cases sounds easy enough.  Wonder if there's not a
bigger problem with WAL records generated e.g. by HOT pruning or such,
during decoding.  Not super likely, but would probably hit exactly the
same, no?

- Andres