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

On Fri, Apr 14, 2017 at 10:33 PM, Petr Jelinek
<petr.jelinek@2ndquadrant.com> 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.

Regards,

-- 
Fujii Masao