Re: logical replication and PANIC during shutdown checkpoint in publisher

On 14/04/17 19:33, Fujii Masao wrote:
> On Fri, Apr 14, 2017 at 10:33 PM, Petr Jelinek
> <petr(dot)jelinek(at)2ndquadrant(dot)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.
>

Ah yes looking at the code, it does exactly that (on master only). Means
that backport will be necessary.

--
Petr Jelinek http://www.2ndQuadrant.com/
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