Re: [HACKERS] Issue with logical replication: MyPgXact->xmin alreadyis valid

On 06.10.2017 15:29, Petr Jelinek wrote:
> On 06/10/17 12:16, Konstantin Knizhnik wrote:
>> When creating logical replication slots we quite often get the following
>> error:
>>
>> ERROR:  cannot build an initial slot snapshot when MyPgXact->xmin
>> already is valid
>>
>> which cause restart of WAL sender.
>> The comment to this line doesn't clarify much:
>>
>>      /* so we don't overwrite the existing value */
>>      if (TransactionIdIsValid(MyPgXact->xmin))
>>          elog(ERROR, "cannot build an initial slot snapshot when
>> MyPgXact->xmin already is valid");
>> I wonder if it is normal situation or something goes wrong?
>>
> Hi,
>
> no it's not normal situation, it seems you are doing something that
> assigns xid before you run the CREATE_REPLICATION_SLOT command on that
> connection.

I have not doing something in this connection: it is wal sender 
executing CREATE_REPLICATION_SLOT replication command.
Please look at the stack in my original e-mail. It shows who and when is 
setting MyPgXact->xmin.
It is GetSnapshotData called because of reloading configuration settings.
And configuration setting are reloaded because our application is 
updating "synchronous_standby_names".


-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company



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