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:  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?


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

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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