RE: [PoC] pg_upgrade: allow to upgrade publisher node

Dear Bharath,

Thank you for reviewing! Before addressing them, I would like to reply some comments.

> 6.
> + if (PQntuples(res) != 1)
> + pg_fatal("could not count the number of logical replication slots");
> +
>
> Not existing a single logical replication slot an error? I think it
> must be if (PQntuples(res) == 0) return;?
>

The query executes "SELECT count(*)...", IIUC it exactly returns 1 row.

> 7. A nit:
> + nslots_on_new = atoi(PQgetvalue(res, 0, 0));
> +
> + if (nslots_on_new)
>
> Just do if(atoi(PQgetvalue(res, 0, 0)) > 0) and get rid of nslots_on_new?

Note that the vaule would be used for upcoming pg_fatal. I prefer current style
because multiple atoi(PQgetvalue(res, 0, 0)) was not so beautiful.

>
> 11.
> +# 2. Generate extra WAL records. Because these WAL records do not get
> consumed
> +# it will cause the upcoming pg_upgrade test to fail.
> +$old_publisher->safe_psql('postgres',
> + "CREATE TABLE tbl AS SELECT generate_series(1, 10) AS a;"
> +);
> +$old_publisher->stop;
>
> This might be a recipie for sporadic test failures - how is it
> guaranteed that the newly generated WAL records aren't consumed.

You mentioned at line 118, but at that time logical replication system is not created.
The subscriber is created at line 163.
Therefore WALs would not be consumed automatically.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED