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

Dear Dilip,

Thank you for reviewing! 

> 
> 1.
> + conn = connectToServer(&new_cluster, "template1");
> +
> + prep_status("Checking for logical replication slots");
> +
> + res = executeQueryOrDie(conn, "SELECT slot_name "
> + "FROM pg_catalog.pg_replication_slots "
> + "WHERE slot_type = 'logical' AND "
> + "temporary IS FALSE;");
> 
> 
> I think we should add some comment saying this query will only fetch
> logical slots because the database name will always be NULL in the
> physical slots.  Otherwise looking at the query it is very confusing
> how it is avoiding the physical slots.

Hmm, the query you pointed out does not check the database of the slot...
We are fetching only logical slots by the condition "slot_type = 'logical'",
I think it is too trivial to describe in the comment.
Just to confirm - pg_replication_slots can see alls the slots even if the database
is not current one.

```
tmp=# SELECT slot_name, slot_type, database FROM pg_replication_slots where database != current_database();
 slot_name | slot_type | database 
-----------+-----------+----------
 test      | logical   | postgres
(1 row)
```

If I misunderstood something, please tell me...

> 2.
> +void
> +get_old_cluster_logical_slot_infos(void)
> +{
> + int dbnum;
> +
> + /* Logical slots can be migrated since PG17. */
> + if (GET_MAJOR_VERSION(old_cluster.major_version) <= 1600)
> + return;
> +
> + pg_log(PG_VERBOSE, "\nsource databases:");
> 
> I think we need to change some headings like "slot info source
> databases:"  Or add an extra message saying printing slot information.
> 
> Before this patch, we were printing all the relation information so
> message ordering was quite clear e.g.
> 
> source databases:
> Database: "template1"
> relname: "pg_catalog.pg_largeobject", reloid: 2613, reltblspace: ""
> relname: "pg_catalog.pg_largeobject_loid_pn_index", reloid: 2683,
> reltblspace: ""
> Database: "postgres"
> relname: "pg_catalog.pg_largeobject", reloid: 2613, reltblspace: ""
> relname: "pg_catalog.pg_largeobject_loid_pn_index", reloid: 2683,
> reltblspace: ""
> 
> But after this patch slot information is also getting printed in a
> similar fashion so it's very confusing now.  Refer
> get_db_and_rel_infos() for how it is fetching all the relation
> information first and then printing them.
> 
> 
> 
> 
> 3. One more problem is that the slot information and the execute query
> messages are intermingled so it becomes more confusing, see the below
> example of the latest messaging.  I think ideally we should execute
> these queries first
> and then print all slot information together instead of intermingling
> the messages.
> 
> source databases:
> executing: SELECT pg_catalog.set_config('search_path', '', false);
> executing: SELECT slot_name, plugin, two_phase FROM
> pg_catalog.pg_replication_slots WHERE wal_status <> 'lost' AND
> database = current_database() AND temporary IS FALSE;
> Database: "template1"
> executing: SELECT pg_catalog.set_config('search_path', '', false);
> executing: SELECT slot_name, plugin, two_phase FROM
> pg_catalog.pg_replication_slots WHERE wal_status <> 'lost' AND
> database = current_database() AND temporary IS FALSE;
> Database: "postgres"
> slotname: "isolation_slot1", plugin: "pgoutput", two_phase: 0
> 
> 4.  Looking at the above two comments I feel that now the order should be like
> - Fetch all the db infos
> get_db_infos()
> - loop
>    get_rel_infos()
>    get_old_cluster_logical_slot_infos()
> 
> -- and now print relation and slot information per database
>  print_db_infos()

Fixed like that. It seems that we go back to old style...
Now the debug prints are like below:

```
source databases:
Database: "template1"
relname: "pg_catalog.pg_largeobject", reloid: 2613, reltblspace: ""
relname: "pg_catalog.pg_largeobject_loid_pn_index", reloid: 2683, reltblspace: ""
Database: "postgres"
relname: "pg_catalog.pg_largeobject", reloid: 2613, reltblspace: ""
relname: "pg_catalog.pg_largeobject_loid_pn_index", reloid: 2683, reltblspace: ""
Logical replication slots within the database:
slotname: "old1", plugin: "test_decoding", two_phase: 0
slotname: "old2", plugin: "test_decoding", two_phase: 0
slotname: "old3", plugin: "test_decoding", two_phase: 0
```


Best Regards,
Hayato Kuroda
FUJITSU LIMITED