RE: POC: enable logical decoding when wal_level = 'replica' without a server restart

Dear Sawada-san,

> Thank you for testing the patch!
> 
> I've reworked the locking part in the patch. The attached v4 patch
> should address all review comments including your previous
> comments[1].

Thanks for making the patch! I resumed to spending time for the project.
Here are my comments.

1.
Just in case - can you modify xlogdesc.c based on your fix?

2.
Currently pg_upgrade has below checking:
```
	if (nslots_on_old > 0 && strcmp(wal_level, "logical") != 0)
		pg_fatal("\"wal_level\" must be \"logical\" but is set to \"%s\"",
				 wal_level);
```

But this can be relaxed because wal_level can be adjusted appropriately. IIUC it
is enough to be higher than "minimal". Is it right?

3.
Currently pg_createsubscriber has below checking:
```
	if (strcmp(wal_level, "logical") != 0)
	{
		pg_log_error("publisher requires \"wal_level\" >= \"logical\"");
		failed = true;
	}
```

I feel the checking is completely not needed, because pg_createsubscriber needs
a streaming standby and wal_level = minimal cannot be set with this node placement.
Thought?

4.
We should update PG_CONTROL_VERSION and pg_controldata as well.

5.
I'm wondering how pg_resetwal handles. Since all the replication slot cannot be
used after the command, logicalDecodingEnabled can be set to false, right?

Best regards,
Hayato Kuroda
FUJITSU LIMITED