RE: Have pg_basebackup write "dbname" in "primary_conninfo"?

Dear Robert,

> > Just FYI - here is an extreme case. And note that I have applied proposed patch.
> >
> > When `pg_basebackup -D data_N2 -R` is used:
> > ```
> > primary_conninfo = 'user=hayato ... dbname=hayato ...
> > ```
> >
> > But when `pg_basebackup -d "" -D data_N2 -R` is used:
> > ```
> > primary_conninfo = 'user=hayato ... dbname=replication
> > ```
> 
> It seems like maybe somebody should look into why this is happening,
> and perhaps fix it.

I think this caused from below part [1] in GetConnection().

If both dbname and connection_string are the NULL, we will enter the else part
and NULL would be substituted - {"dbnmae", NULL} key-value pair is generated
only here.

Then, in PQconnectdbParams()->PQconnectStartParams->pqConnectOptions2(),
the strange part would be found and replaced to the username [2].

I think if both the connection string and the dbname are NULL, it should be
considered as the physical replication connection. here is a patch to fix it.
After the application, below two examples can output "dbname=replication".
You can also confirm.

```
pg_basebackup -D data_N2 -U postgres
pg_basebackup -D data_N2 -R -v

-> primary_conninfo = 'user=postgres ... dbname=replication ...
```

[1]
```
    else
    {
        keywords = pg_malloc0((argcount + 1) * sizeof(*keywords));
        values = pg_malloc0((argcount + 1) * sizeof(*values));
        keywords[i] = "dbname";
        values[i] = dbname;
        i++;
    }
```

[2]
```
    /*
     * If database name was not given, default it to equal user name
     */
    if (conn->dbName == NULL || conn->dbName[0] == '\0')
    {
        free(conn->dbName);
        conn->dbName = strdup(conn->pguser);
        if (!conn->dbName)
            goto oom_error;
    }
```


Best Regards,
Hayato Kuroda
FUJITSU LIMITED
https://www.fujitsu.com/