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

On Tue, Mar 19, 2024 at 8:48 PM Hayato Kuroda (Fujitsu)
<kuroda.hayato@fujitsu.com> wrote:
>
> Dear Sawada-san,
>
> Thanks for giving comments!
>
> > This behavior makes sense to me. But do we want to handle the case of
> > using environment variables too?
>
> Yeah, v5 does not consider which libpq parameters are specified by environment
> variables. Such a variable should be used when the dbname is not expressly written
> in the connection string.
> Such a path was added in the v6 patch. If the dbname is not determined after
> parsing the connection string, we call PQconndefaults() to get settings from
> environment variables and service files [1], then start to search dbname again.
> Below shows an example.

Thank you for updating the patch!

>
> ```
> PGPORT=5431 PGUSER=kuroda PGDATABASE=postgres pg_basebackup -D data_N2 -R -v
> ->
> primary_conninfo = 'user=kuroda ... port=5431 ... dbname=postgres ... '
> ```
>
> > IIUC,
> >
> > pg_basebackup -D tmp -d "user=masahiko dbname=test_db"
> >
> > is equivalent to:
> >
> > PGDATABASE="user=masahiko dbname=test_db" pg_basebackup -D tmp
>
> The case won't work. I think You assumed that expanded_dbname like
> PQconnectdbParams() [2] can be used for enviroment variables, but it is not correct
> - it won't parse as connection string again.
>
> In the libpq layer, connection parameters are parsed in PQconnectStartParams()->conninfo_array_parse().
> When expand_dbname is specified, the entry "dbname" is firstly checked and
> parsed its value. They are done at fe-connect.c:5846.
>
> The environment variables are checked and parsed in conninfo_add_defaults(), which
> is called from conninfo_array_parse(). However, it is done at fe-connect.c:5956 - the
> expand_dbname has already been done at that time. This means there is no chance
> that PGDATABASE is parsed as an expanded style.
>

Thank you for pointing it out. I tested the use of PGDATABASE with
pg_basebackup and somehow missed the fact you explained.

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com