Re: Libpq support to connect to standby server as priority

On Mon, Feb 8, 2021 at 8:17 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
>
> >
> > I think what we want to do is mark default_transaction_read_only as
> > GUC_REPORT, instead. That will give a reliable report of what the
> > state of its GUC is, and you can combine it with is_hot_standby
> > to decide whether the session should be considered read-only.
> > If you don't get those two GUC values during connection, then you
> > can fall back on "SHOW transaction_read_only".
> >
>
> I have made a patch for the above with the changes suggested and
> rebased it with the head code.
> Attached v21 patch which has the changes for the same.
> Thoughts?

Further to my other doc change feedback, I can only spot the following
minor things (otherwise the changes that you have made seek OK to me).

1) doc/src/sgml/protocol.sgml

<varname>default_transaction_read_only</varname> and
<varname>in_hot_standby</varname> were not reported by releases before
14.)

should be:

<varname>default_transaction_read_only</varname> and
<varname>in_hot_standby</varname> were not reported by releases before
14.0)

2) doc/src/sgml/high-availability,sgml

<para>
During hot standby, the parameter <varname>in_hot_standby</varname> and
<varname>default_transaction_read_only</varname> are always true and may
not be changed.

should be:

<para>
During hot standby, the parameters <varname>in_hot_standby</varname> and
<varname>transaction_read_only</varname> are always true and may
not be changed.

[I believe that there's only checks on attempts to change
"transaction_read_only" when in hot_standby, not
"default_transaction_read_only"; see check_transaction_read_only()]

3) src/interfaces/libpq/fe-connect.c

In rejectCheckedReadOrWriteConnection() and
rejectCheckedStandbyConnection(), now that host and port info are
emitted separately and are not included in each error message string
(as parameters in a format string), I think those functions should use
appendPQExpBufferStr() instead of appendPQExpBuffer(), as it's more
efficient if there is just a single string argument.

Regards,
Greg Nancarrow
Fujitsu Australia