Re: pgsql: Add libpq parameter 'channel_binding'.

On Mon, Sep 30, 2019 at 03:45:39PM +0900, Michael Paquier wrote:
> If the server publishes SCRAM-SHA-256-PLUS and the server does not
> support channel binding, then we get this error message:
> "channel binding is required, but server did not offer an
> authentication method that supports channel binding."
> So that's the part which is wrong.
> 
> Now, I am not completely sure that the suggested change is completely
> right either as we would get an error in this scenario when
> channel_binding is "prefer" or "require".  For "require", this error
> message is fine.  However, for "prefer", shouldn't we do what we do on
> HEAD, aka *not* select SCRAM-SHA-256-PLUS and switch to SCRAM-SHA-256?
> This would have the advantage to make the connection work with default
> parameters.

So, something like the attached looks better to me.  Using a server
which publishes SCRAM-SHA-256-PLUS, I get the following over SSL:
1) client supports channel binding:
1-1) channel_binding = disable => OK, with SCRAM-SHA-256 
1-2) channel_binding = prefer => OK, with SCRAM-SHA-256-PLUS
1-3) channel_binding = require => OK, with SCRAM-SHA-256-PLUS
2) client does not support channel binding
2-1) channel_binding = disable => OK, with SCRAM-SHA-256
2-2) channel_binding = prefer => OK, with SCRAM-SHA-256
2-3) channel_binding = require => failure with new error message,
instead of the confusing one.

The bug is with 2-3, and Tom's suggestion would have switched 2-2 to a
failure (2-2 works on HEAD).
--
Michael