Letting the client choose the protocol to use during a SASL exchange

Hi all,

There is still one open item pending for SCRAM that has not been
treated which is mentioned here:
https://www.postgresql.org/message-id/b081887e-1712-3aa4-7dbe-e012333d50e4@iki.fi

When doing an authentication with SASL, the server decides what is the
mechanism that the client has to use. As SCRAM-SHA-256 is only one of
such mechanisms, it would be nice to have something more generic and
have the server return to the client a list of protocols that the
client can choose from. And also the server achnowledge which protocol
is going to be used.

Note that RFC4422 has some content on the matter
https://tools.ietf.org/html/rfc4422#section-3.1:
Mechanism negotiation is protocol specific.

Commonly, a protocol will specify that the server advertises
supported and available mechanisms to the client via some facility
provided by the protocol, and the client will then select the "best"
mechanism from this list that it supports and finds suitable.

So once the server sends back the list of mechanisms that are
supported, the client is free to use what it wants.

On HEAD, a 'R' message with AUTH_REQ_SASL followed by
SCRAM_SHA256_NAME is sent to let the client know what is the mechanism
to use for the SASL exchange. In the future, this should be extended
so as a list of names is sent, for example a comma-separated list, but
we are free to choose the format we want here. With this list at hand,
the client can then choose the protocol it thinks is the best. Still,
there is a gap with our current implementation because the server
expects the first message from the client to have a SCRAM format, but
that's true only if SCRAM-SHA-256 is used as mechanism.

In order to cover this gap, it seems to me that we need to have an
intermediate state before the server is switched to FE_SCRAM_INIT so
as the mechanism used is negotiated between the two parties. Once the
protocol negotiation is done, the server can then move on with the
mechanism to use. This would be important in the future to allow more
SASL mechanisms to work. I am adding an open item for that.

For extensibility, we may also want to revisit the choice of defining
'scram' in pg_hba.conf instead of 'sasl'...

Thoughts?
--
Michael