Add more sanity checks in SASL exchanges

The following checks are added, to make the SASL infrastructure more
aware of defects when implementing new mechanisms:
- Detect that no output is generated by a mechanism if an exchange fails
in the backend, failing if there is a message waiting to be sent.
- Handle zero-length messages in the frontend.  The backend handles that
already, and SCRAM would complain if sending empty messages as this is
not authorized for this mechanism, but other mechanisms may want this
capability (the SASL specification allows that).
- Make sure that a mechanism generates a message in the middle of the
exchange in the frontend.

SCRAM, as implemented, respects all these requirements already, and the
recent refactoring of SASL done in 9fd8557 helps in documenting that in
a cleaner way.

Analyzed-by: Jacob Champion
Author: Michael Paquier
Reviewed-by: Jacob Champion
Discussion: https://postgr.es/m/3d2a6f5d50e741117d6baf83eb67ebf1a8a35a11.camel@vmware.com

src/backend/libpq/auth-sasl.c

@@ -171,6 +171,13 @@ CheckSASLAuth(const pg_be_sasl_mech *mech, Port *port, char *shadow_pass,
 
 		if (output)
 		{
+			/*
+			 * PG_SASL_EXCHANGE_FAILURE with some output is forbidden by SASL.
+			 * Make sure here that the mechanism used got that right.
+			 */
+			if (result == PG_SASL_EXCHANGE_FAILURE)
+				elog(ERROR, "output message found after SASL exchange failure");
+
 			/*
 			 * Negotiation generated data to be sent to the client.
 			 */