Give nicer error message when connecting to a v10 server requiring SCRAM.

This is just to give the user a hint that they need to upgrade, if they try
to connect to a v10 server that uses SCRAM authentication, with an older
client.

Commit to all stable branches, but not master.

Discussion: https://www.postgresql.org/message-id/bbf45d92-3896-eeb7-7399-2111d517261b@pivotal.io

Author: hlinnaka

src/include/libpq/pqcomm.h

@@ -172,6 +172,8 @@ extern bool Db_user_namespace;
 #define AUTH_REQ_GSS		7	/* GSSAPI without wrap() */
 #define AUTH_REQ_GSS_CONT	8	/* Continue GSS exchanges */
 #define AUTH_REQ_SSPI		9	/* SSPI negotiate without wrap() */
+#define AUTH_REQ_SASL	   10	/* SASL authentication. Not supported before
+								 * libpq version 10. */
 
 typedef uint32 AuthRequest;
 

src/interfaces/libpq/fe-auth.c

@@ -703,6 +703,19 @@ pg_fe_sendauth(AuthRequest areq, PGconn *conn)
 				return STATUS_ERROR;
 			break;
 
+			/*
+			 * SASL authentication was introduced in version 10. Older
+			 * versions recognize the request only to give a nicer error
+			 * message. We call it "SCRAM authentication" in the error, rather
+			 * than SASL, because SCRAM is more familiar to users, and it's
+			 * the only SASL authentication mechanism that has been
+			 * implemented as of this writing, anyway.
+			 */
+		case AUTH_REQ_SASL:
+			printfPQExpBuffer(&conn->errorMessage,
+							  libpq_gettext("SCRAM authentication requires libpq version 10 or above\n"));
+			return STATUS_ERROR;
+
 		default:
 			printfPQExpBuffer(&conn->errorMessage,
 			libpq_gettext("authentication method %u not supported\n"), areq);