libpq: Handle NegotiateProtocolVersion message

Before, receiving a NegotiateProtocolVersion message would result in a
confusing error message like

    expected authentication request from server, but received v

This adds proper handling of this protocol message and produces an
on-topic error message from it.

Reviewed-by: Jacob Champion <jchampion@timescale.com>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/f9c7862f-b864-8ef7-a861-c4638c83e209%40enterprisedb.com

src/interfaces/libpq/fe-connect.c

@@ -3194,10 +3194,11 @@ keep_going:						/* We will come back to here until there is
 
 				/*
 				 * Validate message type: we expect only an authentication
-				 * request or an error here.  Anything else probably means
+				 * request, NegotiateProtocolVersion, or an error here.
-				 * it's not Postgres on the other end at all.
+				 * Anything else probably means it's not Postgres on the other
+				 * end at all.
 				 */
-				if (!(beresp == 'R' || beresp == 'E'))
+				if (!(beresp == 'R' || beresp == 'v' || beresp == 'E'))
 				{
 					libpq_append_conn_error(conn, "expected authentication request from server, but received %c",
 									   beresp);
@@ -3214,14 +3215,15 @@ keep_going:						/* We will come back to here until there is
 				/*
 				 * Try to validate message length before using it.
 				 * Authentication requests can't be very large, although GSS
-				 * auth requests may not be that small.  Errors can be a
+				 * auth requests may not be that small.  Same for
+				 * NegotiateProtocolVersion.  Errors can be a
 				 * little larger, but not huge.  If we see a large apparent
 				 * length in an error, it means we're really talking to a
 				 * pre-3.0-protocol server; cope.  (Before version 14, the
 				 * server also used the old protocol for errors that happened
 				 * before processing the startup packet.)
 				 */
-				if (beresp == 'R' && (msgLength < 8 || msgLength > 2000))
+				if ((beresp == 'R' || beresp == 'v') && (msgLength < 8 || msgLength > 2000))
 				{
 					libpq_append_conn_error(conn, "expected authentication request from server, but received %c",
 									   beresp);
@@ -3351,6 +3353,16 @@ keep_going:						/* We will come back to here until there is
 
 					goto error_return;
 				}
+				else if (beresp == 'v')
+				{
+					if (pqGetNegotiateProtocolVersion3(conn))
+					{
+						goto error_return;
+					}
+					/* OK, we read the message; mark data consumed */
+					conn->inStart = conn->inCursor;
+					goto error_return;
+				}
 
 				/* It is an authentication request. */
 				conn->auth_req_received = true;

src/interfaces/libpq/fe-protocol3.c

@@ -1385,6 +1385,61 @@ reportErrorPosition(PQExpBuffer msg, const char *query, int loc, int encoding)
 }
 
 
+/*
+ * Attempt to read a NegotiateProtocolVersion message.
+ * Entry: 'v' message type and length have already been consumed.
+ * Exit: returns 0 if successfully consumed message.
+ *		 returns EOF if not enough data.
+ */
+int
+pqGetNegotiateProtocolVersion3(PGconn *conn)
+{
+	int			tmp;
+	ProtocolVersion their_version;
+	int			num;
+	PQExpBufferData buf;
+
+	if (pqGetInt(&tmp, 4, conn) != 0)
+		return EOF;
+	their_version = tmp;
+
+	if (pqGetInt(&num, 4, conn) != 0)
+		return EOF;
+
+	initPQExpBuffer(&buf);
+	for (int i = 0; i < num; i++)
+	{
+		if (pqGets(&conn->workBuffer, conn))
+		{
+			termPQExpBuffer(&buf);
+			return EOF;
+		}
+		if (buf.len > 0)
+			appendPQExpBufferChar(&buf, ' ');
+		appendPQExpBufferStr(&buf, conn->workBuffer.data);
+	}
+
+	if (their_version < conn->pversion)
+		appendPQExpBuffer(&conn->errorMessage,
+						  libpq_gettext("protocol version not supported by server: client uses %u.%u, server supports up to %u.%u\n"),
+						  PG_PROTOCOL_MAJOR(conn->pversion), PG_PROTOCOL_MINOR(conn->pversion),
+						  PG_PROTOCOL_MAJOR(their_version), PG_PROTOCOL_MINOR(their_version));
+	if (num > 0)
+		appendPQExpBuffer(&conn->errorMessage,
+						  libpq_ngettext("protocol extension not supported by server: %s\n",
+										 "protocol extensions not supported by server: %s\n", num),
+						  buf.data);
+
+	/* neither -- server shouldn't have sent it */
+	if (!(their_version < conn->pversion) && !(num > 0))
+		appendPQExpBuffer(&conn->errorMessage,
+						  libpq_gettext("invalid %s message"), "NegotiateProtocolVersion");
+
+	termPQExpBuffer(&buf);
+	return 0;
+}
+
+
 /*
  * Attempt to read a ParameterStatus message.
  * This is possible in several places, so we break it out as a subroutine.

src/interfaces/libpq/libpq-int.h

@@ -685,6 +685,7 @@ extern void pqParseInput3(PGconn *conn);
 extern int	pqGetErrorNotice3(PGconn *conn, bool isError);
 extern void pqBuildErrorMessage3(PQExpBuffer msg, const PGresult *res,
 								 PGVerbosity verbosity, PGContextVisibility show_context);
+extern int	pqGetNegotiateProtocolVersion3(PGconn *conn);
 extern int	pqGetCopyData3(PGconn *conn, char **buffer, int async);
 extern int	pqGetline3(PGconn *conn, char *s, int maxlen);
 extern int	pqGetlineAsync3(PGconn *conn, char *buffer, int bufsize);