standby promote: fall back to "pg_ctl promote" if necessary

From PostgreSQL 12, the SQL-level function "pg_promote()" can be used
to promote a PostgreSQL instance, however usage is restricted to
superusers and users to whom explicit execution permission for this
function has been granted.

Therefore, if execution permission is not available, fall back to
"pg_ctl promote".

dbutils.c

@@ -1723,6 +1723,43 @@ get_timeline_history(PGconn *repl_conn, TimeLineID tli)
 /* user/role information functions */
 /* =============================== */
 
+
+bool
+can_execute_pg_promote(PGconn *conn)
+{
+	PQExpBufferData query;
+	PGresult   *res;
+	bool		has_pg_promote= false;
+
+	/* pg_promote() available from PostgreSQL 12 */
+	if(PQserverVersion(conn) < 120000)
+		return false;
+
+	initPQExpBuffer(&query);
+	appendPQExpBufferStr(&query,
+						 " SELECT pg_catalog.has_function_privilege( "
+						 "    CURRENT_USER, "
+						 "    'pg_catalog.pg_promote(bool,int)', "
+						 "    'execute' "
+						 " )");
+
+	res = PQexec(conn, query.data);
+
+	if (PQresultStatus(res) != PGRES_TUPLES_OK)
+	{
+		log_db_error(conn, query.data,
+					 _("can_execute_pg_promote(): unable to query user function privilege"));
+	}
+	else
+	{
+		has_pg_promote = atobool(PQgetvalue(res, 0, 0));
+	}
+	termPQExpBuffer(&query);
+
+	return has_pg_promote;
+}
+
+
 bool
 connection_has_pg_settings(PGconn *conn)
 {