MDEV-11016 wsrep_node_is_ready() check is too strict

Problem:-
  The condition that checks for node readiness is too strict as it does
  not allow SELECTs even if these selects do not access any tables.
  For example,if we run
       SELECT 1;
  OR
       SELECT @@max_allowed_packet;
  Solution:-
  We need not to report this error when all_tables(lex->query_tables)
  is NULL:

mysql-test/suite/galera/r/galera_var_dirty_reads.result

@@ -18,6 +18,19 @@ SET @@session.wsrep_dirty_reads=ON;
 SELECT * FROM t1;
 i
 1
+SET @@session.wsrep_dirty_reads=OFF;
+SELECT 2;
+2
+2
+SELECT @@max_allowed_packet;
+@@max_allowed_packet
+4194304
+SELECT 2+2 from DUAL;
+2+2
+4
+SELECT sysdate() from DUAL;
+sysdate()
+2016-10-28 23:13:06
 SELECT * FROM t1;
 i
 1

mysql-test/suite/galera/t/galera_var_dirty_reads.test

@@ -36,6 +36,13 @@ SET @@session.wsrep_dirty_reads=ON;
 
 SELECT * FROM t1;
 
+#Select query which does not access table should be allowed MDEV-11016
+SET @@session.wsrep_dirty_reads=OFF;
+SELECT 2;
+SELECT @@max_allowed_packet;
+SELECT 2+2 from DUAL;
+SELECT sysdate() from DUAL;
+
 --disable_query_log
 --eval SET @@global.wsrep_cluster_address = '$wsrep_cluster_address_saved'
 --enable_query_log

sql/sql_parse.cc

@@ -2650,12 +2650,15 @@ mysql_execute_command(THD *thd)
 
     /*
       Bail out if DB snapshot has not been installed. We however, allow SET,
-      SHOW and SELECT queries (only if wsrep_dirty_reads is set).
+      SHOW and SELECT queries (only if wsrep_dirty_reads is set or when it
+      does not access any table).
     */
     if (lex->sql_command != SQLCOM_SET_OPTION  &&
         !wsrep_is_show_query(lex->sql_command) &&
         !(thd->variables.wsrep_dirty_reads     &&
           lex->sql_command == SQLCOM_SELECT)   &&
+        !(lex->sql_command == SQLCOM_SELECT    &&
+          !all_tables)                         &&
         !wsrep_node_is_ready(thd))
       goto error;
   }