Index: /PS5518/branches/PS-r1331-rpl-circular/mysql-test/suite/rpl/t/rpl_circular_event_detect.cnf
===================================================================
--- /PS5518/branches/PS-r1331-rpl-circular/mysql-test/suite/rpl/t/rpl_circular_event_detect.cnf	(revision 0)
+++ /PS5518/branches/PS-r1331-rpl-circular/mysql-test/suite/rpl/t/rpl_circular_event_detect.cnf	(revision 1861)
@@ -0,0 +1,10 @@
+!include ../my.cnf
+
+[mysqld.1]
+log-slave-updates
+innodb
+
+[mysqld.2]
+log-slave-updates
+innodb
+
Index: /PS5518/branches/PS-r1331-rpl-circular/mysql-test/suite/rpl/t/rpl_circular_event_detect.test
===================================================================
--- /PS5518/branches/PS-r1331-rpl-circular/mysql-test/suite/rpl/t/rpl_circular_event_detect.test	(revision 0)
+++ /PS5518/branches/PS-r1331-rpl-circular/mysql-test/suite/rpl/t/rpl_circular_event_detect.test	(revision 1861)
@@ -0,0 +1,86 @@
+# ==== Purpose ====
+#
+# Confirm abnormal event with another server_id would be passed
+# among the circular replacted hosts, and it should be detected
+# with warning information.
+#
+#
+# ==== Related bugs and worklogs ====
+#
+
+# 1. setup environment: 
+#    server1(server_id=1)  <---(dual master)--> server2(server_id=2)
+# 2. create table t on server1, and it should be replicated to server2
+# 3. stop slave on server1
+# 4. insert one record on server1
+# 5. change server_id from 1 to 3 on server1
+# 6. start slave on server1
+# 7. expected record more than 1 on server1 (also warning of dual master for event)
+
+
+--source include/have_innodb.inc
+# Use wait_for_slave_to_(start|stop) for current connections
+let $keep_connection= 1;
+
+# Set up circular ring and new names for servers
+--echo [set up circular replication on two servers]
+--let $rpl_topology= 1->2->1
+--source include/rpl_init.inc
+--echo
+
+# Preparing data on server1.
+--echo [preparing data on server1]
+--connection server_1
+
+# make sure log_slave_updates is on
+#SET session log_slave_updates=on;
+
+#CREATE TABLE IF NOT EXISTS t (a INT NOT NULL AUTO_INCREMENT, b VARCHAR(100), PRIMARY KEY(a)) ENGINE=InnoDB;
+CREATE TABLE IF NOT EXISTS t (a VARCHAR(100)) ENGINE=InnoDB;
+
+--echo [skip create table events on server1]
+#CHANGE MASTER TO master_log_pos=276;
+
+call mtr.add_suppression("Detect dual master circular topological structure *");
+
+--real_sleep 1
+STOP SLAVE;
+
+--connection server_2
+STOP SLAVE;
+
+--connection server_1
+INSERT INTO t values('abc');
+
+SET GLOBAL SERVER_ID = 3;
+START SLAVE;
+
+--connection server_2
+START SLAVE;
+
+
+#--real_sleep 3
+
+--connection server_1
+let $wait_condition= SELECT COUNT(*) > 10 FROM t;
+--source include/wait_condition.inc
+#select * from t;
+
+--connection server_2
+#select * from t;
+
+--echo
+--echo *** detect event with circle A->B->A  ***
+--echo
+
+--disable_warnings
+
+# Clean up environment
+--echo [clean up]
+
+--connection server_1
+SET GLOBAL SERVER_ID = 1;
+DROP TABLE IF EXISTS  t;
+
+--source include/rpl_reset.inc
+--source include/rpl_end.inc
Index: /PS5518/branches/PS-r1331-rpl-circular/mysql-test/suite/rpl/r/rpl_circular_event_detect.result
===================================================================
--- /PS5518/branches/PS-r1331-rpl-circular/mysql-test/suite/rpl/r/rpl_circular_event_detect.result	(revision 0)
+++ /PS5518/branches/PS-r1331-rpl-circular/mysql-test/suite/rpl/r/rpl_circular_event_detect.result	(revision 1861)
@@ -0,0 +1,21 @@
+[set up circular replication on two servers]
+include/rpl_init.inc [topology=1->2->1]
+
+[preparing data on server1]
+CREATE TABLE IF NOT EXISTS t (a VARCHAR(100)) ENGINE=InnoDB;
+[skip create table events on server1]
+call mtr.add_suppression("Detect dual master circular topological structure *");
+STOP SLAVE;
+STOP SLAVE;
+INSERT INTO t values('abc');
+SET GLOBAL SERVER_ID = 3;
+START SLAVE;
+START SLAVE;
+
+*** detect event with circle A->B->A  ***
+
+[clean up]
+SET GLOBAL SERVER_ID = 1;
+DROP TABLE IF EXISTS  t;
+include/rpl_reset.inc
+include/rpl_end.inc
Index: /PS5518/branches/PS-r1331-rpl-circular/sql/repl_failsafe.h
===================================================================
--- /PS5518/branches/PS-r1331-rpl-circular/sql/repl_failsafe.h	(revision 1824)
+++ /PS5518/branches/PS-r1331-rpl-circular/sql/repl_failsafe.h	(revision 1861)
@@ -37,6 +37,7 @@
 int find_recovery_captain(THD* thd, MYSQL* mysql);
 
 extern HASH slave_list;
+extern bool rpl_circular_warning;
 
 bool show_slave_hosts(THD* thd);
 void init_slave_list();
Index: /PS5518/branches/PS-r1331-rpl-circular/sql/repl_failsafe.cc
===================================================================
--- /PS5518/branches/PS-r1331-rpl-circular/sql/repl_failsafe.cc	(revision 1824)
+++ /PS5518/branches/PS-r1331-rpl-circular/sql/repl_failsafe.cc	(revision 1861)
@@ -151,6 +151,8 @@
   mysql_mutex_lock(&LOCK_slave_list);
   unregister_slave(thd,0,0);
   res= my_hash_insert(&slave_list, (uchar*) si);
+  /*when new slave registerd, rpl circular becomes possible*/
+  rpl_circular_warning = true;
   mysql_mutex_unlock(&LOCK_slave_list);
   return res;
 
Index: /PS5518/branches/PS-r1331-rpl-circular/sql/slave.cc
===================================================================
--- /PS5518/branches/PS-r1331-rpl-circular/sql/slave.cc	(revision 1824)
+++ /PS5518/branches/PS-r1331-rpl-circular/sql/slave.cc	(revision 1861)
@@ -75,6 +75,9 @@
 my_bool replicate_same_server_id;
 ulonglong relay_log_space_limit = 0;
 
+/*wheather to show warning message when rpl circular detected*/
+bool rpl_circular_warning = false;
+
 /*
   When slave thread exits, we need to remember the temporary tables so we
   can re-use them on slave start.
@@ -4060,6 +4063,25 @@
   }
   else
   {
+    if (rpl_circular_warning && s_id !=::server_id)
+    {
+      /*
+        if replicated event's server_id is neither the same with current
+        instance's id, nor id in slave_list, a warning should be given
+        for dual mastser topological structure.
+      */ 
+      mysql_mutex_lock(&LOCK_slave_list);
+      if (!my_hash_search(&slave_list, (uchar*)&s_id, 4) &&
+          my_hash_search(&slave_list, (uchar*)&mi->master_id, 4))
+      {
+        sql_print_warning("Detect dual master circular topological structure: "
+          " event_sid=%ld, server id=%ld, master(slave) id=%ld ",
+          s_id, ::server_id, mi->master_id);
+        rpl_circular_warning = false;
+      }
+      mysql_mutex_unlock(&LOCK_slave_list);
+    }
+
     /* write the event to the relay log */
     if (likely(!(rli->relay_log.appendv(buf,event_len,0))))
     {