BUG#16438800 - SLAVE_MAX_ALLOWED_PACKET NOT HONORED ON SLAVE IO CONNECT

      
Problem - When the slave was disconnected from the master, under certain 
          conditions, upon reconnect, it will report that it received a 
          packet larger the slave_max_allowed_packet which causes the
          replication to stop.
 
Analysis -The reason of this failure is that on reconnect
          the slave sets the max_allowed_packet from the master's mi->mysql
          object which keeps the max_allowed_packet as 1MB. This causes the 
          slave to report such error on recieving packet bigger than 1MB. 
          START SLAVE on the slave fixes the problem since it restarts
          slave threads which initializes the max_allowed_packet to
          slave_max_allowed_packet.
      
Fix - The problem is fixed by some code refactoring and introduction of a new
      function which updates the max_allowed_packet for the THD object of the
      slave thread and the mysql->options max_allowed_packet.

sql/slave.cc

@@ -158,6 +158,37 @@ static int terminate_slave_thread(THD *thd,
                                   volatile uint *slave_running,
                                   bool skip_lock);
 static bool check_io_slave_killed(THD *thd, Master_info *mi, const char *info);
+/*
+  Function to set the slave's max_allowed_packet based on the value
+  of slave_max_allowed_packet.
+
+    @in_param    thd    Thread handler for slave
+    @in_param    mysql  MySQL connection handle
+*/
+
+static void set_slave_max_allowed_packet(THD *thd, MYSQL *mysql)
+{
+  DBUG_ENTER("set_slave_max_allowed_packet");
+  // thd and mysql must be valid
+  DBUG_ASSERT(thd && mysql);
+
+  thd->variables.max_allowed_packet= slave_max_allowed_packet;
+  thd->net.max_packet_size= slave_max_allowed_packet;
+  /*
+    Adding MAX_LOG_EVENT_HEADER_LEN to the max_packet_size on the I/O
+    thread and the mysql->option max_allowed_packet, since a
+    replication event can become this much  larger than
+    the corresponding packet (query) sent from client to master.
+  */
+  thd->net.max_packet_size+= MAX_LOG_EVENT_HEADER;
+  /*
+    Skipping the setting of mysql->net.max_packet size to slave
+    max_allowed_packet since this is done during mysql_real_connect.
+  */
+  mysql->options.max_allowed_packet=
+    slave_max_allowed_packet+MAX_LOG_EVENT_HEADER;
+  DBUG_VOID_RETURN;
+}
 
 /*
   Find out which replications threads are running
@@ -2072,12 +2103,6 @@ static int init_slave_thread(THD* thd, SLAVE_THD_TYPE thd_type)
     SYSTEM_THREAD_SLAVE_SQL : SYSTEM_THREAD_SLAVE_IO;
   thd->security_ctx->skip_grants();
   my_net_init(&thd->net, 0);
-/*
-  Adding MAX_LOG_EVENT_HEADER_LEN to the max_allowed_packet on all
-  slave threads, since a replication event can become this much larger
-  than the corresponding packet (query) sent from client to master.
-*/
-  thd->variables.max_allowed_packet= slave_max_allowed_packet;
   thd->slave_thread = 1;
   thd->enable_slow_log= opt_log_slow_slave_statements;
   set_slave_thread_options(thd);
@@ -2828,13 +2853,6 @@ pthread_handler_t handle_slave_io(void *arg)
                           mi->user, mi->host, mi->port,
 			  IO_RPL_LOG_NAME,
 			  llstr(mi->master_log_pos,llbuff));
-  /*
-    Adding MAX_LOG_EVENT_HEADER_LEN to the max_packet_size on the I/O
-    thread, since a replication event can become this much larger than
-    the corresponding packet (query) sent from client to master.
-  */
-    thd->net.max_packet_size= slave_max_allowed_packet;
-    mysql->net.max_packet_size= thd->net.max_packet_size+= MAX_LOG_EVENT_HEADER;
   }
   else
   {
@@ -4231,7 +4249,7 @@ static int connect_to_master(THD* thd, MYSQL* mysql, Master_info* mi,
   ulong err_count=0;
   char llbuff[22];
   DBUG_ENTER("connect_to_master");
-
+  set_slave_max_allowed_packet(thd, mysql);
 #ifndef DBUG_OFF
   mi->events_till_disconnect = disconnect_slave_event_count;
 #endif