bug#19402 SQL close to the size of the max_allowed_packet fails on the slave

comments are fixed as was suggested in reviews.

sql/log_event.h

@@ -147,8 +147,10 @@ struct sql_ex_info
 #define EXEC_LOAD_HEADER_LEN   4
 #define DELETE_FILE_HEADER_LEN 4
 
-/* amount of byte is contribution of replicaton protocol to a query
-   received from user when the query goes to binlog */
+/* 
+  Max number of possible extra bytes in a replication event compared to a
+  packet (i.e. a query) sent from client to master.
+*/
 #define MAX_LOG_EVENT_HEADER   (LOG_EVENT_HEADER_LEN + /* write_header */ \
 				QUERY_HEADER_LEN     + /* write_data */   \
 				NAME_LEN + 1)

sql/slave.cc

@@ -2618,8 +2618,13 @@ static int init_slave_thread(THD* thd, SLAVE_THD_TYPE thd_type)
     SYSTEM_THREAD_SLAVE_SQL : SYSTEM_THREAD_SLAVE_IO; 
   thd->host_or_ip= "";
   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= global_system_variables.max_allowed_packet
-    + MAX_LOG_EVENT_HEADER;  /* reentering secured through using global */
+    + MAX_LOG_EVENT_HEADER;  /* note, incr over the global not session var */
   thd->net.read_timeout = slave_net_timeout;
   thd->master_access= ~(ulong)0;
   thd->priv_user = 0;
@@ -3156,7 +3161,11 @@ extern "C" pthread_handler_decl(handle_slave_io,arg)
 			  mi->host, mi->port,
 			  IO_RPL_LOG_NAME,
 			  llstr(mi->master_log_pos,llbuff));
-    /* post-net-init for slave */
+  /*
+    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.
+  */
     mysql->net.max_packet_size= thd->net.max_packet_size+= MAX_LOG_EVENT_HEADER;
   }
   else

sql/sql_repl.cc

@@ -420,7 +420,11 @@ impossible position";
     goto err;
   }
   packet->set("\0", 1, &my_charset_bin);
-  /* dump thread  the whole header size of query_log_event */
+  /*
+    Adding MAX_LOG_EVENT_HEADER_LEN, since a binlog event can become
+    this larger than the corresponding packet (query) sent 
+    from client to master.
+  */
   thd->variables.max_allowed_packet+= MAX_LOG_EVENT_HEADER;
 
   while (!net->error && net->vio != 0 && !thd->killed)