MDEV-16146: MariaDB slave stops with following errors.

Problem:
========
180511 11:07:58 [ERROR] Slave I/O: Unexpected master's heartbeat data:
heartbeat is not compatible with local info;the event's data: log_file_name
mysql-bin.000009 log_pos 1054262041, Error_code: 1623

Analysis:
=========
In replication setup when master server doesn't have any events to send to
slave server it sends an 'Heartbeat_log_event'. This event carries the
current binary log filename and offset details. The offset values is stored
within 4 bytes of event header. When the size of binary log is higher than
UINT32_MAX the log_pos values will not fit in 4 bytes memory.  It overflows
and hence slave stops with an error.

Fix:
===
Since we cannot extend the common_header of Log_event class, a greater than
4GB value of Log_event::log_pos is made to be transported with a HeartBeat
event's sub-header.  Log_event::log_pos in such case is set to zero to
indicate that the 8 byte sub-header is allocated in the event.

In case of cross version replication following behaviour is expected

OLD - Server without fix
NEW - Server with fix

OLD<->NEW : works bidirectionally as long as the binlog offset is
            (normally) within 4GB.

When log_pos > UINT32_MAX
OLD->NEW  : The 'log_pos' is bound to overflow and NEW slave may report
            an invalid event/incompatible heart beat event error.
NEW->OLD  : Since patched server sets log_pos=0 on overflow, OLD slave will
            report invalid event error.

1 files changed, 27 insertions, 4 deletions

diff --git a/sql/sql_repl.cc b/sql/sql_repl.cc
index 59a3f686e45..7ff0e27b008 100644
--- a/sql/sql_repl.cc
+++ b/sql/sql_repl.cc
@@ -32,6 +32,7 @@
 #include "debug_sync.h"
 #include "log.h"                                // get_gtid_list_event
 
+
 enum enum_gtid_until_state {
   GTID_UNTIL_NOT_DONE,
   GTID_UNTIL_STOP_AFTER_STANDALONE,
@@ -781,7 +782,7 @@ get_slave_until_gtid(THD *thd, String *out_str)
   @param event_coordinates  binlog file name and position of the last
                             real event master sent from binlog
 
-  @note 
+  @note
     Among three essential pieces of heartbeat data Log_event::when
     is computed locally.
     The  error to send is serious and should force terminating
@@ -795,6 +796,8 @@ static int send_heartbeat_event(binlog_send_info *info,
   DBUG_ENTER("send_heartbeat_event");
 
   ulong ev_offset;
+  char sub_header_buf[HB_SUB_HEADER_LEN];
+  bool sub_header_in_use=false;
   if (reset_transmit_packet(info, info->flags, &ev_offset, &info->errmsg))
     DBUG_RETURN(1);
 
@@ -815,18 +818,38 @@ static int send_heartbeat_event(binlog_send_info *info,
   ulong event_len = ident_len + LOG_EVENT_HEADER_LEN +
     (do_checksum ? BINLOG_CHECKSUM_LEN : 0);
   int4store(header + SERVER_ID_OFFSET, global_system_variables.server_id);
+  DBUG_EXECUTE_IF("simulate_pos_4G",
+  {
+    const_cast<event_coordinates *>(coord)->pos= (UINT_MAX32 + (ulong)1);
+    DBUG_SET("-d, simulate_pos_4G");
+  };);
+  if (coord->pos <= UINT_MAX32)
+  {
+    int4store(header + LOG_POS_OFFSET, coord->pos);  // log_pos
+  }
+  else
+  {
+    // Set common_header.log_pos=0 to indicate its overflow
+    int4store(header + LOG_POS_OFFSET, 0);
+    sub_header_in_use= true;
+    int8store(sub_header_buf, coord->pos);
+    event_len+= HB_SUB_HEADER_LEN;
+  }
+
   int4store(header + EVENT_LEN_OFFSET, event_len);
   int2store(header + FLAGS_OFFSET, 0);
 
-  int4store(header + LOG_POS_OFFSET, coord->pos);  // log_pos
-
   packet->append(header, sizeof(header));
-  packet->append(p, ident_len);             // log_file_name
+  if (sub_header_in_use)
+    packet->append(sub_header_buf, sizeof(sub_header_buf));
+  packet->append(p, ident_len);                    // log_file_name
 
   if (do_checksum)
   {
     char b[BINLOG_CHECKSUM_LEN];
     ha_checksum crc= my_checksum(0, (uchar*) header, sizeof(header));
+    if (sub_header_in_use)
+      crc= my_checksum(crc, (uchar*) sub_header_buf, sizeof(sub_header_buf));
     crc= my_checksum(crc, (uchar*) p, ident_len);
     int4store(b, crc);
     packet->append(b, sizeof(b));