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, 13 insertions, 4 deletions

diff --git a/sql/log_event.cc b/sql/log_event.cc
index c32f31db1f6..94b2af20354 100644
--- a/sql/log_event.cc
+++ b/sql/log_event.cc
@@ -14324,14 +14324,23 @@ st_print_event_info::st_print_event_info()
 #endif
 
 #if defined(HAVE_REPLICATION) && !defined(MYSQL_CLIENT)
-Heartbeat_log_event::Heartbeat_log_event(const char* buf, uint event_len,
+Heartbeat_log_event::Heartbeat_log_event(const char* buf, ulong event_len,
                     const Format_description_log_event* description_event)
   :Log_event(buf, description_event)
 {
   uint8 header_size= description_event->common_header_len;
-  ident_len = event_len - header_size;
-  set_if_smaller(ident_len,FN_REFLEN-1);
-  log_ident= buf + header_size;
+  if (log_pos == 0)
+  {
+    log_pos= uint8korr(buf + header_size);
+    log_ident= buf + header_size + HB_SUB_HEADER_LEN;
+    ident_len= event_len - (header_size + HB_SUB_HEADER_LEN);
+  }
+  else
+  {
+    log_ident= buf + header_size;
+    ident_len = event_len - header_size;
+  }
+
 }
 #endif