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, 11 insertions, 2 deletions

diff --git a/sql/log_event.h b/sql/log_event.h
index 3fc44a9669f..bd40795a7fb 100644
--- a/sql/log_event.h
+++ b/sql/log_event.h
@@ -570,6 +570,14 @@ class String;
 #define MARIA_SLAVE_CAPABILITY_MINE MARIA_SLAVE_CAPABILITY_GTID
 
 
+/*
+  When the size of 'log_pos' within Heartbeat_log_event exceeds UINT32_MAX it
+  cannot be accommodated in common_header, as 'log_pos' is of 4 bytes size. In
+  such cases, sub_header, of size 8 bytes will hold larger 'log_pos' value.
+*/
+#define HB_SUB_HEADER_LEN 8
+
+
 /**
   @enum Log_event_type
 
@@ -5160,12 +5168,13 @@ static inline bool copy_event_cache_to_file_and_reinit(IO_CACHE *cache,
 class Heartbeat_log_event: public Log_event
 {
 public:
-  Heartbeat_log_event(const char* buf, uint event_len,
+  uint8 hb_flags;
+  Heartbeat_log_event(const char* buf, ulong event_len,
                       const Format_description_log_event* description_event);
   Log_event_type get_type_code() { return HEARTBEAT_LOG_EVENT; }
   bool is_valid() const
     {
-      return (log_ident != NULL &&
+      return (log_ident != NULL && ident_len <= FN_REFLEN-1 &&
               log_pos >= BIN_LOG_HEADER_SIZE);
     }
   const char * get_log_ident() { return log_ident; }