MDEV-36810 Deduplicate LOG_EVENT_OFFSET

`LOG_EVENT_OFFSET` is a duplicate of `EVENT_TYPE_OFFSET`
and `BIN_LOG_HEADER_SIZE` depending on the context.

While there, centralize `BIN_LOG_HEADER_SIZE` to `sql/rpl_constants.h`
(previously from `sql/unireg.h` and `client/mysqlbinlog.cc`)

Reviewed-by: Brandon Nesterenko <brandon.nesterenko@mariadb.com>

client/mysqlbinlog.cc

@@ -70,7 +70,6 @@ extern "C" unsigned char *mysql_net_store_length(unsigned char *packet, size_t l
 
 Rpl_filter *binlog_filter= 0;
 
-#define BIN_LOG_HEADER_SIZE	4
 #define PROBE_HEADER_LEN	(EVENT_LEN_OFFSET+4)
 
 /* Needed for Rpl_filter */

sql/log.cc

@@ -5956,8 +5956,11 @@ int MYSQL_BIN_LOG::new_file_impl(bool commit_by_rotate)
       We log the whole file name for log file as the user may decide
       to change base names at some point.
     */
-    Rotate_log_event r(new_name + dirname_length(new_name), 0, LOG_EVENT_OFFSET,
+    Rotate_log_event r(
-                       is_relay_log ? Rotate_log_event::RELAY_LOG : 0);
+      &new_name[dirname_length(new_name)], /* strlen() */ 0,
+      BIN_LOG_HEADER_SIZE,
+      is_relay_log ? Rotate_log_event::RELAY_LOG : 0
+    );
     enum_binlog_checksum_alg checksum_alg = BINLOG_CHECKSUM_ALG_UNDEF;
     /*
       The current relay-log's closing Rotate event must have checksum

sql/log_event.h

@@ -92,8 +92,6 @@ class String;
 #define LOG_READ_CHECKSUM_FAILURE -8
 #define LOG_READ_DECRYPT -9
 
-#define LOG_EVENT_OFFSET 4
-
 /*
    3 is MySQL 4.x; 4 is MySQL 5.0.0.
    Compared to version 3, version 4 has:

sql/rpl_constants.h

@@ -20,6 +20,9 @@
 #include <my_sys.h>
 #include <my_crypt.h>
 
+/** Size of the `"\xfe""bin"` magic mumber */
+#define BIN_LOG_HEADER_SIZE 4
+
 /**
    Enumeration of the incidents that can occur for the server.
  */

sql/sql_repl.cc

@@ -2261,7 +2261,9 @@ send_event_to_slave(binlog_send_info *info, Log_event_type event_type,
     return "Failed on my_net_write()";
   }
 
-  DBUG_PRINT("info", ("log event code %d", (*packet)[LOG_EVENT_OFFSET+1] ));
+  DBUG_PRINT("info", ("log event code %d",
+    (*packet)[/* Replication protocol status byte */ 1 + EVENT_TYPE_OFFSET]
+  ));
   if (event_type == LOAD_EVENT)
   {
     if (send_file(info->thd))
@@ -2528,7 +2530,9 @@ static int send_format_descriptor_event(binlog_send_info *info, IO_CACHE *log,
     DBUG_RETURN(1);
   }
 
-  event_type= (Log_event_type)((uchar)(*packet)[LOG_EVENT_OFFSET+ev_offset]);
+  event_type= static_cast<Log_event_type>(
+    static_cast<unsigned char>((*packet)[ev_offset + EVENT_TYPE_OFFSET])
+  );
 
   /*
     The packet has offsets equal to the normal offsets in a
@@ -2663,7 +2667,9 @@ static int send_format_descriptor_event(binlog_send_info *info, IO_CACHE *log,
     DBUG_RETURN(1);
   }
 
-  event_type= (Log_event_type)((uchar)(*packet)[LOG_EVENT_OFFSET + ev_offset]);
+  event_type= static_cast<Log_event_type>(
+    static_cast<unsigned char>((*packet)[ev_offset + EVENT_TYPE_OFFSET])
+  );
   if (event_type == START_ENCRYPTION_EVENT)
   {
     Start_encryption_log_event *sele= (Start_encryption_log_event *)
@@ -2931,8 +2937,10 @@ static int send_events(binlog_send_info *info, IO_CACHE* log, LOG_INFO* linfo,
       return 1;
     }
 
-    Log_event_type event_type=
+  Log_event_type event_type= static_cast<Log_event_type>(
-        (Log_event_type)((uchar)(*packet)[LOG_EVENT_OFFSET+ev_offset]);
+    static_cast<unsigned char>((*packet)[ev_offset + EVENT_TYPE_OFFSET])
+  );
+
 
 #ifndef DBUG_OFF
     if (info->dbug_reconnect_counter > 0)

sql/unireg.h

@@ -144,14 +144,6 @@
 */
 #define MIN_TURBOBM_PATTERN_LEN 3
 
-/* 
-   Defines for binary logging.
-   Do not decrease the value of BIN_LOG_HEADER_SIZE.
-   Do not even increase it before checking code.
-*/
-
-#define BIN_LOG_HEADER_SIZE    4 
-
 #define DEFAULT_KEY_CACHE_NAME "default"