MDEV-32862 MYSQL struct in C/C and server differs

move MYSQL::fields down, replacing MYSQL::unused5 this way only MYSQL::fields and MYSQL::field_alloc will still have different offset in C/C and the server, but all other MYSQL members will get back in sync. luckily, plugins shouldn't need MYSQL::fields or MYSQL::field_alloc added a check to ensure both MYSQL structures are always of the same size.

Author: vuvova

include/mysql.h

@@ -274,7 +274,6 @@ typedef struct st_mysql
  char*host,*user,*passwd,*unix_socket,*server_version,*host_info;
  char *info, *db;
  const struct charset_info_st *charset;
- MYSQL_FIELD*fields;
  MEM_ROOTfield_alloc;
  my_ulonglong affected_rows;
  my_ulonglong insert_id;/* id if insert on table with NEXTNR */
@@ -296,7 +295,8 @@ typedef struct st_mysql
  /* session-wide random string */
  char scramble[SCRAMBLE_LENGTH+1];
  my_bool auto_local_infile;
- void *unused2, *unused3, *unused4, *unused5;
+ void *unused2, *unused3, *unused4;
+ MYSQL_FIELD*fields;
 
  LIST *stmts; /* list of all statements */
  const struct st_mysql_methods *methods;

tests/mysql_client_fw.c

@@ -1426,6 +1426,14 @@ int main(int argc, char **argv)
  tests_to_run[i]= NULL;
  }
 
+#ifdef _WIN32
+ /* must be the same in C/C and embedded, 1208 on 64bit, 968 on 32bit */
+ compile_time_assert(sizeof(MYSQL) == 60*sizeof(void*)+728);
+#else
+ /* must be the same in C/C and embedded, 1272 on 64bit, 964 on 32bit */
+ compile_time_assert(sizeof(MYSQL) == 77*sizeof(void*)+656);
+#endif
+
  if (mysql_server_init(embedded_server_arg_count,
  embedded_server_args,
  (char**) embedded_server_groups))