[MySQL Bug] 在RBR复制模式下使用Merge引擎导致备库Crash的bug
[MySQL Bug] 在RBR复制模式下使用Merge引擎导致备库Crash的bug,而在table_def::compatible_with中：can_convert_field_to(field,
最近碰到一个很蛋疼的BUG，使用merge引擎，row模式下复制，极大的概率crash备库。
我的测试环境是5.1的主库，5.5的备库
test case：
crash的原因非常简单，实际上在很早之前这个bug就fix了(MySQL Bug#47103)， 可能是官方疏忽，又或者是Merge存储引擎用的人少，没有将这个patch backport到5.5中。以下是个简单的介绍。
我们知道每个Binlog的行事件都有一个table_map_event。
在函数Table_map_log_event::do_apply_event中，会根据table map event的内容初始化table_list结构体，并构建table_list->m_tabledef。
这里的table_list->m_tabledef.m_field_metadata是被分配内存的。然后将该table_list挂到rli->tables_to_lock上
而在函数Rows_log_event::do_apply_event中，当调用open_and_lock_tables实际打开表时，还会打开相依赖的子表，并将这些table_list挂到rli->tables_to_lock之后(链接指针为next_global)，但要注意，这里打开的依赖字表时，其table_list->m_tabledef.m_field_metadata实0x0，并没有分配空间
随后，在打开表后，要检查table map里的定义和实际打开表的定义是否相容。
而在table_def::compatible_with中：can_convert_field_to(field, type(col), field_metadata(col), rli, m_flags, &order)
其中type和field_metadata都会涉及到对m_field_metadata的引用，，因此直接产生段错误，导致crash。
官方的fix见
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