MySQL 派生表查询导致 Crash 的根源分析与解决方案
一、问题发现
在之前的 MySQL 8.0.32 使用中，发现使用以下带有派生表的 SQL 会导致 MySQL Crash，以下的sequence_table(2)替换为任何非常量表都行：
仅 MySQL 8.0.32 版本有影响。
EXPLAIN FORMAT=TREE
select
trim(ref_15.c_ogj),
0<>0 as c_lrcm63eani
from
(select
0<>0 as c_ogj
from
sequence_table(2) t1
where 0<>0
order by c_ogj asc) as ref_15;
Crash 的堆栈如下：
Thread 55 "mysqld" received signal SIGSEGV, Segmentation fault.
Item_view_ref::used_tables (this=0x7fff2418f410)
at sql/item.h:6670
6670 table_map inner_map = ref_item()->used_tables(); ==> ref_item()为空指针，因此crash了
(gdb) bt
#0 Item_view_ref::used_tables (this=0x7fff2418f410)
at sql/item.h:6670
#1 0x0000555558e978d1 in Item::const_item (this=0x7fff2418f410)
at sql/item.h:2342
#2 0x0000555558ecc765 in Item_ref::print (this=0x7fff2418f410, thd=0x7fff24001050,
str=0x7fffc83ee7e0, query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/item.cc:9993
#3 0x000055555903b839 in Item_func_trim::print (this=0x7fff24120d20, thd=0x7fff24001050,
str=0x7fffc83ee7e0, query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/item_strfunc.cc:3244
#4 0x0000555558ea7fc5 in Item::print_item_w_name (this=0x7fff24120d20, thd=0x7fff24001050,
str=0x7fffc83ee7e0, query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/item.cc:727
#5 0x00005555593f18c0 in Query_block::print_item_list (this=0x7fff24120768, thd=0x7fff24001050,
str=0x7fffc83ee7e0, query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/sql_lex.cc:4041
#6 0x00005555593efb50 in Query_block::print_query_block (this=0x7fff24120768,
thd=0x7fff24001050, str=0x7fffc83ee7e0,
query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/sql_lex.cc:3614
#7 0x00005555593efa3d in Query_block::print (this=0x7fff24120768, thd=0x7fff24001050,
str=0x7fffc83ee7e0, query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/sql_lex.cc:3598
#8 0x00005555593ee556 in Query_expression::print (this=0x7fff24120670, thd=0x7fff24001050,
str=0x7fffc83ee7e0, query_type=(QT_TO_SYSTEM_CHARSET | QT_SHOW_SELECT_NUMBER))
at sql/sql_lex.cc:3232
#9 0x0000555559a89c2c in print_query_for_explain (query_thd=0x7fff24001050,
unit=0x7fff24120670, str=0x7fffc83ee7e0)
at sql/opt_explain.cc:2288
#10 0x0000555559a10b11 in PrintQueryPlan[abi:cxx11](THD*, THD const*, Query_expression*) (
ethd=0x7fff24001050, query_thd=0x7fff24001050, unit=0x7fff24120670)
at sql/join_optimizer/explain_access_path.cc:1894
#11 0x0000555559a8985a in ExplainIterator (ethd=0x7fff24001050, query_thd=0x7fff24001050,
unit=0x7fff24120670) at sql/opt_explain.cc:2205
#12 0x0000555559a89e91 in explain_query (explain_thd=0x7fff24001050, query_thd=0x7fff24001050,
unit=0x7fff24120670) at sql/opt_explain.cc:2359
#13 0x000055555955cd46 in Sql_cmd_dml::execute_inner (this=0x7fff24165630, thd=0x7fff24001050)
二、问题调查过程
调查执行 SQL 的 optimize 的过程，分析发现该 SQL 的 SQL 变换情况如下：
以下的 trim(ref_15.c_ogj) 执行完 find_order_in_list 后，Item_func_trim的args[0]->m_ref_item[0] 等于0<>0 as c_lrcm63eani，而不是0<>0 as c_ogj，这是因为c_lrcm63eani和c_ogj的名字都一样，都是0<>0，在find_order_in_list函数里面由于名字一样因此内层字段被外层替代了。而后在Item::clean_up_after_removal执行的时候，Item_func_ne即c_lrcm63eani因为出现了2次，因此执行了2次decrement_ref_count()，然而在Query_block::delete_unused_merged_columns函数却把0<>0 as c_lrcm63eani的Item置为空了，因为这个时候c_lrcm63eani的item->decrement_ref_count()以后ref_count()为0因此继续执行Item::clean_up_after_removal了。
EXPLAIN FORMAT=TREE
select
trim(ref_15.c_ogj),
0<>0 as c_lrcm63eani
from
(select
0<>0 as c_ogj
from
sequence_table(2) t1
where 0<>0
order by c_ogj asc) as ref_15;
查看函数调用过程发现 Query_block 在 prepare 的时候执行了 delete_unused_merged_columns，
-- 函数调用过程： Query_block::prepare -> Query_block::apply_local_transforms -> Query_block::delete_unused_merged_columns
bool find_order_in_list() {
if (select_item != not_found_item) {
if ((*order->item)->real_item() != (*select_item)->real_item()) {
Item::Cleanup_after_removal_context ctx(
thd->lex->current_query_block());
(*order->item)
->walk(&Item::clean_up_after_removal, walk_options, ==>Item_func_ne执行了2次，也执行了2次decrement_ref_count()
pointer_cast<uchar *>(&ctx));
}
}
}
bool Query_block::apply_local_transforms(THD *thd, bool prune) {
DBUG_TRACE;
assert(first_execution);
-- 这个函数把((Item_func *)&fields[0][0])->args[0]->m_ref_item[0]给删了
if (derived_table_count) delete_unused_merged_columns(&m_table_nest);
}
void Query_block::delete_unused_merged_columns(
mem_root_deque<Table_ref *> *tables) {
DBUG_TRACE;
for (Table_ref *tl : *tables) {
if (tl->nested_join == nullptr) continue;
if (tl->is_merged()) {
for (Field_translator *transl = tl->field_translation;
transl < tl->field_translation_end; transl++) {
Item *const item = transl->item;
// Decrement the ref count as its no more used in
// select list.
if (item->decrement_ref_count()) continue; -- 因为执行完decrement_ref_count()以后返回的m_ref_count=0因此不会跳出这个循环
// Cleanup the item since its not referenced from
// anywhere.
assert(item->fixed);
Item::Cleanup_after_removal_context ctx(this);
item->walk(&Item::clean_up_after_removal, walk_options,
pointer_cast<uchar *>(&ctx));
transl->item = nullptr; -- 这个地方把Item_view_ref引用的Item_func_ne对象置为空了，即把trim函数参数的c_lrcm63eani列删除了
}
}
delete_unused_merged_columns(&tl->nested_join->m_tables);
}
}
三、解决方案
通过上面的分析，我们可以发现问题在于多执行了一次Item::clean_up_after_removal，随后在 MySQL 最新代码尝试执行以上 SQL 发现该 BUG 已经被修复，找到相关修复代码，可以发现以下修复代码。
相关commit ID号为： 2171a1260e2cdbbd379646be8ff6413a92fd48f4
-- 相关修复代码如下：
@@ -7575,7 +7865,6 @@ bool Item::clean_up_after_removal(uchar *arg) {
if (reference_count() > 1) {
(void)decrement_ref_count();
+ ctx->stop_at(this);
}
return false;
}
修改完查看一下这个函数的堆栈信息：
#0 Item::clean_up_after_removal (this=0x2,
arg=0x41 <error: Cannot access memory at address 0x41>)
at sql/item.cc:9236
#1 0x0000555558fea5a8 in Item::walk (this=0x7fff2c338db8, processor=&virtual table offset 864,
walk=7, arg=0x7fffc83ee4b0 "") at sql/item.h:2543
#2 0x00005555596cc6f2 in find_order_in_list (thd=0x7fff2c001070, ref_item_array=...,
tables=0x7fff2c330b90, order=0x7fff2c32eae8, fields=0x7fff2c32fb20, is_group_field=false,
is_window_order=false) at sql/sql_resolver.cc:4625
#3 0x00005555596cd0ae in setup_order (thd=0x7fff2c001070, ref_item_array=...,
tables=0x7fff2c330b90, fields=0x7fff2c32fb20, order=0x7fff2c32eae8)
at sql/sql_resolver.cc:4811
#4 0x00005555596bf528 in Query_block::prepare (this=0x7fff2c32fae0, thd=0x7fff2c001070,
insert_field_list=0x0) at sql/sql_resolver.cc:400
#5 0x00005555597d035d in Query_expression::prepare (this=0x7fff2c32f9e8, thd=0x7fff2c001070,
sel_result=0x7fff2c33b2a8, insert_field_list=0x0, added_options=0, removed_options=0)
at sql/sql_union.cc:758
#6 0x0000555559590772 in Table_ref::resolve_derived (this=0x7fff2c339790, thd=0x7fff2c001070,
apply_semijoin=true) at sql/sql_derived.cc:451
#7 0x00005555596c2a80 in Query_block::resolve_placeholder_tables (this=0x7fff2c333f08,
thd=0x7fff2c001070, apply_semijoin=true)
at sql/sql_resolver.cc:1408
#8 0x00005555596bea62 in Query_block::prepare (this=0x7fff2c333f08, thd=0x7fff2c001070,
insert_field_list=0x0) at sql/sql_resolver.cc:265
对于0<>0 as c_lrcm63eani这个Item_func_ne对象，执行到Item::clean_up_after_removal的时候，因为reference_count() > 1因此会执行新添加的ctx->stop_at(this)，等到下一次再执行到这个Item_func_ne的clean_up_after_removal()函数的时候，就会因为ctx->is_stopped(this)而直接返回，不再执行一次decrement_ref_count()，从而避免了执行后面的transl->item = nullptr。
bool find_order_in_list() {
if (select_item != not_found_item) {
if ((*order->item)->real_item() != (*select_item)->real_item()) {
Item::Cleanup_after_removal_context ctx(
thd->lex->current_query_block());
(*order->item)
->walk(&Item::clean_up_after_removal, walk_options, -- Item_func_ne执行了2次，而只执行了一次decrement_ref_count()
pointer_cast<uchar *>(&ctx));
}
}
}
void Query_block::delete_unused_merged_columns(
mem_root_deque<Table_ref *> *tables) {
DBUG_TRACE;
for (Table_ref *tl : *tables) {
if (tl->nested_join == nullptr) continue;
if (tl->is_merged()) {
for (Field_translator *transl = tl->field_translation;
transl < tl->field_translation_end; transl++) {
Item *const item = transl->item;
// Decrement the ref count as its no more used in
// select list.
if (item->decrement_ref_count()) continue; 因为执行完decrement_ref_count()以后返回的m_ref_count=1因此不会继续执行后面的置空设置
// Cleanup the item since its not referenced from
// anywhere.
assert(item->fixed);
Item::Cleanup_after_removal_context ctx(this);
item->walk(&Item::clean_up_after_removal, walk_options,
pointer_cast<uchar *>(&ctx));
transl->item = nullptr; ==>这个地方不会运行到
}
}
delete_unused_merged_columns(&tl->nested_join->m_tables);
}
}
四、问题总结
通过以上分析我们可以发现，对于复杂的 SQL 会执行复杂的 Item 变换和删除不需要的 Item，但是正是由于这样才更容易导致 Crash 的出现。分析类似这样的 Crash 问题的时候，因为涉及代码量大，代码逻辑复杂往往很难找到相关修复代码，因此需要对代码运行流程比较熟悉，同时要有相关复杂问题解决的经验才能更好的应对这类问题。
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