从零开始学习MySQL调试跟踪（2）

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• GreatSQL是MySQL的国产分支版本，使用上与MySQL一致。
• 作者：  Yejinrong/叶金荣
• 文章来源：GreatSQL社区投稿

1. 启用coredump
2. 制造一个coredump场景
3. 真实故障场景分析跟踪

上一篇文档介绍了如何构建gdb跟踪调试环境，本文介绍如何根据错误日志信息，跟踪定位问题可能的原因，以及如何利用coredump文件查找问题线索。

1. 启用coredump

程序运行过程中可能会异常终止或崩溃，OS会把程序挂掉时的内存状态记录下来，写入core文件，这就叫 coredump，通过gdb结合core文件可以方便地进行调试。

利用core文件中保留的异常堆栈文件，能够帮助研发同学更快定位问题。因此，如果某些故障断断续续会出现，建议阶段性开启coredump功能。

想要开启coredump，需要先修改OS层的几个设置：

$ ulimit -c unlimited
$ sysctl -w fs.suid_dumpable=2
$ echo "core.%p.%e.%s" > /proc/sys/kernel/core_pattern

同时，将这些修改持久化到相应文件中（假定MySQL/GreatSQL服务进程的属主用户是  mysql）：

$ echo "mysql - core unlimited" >> /etc/security/limits.conf
$ echo "fs.suid_dumpable=2" >> /etc/sysctl.conf
$ echo "kernel.core_pattern=core.%e.%p.%t" >> /etc/sysctl.conf
$ sysctl -p

接下来，修改 my.cnf 配置文件，增加以下两行内容：

core_file
innodb_buffer_pool_in_core_file=OFF

然后重启GreatSQL服务进程，即可生效，查询确认下：

mysql> show global variables like '%core%';
+---------------------------------+-------+
| Variable_name | Value |
+---------------------------------+-------+
| core_file | ON |
| innodb_buffer_pool_in_core_file | OFF |
+---------------------------------+-------+

这样设置完成后，需要的话会在 datadir 目录下生成core文件。

2. 制造一个coredump场景

我们可以给mysqld进程发送 SIGSEGV(11) 信号，即可模拟出coredump的场景，例如：

$ kill -s SIGSEGV `pidof mysqld`

这时查看GreatSQL错误日志文件，以及core文件，就会发现有coredump：

$ls -la
...
-rw------- 1 mysql mysql 1081147392 Feb 20 22:36 core.mysqld-debug.2658134.1676903816
...

Thread pointer: 0x0
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
stack_bottom = 0 thread_stack 0x80000
/usr/local/GreatSQL-8.0.25-16-Linux-glibc2.28-x86_64/bin/mysqld-debug(my_print_stacktrace(unsigned char const*, unsigned long)+0x43) [0x4b04
d26]
/usr/local/GreatSQL-8.0.25-16-Linux-glibc2.28-x86_64/bin/mysqld-debug(handle_fatal_signal+0x2cb) [0x39a7d22]
/lib64/libpthread.so.0(+0x12c20) [0x7fc3e669ac20]
/lib64/libc.so.6(__poll+0x51) [0x7fc3e45c4a41]
/usr/local/GreatSQL-8.0.25-16-Linux-glibc2.28-x86_64/bin/mysqld-debug(Mysqld_socket_listener::listen_for_connection_event()+0x57) [0x3995195
]
/usr/local/GreatSQL-8.0.25-16-Linux-glibc2.28-x86_64/bin/mysqld-debug(Connection_acceptor<Mysqld_socket_listener>::connection_event_loop()+0
x30) [0x355a024]
/usr/local/GreatSQL-8.0.25-16-Linux-glibc2.28-x86_64/bin/mysqld-debug(mysqld_main(int, char**)+0x27d2) [0x354e4a6]
/usr/local/GreatSQL-8.0.25-16-Linux-glibc2.28-x86_64/bin/mysqld-debug(main+0x20) [0x32de906]
/lib64/libc.so.6(__libc_start_main+0xf3) [0x7fc3e44f6493]
/usr/local/GreatSQL-8.0.25-16-Linux-glibc2.28-x86_64/bin/mysqld-debug(_start+0x2e) [0x32de82e]
Please help us make Percona Server better by reporting any
bugs at https://bugs.percona.com/
...

在一线的同学，如果需要向研发寻求支持或报告故障时，可以先参考这篇文章 MySQL报障之coredump收集处理流程，需要采集其他几个信息：

• 故障时刻的error log。
• 故障产生的core文件。
• 如果有general log的话，也采集起来（故障时刻往前约1小时或10万行日志）。
• 导致core发生涉及到的表DDL以及相应的SQL语句，有必要的话，可能还要同时提供真实数据（或样例数据）。

3. 真实故障场景分析跟踪

在GreatSQL 8.0.25-15版本（上一个版本）中，InnoDB并行查询功能在特定场景下存在bug，会导致crash，相应的日志见下：

mysqld-debug: /opt/greatsql-8.0.25/sql/item.cc:6047: virtual void Item_field::make_field(Send_field*): Assertion `item_name.is_set()' failed.
01:59:20 UTC - mysqld got signal 6 ;
Most likely, you have hit a bug, but this error can also be caused by malfunctioning hardware.

Trying to get some variables.
Some pointers may be invalid and cause the dump to abort.
Query (7fb4a9a65028): SELECT ... FROM t1 WHERE ... #<-- 这是触发bug的SQL语句
Connection ID (thread ID): 8
Status: NOT_KILLED

按照上面所说的方法，我们采集了所有相关信息，并能在测试环境重现上述故障。

接下来，我们利用gdb来定位分析问题原因：

$ gdb path/bin/mysqld-debug path/core.mysqld-debug.2657287.1657270311
GNU gdb (GDB) Red Hat Enterprise Linux 9.2-4.el8
...
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./bin/mysqld-debug...
...
[New LWP 2675795]
[New LWP 2675825]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
--Type <RET> for more, q to quit, c to continue without paging--
Core was generated by `./bin/mysqld-debug --defaults-extra-file=./my.cnf'.
Program terminated with signal SIGABRT, Aborted.
#0 __pthread_kill (threadid=<optimized out>, signo=6) at ../sysdeps/unix/sysv/linux/pthread_kill.c:56
56 return (INTERNAL_SYSCALL_ERROR_P (val, err)
[Current thread is 1 (Thread 0x7fb4f7aa7700 (LWP 2676055))]
(gdb)
(gdb) b *0x3338758 #<-- 上面记下的指针值，前面加个 "*" 号，在这里打上断点
Breakpoint 1 at 0x3338758: file /opt/greatsql-8.0.25/sql/item.cc, line 6048. #<-- 指向可能触发问题的源码位置
(gdb)
(gdb) bt #<-- 打印详细backtrace信息
#0 __pthread_kill (threadid=<optimized out>, signo=6) at ../sysdeps/unix/sysv/linux/pthread_kill.c:56
#1 0x0000000004b04f1d in my_write_core (sig=6) at /opt/greatsql-8.0.25/mysys/stacktrace.cc:409
#2 0x00000000039a7f84 in handle_fatal_signal (sig=6) at /opt/greatsql-8.0.25/sql/signal_handler.cc:199
#3 <signal handler called>
#4 __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#5 0x00007fb512524db5 in __GI_abort () at abort.c:79
#6 0x00007fb512524c89 in __assert_fail_base (fmt=0x7fb51268d698 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=0x57a1835 "item_name.is_set()",
file=0x57a1400 "/opt/greatsql-8.0.25/sql/item.cc", line=6047, function=<optimized out>) at assert.c:92
#7 0x00007fb512532a76 in __GI___assert_fail (assertion=0x57a1835 "item_name.is_set()", file=0x57a1400 "/opt/greatsql-8.0.25/sql/item.cc", line=6047,
function=0x57a3a40 "virtual void Item_field::make_field(Send_field*)") at assert.c:101
#8 0x0000000003338758 in Item_field::make_field (this=0x7fb4a9b5bcf8, tmp_field=0x7fb4f7aa2380) at /opt/greatsql-8.0.25/sql/item.cc:6047
#9 0x00000000036977ab in THD::send_result_metadata (this=0x7fb4a9a0b000, list=..., flags=5) at /opt/greatsql-8.0.25/sql/sql_class.cc:2824
#10 0x00000000035f3ff9 in Query_result_send::send_result_set_metadata (this=0x7fb4a9a0fda0, thd=0x7fb4a9a0b000, list=..., flags=5)
at /opt/greatsql-8.0.25/sql/query_result.cc:76
#11 0x00000000038d057b in Query_expression::ExecuteIteratorQuery (this=0x7fb4a9a65178, thd=0x7fb4a9a0b000) at /opt/greatsql-8.0.25/sql/sql_union.cc:1150
#12 0x00000000038d0d7d in Query_expression::execute (this=0x7fb4a9a65178, thd=0x7fb4a9a0b000) at /opt/greatsql-8.0.25/sql/sql_union.cc:1321
#13 0x000000000381db25 in Sql_cmd_dml::execute_inner (this=0x7fb4a9a0fd68, thd=0x7fb4a9a0b000) at /opt/greatsql-8.0.25/sql/sql_select.cc:814
#14 0x000000000381cfab in Sql_cmd_dml::execute (this=0x7fb4a9a0fd68, thd=0x7fb4a9a0b000) at /opt/greatsql-8.0.25/sql/sql_select.cc:585
#15 0x00000000037a1a2b in mysql_execute_command (thd=0x7fb4a9a0b000, first_level=true) at /opt/greatsql-8.0.25/sql/sql_parse.cc:4684
#16 0x00000000037a3a1d in dispatch_sql_command (thd=0x7fb4a9a0b000, parser_state=0x7fb4f7aa41d0, update_userstat=false)
at /opt/greatsql-8.0.25/sql/sql_parse.cc:5284
#17 0x0000000003799819 in dispatch_command (thd=0x7fb4a9a0b000, com_data=0x7fb4f7aa5370, command=COM_QUERY) at /opt/greatsql-8.0.25/sql/sql_parse.cc:1940
#18 0x0000000003797c48 in do_command (thd=0x7fb4a9a0b000) at /opt/greatsql-8.0.25/sql/sql_parse.cc:1388
#19 0x0000000003991168 in handle_connection (arg=0x7fb4ba094500) at /opt/greatsql-8.0.25/sql/conn_handler/connection_handler_per_thread.cc:307
#20 0x00000000052e4b22 in pfs_spawn_thread (arg=0x7fb511e44320) at /opt/greatsql-8.0.25/storage/perfschema/pfs.cc:2899
#21 0x00007fb5146c017a in start_thread (arg=<optimized out>) at pthread_create.c:479
#22 0x00007fb5125ffdc3 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

有了这些信息，研发同学再去跟踪定位问题根源就会方便很多。

本文简单演示了如何利用core文件去跟踪定位分析可能导致crash的原因，更多有趣实用的方法还有待进一步挖掘，一起探索新世界吧。