啃完O'reilly的《高性能mysql》、姜老师的《MySQL技术内幕》，再加上个2,3年的实战经验，就基本可以成为一名能独立处理问题的DBA了。但有些时候遇到些很刁钻的疑难杂症的话，那就束手无策了。所以要想技术水平更进一步的话，源码调试是避不开的。
GDB 简介
GDB 是 Linux 系统中，非常常见的调试工具，它有以下功能：
- Start your program, specifying anything that might affect its behavior.
- Make your program stop on specified conditions.
- Examine what has happened, when your program has stopped.
- Change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another.
常用的参数命令：
- info threads：查看全部线程
- thread n：指定某个线程
- b：在某处打断点
- c：继续往下走
- s：执行一行代码，如果代码函数调用，则进入函数
- n：执行一行代码，函数调用不进入
- p：打印某个变量值
- list：打印代码的文本信息
- bt：查看某个线程的栈帧
- info b：查看当前所有断点信息
调试环境搭建
直接在 linux 下面使用 gdb，这种应该是目前市面上最简单有效的方式。
1. 安装gdb
yum install -y cmake make gcc gcc-c++ ncurses-devel bison gdb
2. 下载、解压源码
wget https://dev.mysql.com/get/Downloads/MySQL-5.7/mysql-boost-5.7.25.tar.gz
tar zxvf mysql-boost-5.7.25.tar.gzmkdir -p /gdb/mysql/
mkdir -p /gdb/data/
3. 安装数据库
cmake -DCMAKE_INSTALL_PREFIX=/gdb/mysql/ -DMYSQL_DATADIR=/gdb/data/ -DSYSCONFDIR=/gdb/mysql/ -DWITH_INNOBASE_STORAGE_ENGINE=1 -DWITH_ARCHIVE_STORAGE_ENGINE=1 -DWITH_BLACKHOLE_STORAGE_ENGINE=1 -DWITH_FEDERATED_STORAGE_ENGINE=1 -DWITH_PARTITION_STORAGE_ENGINE=1 -DMYSQL_UNIX_ADDR=/gdb/mysql/mysql3.sock -DMYSQL_TCP_PORT=3306 -DENABLED_LOCAL_INFILE=1 -DEXTRA_CHARSETS=all -DDEFAULT_CHARSET=utf8 -DDEFAULT_COLLATION=utf8_general_ci -DMYSQL_USER=mysql -DWITH_BINLOG_PREALLOC=ON -DWITH_BOOST=/gdb/mysql-5.7.25/boost/boost_1_59_0 -DWITH_DEBUG=1
-DWITH_DEBUG=1 是最关键的，它的作用是开启DBUG
make&&make install
4. 初始化数据库
vim /etc/my.cnf
#简易配置下my.cnf文件
[client]
port = 3306
socket = /gdb/data/mysqld.sock
[mysqld]
port = 3306
socket =/gdb/data/mysqld.sock
skip-external-locking
key_buffer_size = 8M
max_allowed_packet = 1M
table_open_cache = 64
sort_buffer_size = 512K
net_buffer_length = 8K
read_buffer_size = 128K
read_rnd_buffer_size = 256K
myisam_sort_buffer_size = 8M
lower_case_table_names=1
innodb_buffer_pool_size=300M
log-bin=mysql-bin
character_set_server=utf8
binlog_format=row
datadir=/gdb/data
log-error =/gdb/data/error.log
pid-file = /gdb/data/mysql.pid
innodb_log_file_size=512M
innodb_log_files_in_group = 3
sql_mode=''
autocommit=1
server-id = 1
max_connections=1500
wait_timeout=70
interactive_timeout=70
skip-name-resolve
[mysqldump]
quick
max_allowed_packet = 16M
[myisamchk]
key_buffer_size = 20M
sort_buffer_size = 20M
read_buffer = 2M
write_buffer = 2M
5. 启动数据库
赋权，以便mysql用户有权限在该目录下生成文件：
chown -R mysql:mysql /gdb/data
初始化数据库命令：
cd /gdb/mysql/bin
./mysqld --initialize --user=mysql --basedir=/gdb/mysql --datadir=/gdb/data
启动数据库：
cd /gdb/mysql/support-files
./mysql.server start
insert 断点调试
1. 查看 mysql 进程 id
[root@ops sql]# ps aux | grep mysql
root 629 0.0 0.0 112724 972 pts/2 S+ 14:52 0:00 grep -E --color=auto mysql
root 20926 0.0 0.0 113312 1628 pts/0 S 11:15 0:00 /bin/sh /gdb/mysql/bin/mysqld_safe --datadir=/gdb/data --pid-file=/gdb/data/mysql.pid
mysql 21357 0.0 5.8 1740820 223820 pts/0 Sl 11:15 0:01 /gdb/mysql/bin/mysqld --basedir=/gdb/mysql --datadir=/gdb/data --plugin-dir=/gdb/mysql/lib/plugin --user=mysql --log-error=/gdb/data/error.log --pid-file=/gdb/data/mysql.pid --socket=/gdb/data/mysqld.sock --port=3306
可以看到此时mysql的进程号为：20926
2. gdb 中 attach mysql 进程
[root@ops ~]# gdb
GNU gdb (GDB) Red Hat Enterprise Linux 7.6.1-119.el7
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
(gdb) attach 21357
Attaching to process 21357
Reading symbols from /gdb/mysql/bin/mysqld...done.
Reading symbols from /lib64/libpthread.so.0...(no debugging symbols found)...done.
[New LWP 21617]
[New LWP 21387]
[New LWP 21386]
[New LWP 21384]
[New LWP 21383]
[New LWP 21382]
[New LWP 21381]
[New LWP 21380]
[New LWP 21379]
[New LWP 21378]
[New LWP 21377]
[New LWP 21376]
[New LWP 21375]
[New LWP 21374]
[New LWP 21373]
[New LWP 21369]
[New LWP 21368]
[New LWP 21367]
[New LWP 21366]
[New LWP 21365]
[New LWP 21364]
[New LWP 21363]
[New LWP 21362]
[New LWP 21361]
[New LWP 21360]
[New LWP 21359]
[New LWP 21358]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
Loaded symbols for /lib64/libpthread.so.0
Reading symbols from /lib64/libcrypt.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib64/libcrypt.so.1
Reading symbols from /lib64/libdl.so.2...(no debugging symbols found)...done.
Loaded symbols for /lib64/libdl.so.2
Reading symbols from /lib64/librt.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib64/librt.so.1
Reading symbols from /lib64/libstdc++.so.6...(no debugging symbols found)...done.
Loaded symbols for /lib64/libstdc++.so.6
Reading symbols from /lib64/libm.so.6...(no debugging symbols found)...done.
Loaded symbols for /lib64/libm.so.6
Reading symbols from /lib64/libgcc_s.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib64/libgcc_s.so.1
Reading symbols from /lib64/libc.so.6...(no debugging symbols found)...done.
Loaded symbols for /lib64/libc.so.6
Reading symbols from /lib64/ld-linux-x86-64.so.2...(no debugging symbols found)...done.
Loaded symbols for /lib64/ld-linux-x86-64.so.2
Reading symbols from /lib64/libfreebl3.so...Reading symbols from /lib64/libfreebl3.so...(no debugging symbols found)...done.
(no debugging symbols found)...done.
Loaded symbols for /lib64/libfreebl3.so
Reading symbols from /lib64/libnss_files.so.2...(no debugging symbols found)...done.
Loaded symbols for /lib64/libnss_files.so.2
Reading symbols from /lib64/libnss_sss.so.2...Reading symbols from /lib64/libnss_sss.so.2...(no debugging symbols found)...done.
(no debugging symbols found)...done.
Loaded symbols for /lib64/libnss_sss.so.2
0x00002b15ce803f0d in poll () from /lib64/libc.so.6
Missing separate debuginfos, use: debuginfo-install glibc-2.17-222.el7.x86_64 libgcc-4.8.5-39.el7.x86_64 libstdc++-4.8.5-39.el7.x86_64 nss-softokn-freebl-3.34.0-2.el7.x86_64 sssd-client-1.16.0-19.el7.x86_64
(gdb)
3. 找到断点
这次看的是 insert 插入的流程，找到 sql_insert.cc 文件：
源码中的函数为：Sql_cmd_insert::mysql_insert
4. 设置断点
(gdb) b Sql_cmd_insert::mysql_insert
Breakpoint 1 at 0x175aed9: file /gdb/mysql-5.7.25/sql/sql_insert.cc, line 423.
然后查看下线程的栈帧：
(gdb) bt
#0 0x00002b15ce803f0d in poll () from /lib64/libc.so.6
#1 0x0000000001667f87 in Mysqld_socket_listener::listen_for_connection_event (this=0x3967430) at /gdb/mysql-5.7.25/sql/conn_handler/socket_connection.cc:852
#2 0x0000000000eb15cc in Connection_acceptor<Mysqld_socket_listener>::connection_event_loop (this=0x4f882e0) at /gdb/mysql-5.7.25/sql/conn_handler/connection_acceptor.h:66
#3 0x0000000000ea904a in mysqld_main (argc=38, argv=0x383c248) at /gdb/mysql-5.7.25/sql/mysqld.cc:5149
#4 0x0000000000ea01bd in main (argc=9, argv=0x7ffc73765b88) at /gdb/mysql-5.7.25/sql/main.cc:25
5. 数据库登陆
gdb断点设置完后，起个新的数据库连接：
会发现此时无法登陆，在gdb中执行next：
(gdb) n
Single stepping until exit from function poll,
which has no line number information.
Mysqld_socket_listener::listen_for_connection_event (this=0x3967430) at /gdb/mysql-5.7.25/sql/conn_handler/socket_connection.cc:859
859 if (retval < 0 && socket_errno != SOCKET_EINTR)
通过输出可以知道数据库处于获取系统 socket 状态。接下来需要跳过的步骤有些多，我们直接使用 continue （直接到下一段可执行代码）
(gdb) c
Continuing.
新起客户端连接成功：
[root@ops bin]# mysql -uroot -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 14
Server version: 5.7.25-debug-log Source distribution
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql>
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