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MYSQL 源码阅读六

2021-03-02 07:02:44 作者：互联网

前期节要

MYSQL源码阅读一

MYSQL源码阅读二

MYSQL源码阅读三

MYSQL 源码阅读四

MYSQL 源码阅读五

上次有两个问题没搞明白

1 是为什么一定要开启调试线程 ? 因为MYSQL是线程模型,当是你也可以调试整个MYSQLD服务进程

使用GDB 要先设置断点,MYSQLD遇到断点所有线程和服务都停了下来. 为此你不调试线程也可以.

不过我们是要跟踪某个线程执行流程,执行了哪些函数? 如果调试进程是无法抓到的,.

2 是为什么没有执行 IF(RC) {...} 里面的代码? 当时折腾了会儿,后来几天想起来估计不符合RC的条件

[root@localhost ~]# gdp -p 6041
(gdb) b do_command
Breakpoint 1 at 0x3616c8e: file /u01/mysql/source/mysql-8.0.20/sql/sql_parse.cc, line 1161.
(gdb) n
Single stepping until exit from function poll,
which has no line number information.

GDB 挂死了!

杀伤GDB 再来,顶多重启系统

调试窗口:


(gdb) b do_command
Breakpoint 1 at 0x3616c8e: file /u01/mysql/source/mysql-8.0.20/sql/sql_parse.cc, line 1161.
(gdb) c
Continuing.

MySQL客户端窗口:


mysql> select * from books;

调试窗口:


[Switching to Thread 0x7f1f940f5700 (LWP 3398)]
Breakpoint 1, do_command (thd=0x7f1f6c006790)
    at /u01/mysql/source/mysql-8.0.20/sql/sql_parse.cc:1161
1161      NET *net = nullptr;

## 啥意思呢?

info thread
* 2    Thread 0x7f1f940f5700 (LWP 3398) "mysqld" do_command (thd=0x7f1f6c006790)

明白了! GDB 调试服务进程时,设置断点函数后, 客户端发出SQL请求, 这时GDB处于死人状态.

为此在设置断点后输入C 命令让GDB继续下去. 然后才让客户端发SQL请求,这时GDB就触发了断点函数,并且自动切换到该线程上, 并且停止1161这行,只要N的命令就可以追踪下去了!

这样就省了我们去查客户端的进程,线程,OS线程麻烦! 不过在生产环境,也就是多用户同时发出SQL语句下,这方法估计不好使.

不过对于我们学习内核源码来说,没有那么多客户端并发操作!

源码代码:


1158 bool do_command(THD *thd) {
1159   bool return_value;
1160   int rc;
1161   NET *net = nullptr;
1162   enum enum_server_command command;
1163   COM_DATA com_data;
1164   DBUG_TRACE;
1165   DBUG_ASSERT(thd->is_classic_protocol());

然后一路火花带闪电N下去, 遇到了RC P一下结果真的为0

(gdb) n
1164      DBUG_TRACE;
(gdb) n
1165      DBUG_ASSERT(thd->is_classic_protocol());
(gdb) n
1171      thd->lex->set_current_select(nullptr);
(gdb) n
1179      thd->clear_error();  // Clear error message
(gdb) n
1180      thd->get_stmt_da()->reset_diagnostics_area();
(gdb) n
1188      net = thd->get_protocol_classic()->get_net();
(gdb) n
1189      my_net_set_read_timeout(net, thd->variables.net_wait_timeout);
(gdb) n
1190      net_new_transaction(net);
(gdb) n
1206      DEBUG_SYNC(thd, "before_do_command_net_read");
(gdb) n
1219      thd->m_server_idle = true;
(gdb) n
1220      rc = thd->get_protocol()->get_command(&com_data, &command);
(gdb) n
1221      thd->m_server_idle = false;
(gdb) n
1223      if (rc) {
(gdb) p rc
$1 = 0
(gdb) n
1256      vio_description(net->vio, desc);
(gdb) n
1257      DBUG_PRINT("info", ("Command on %s = %d (%s)", desc, command,
(gdb) n
1260      DBUG_PRINT("info", ("packet: '%*.s'; command: %d",
(gdb) n
1263      if (thd->get_protocol_classic()->bad_packet)
(gdb) n
1267      thd->get_protocol_classic()->get_output_packet()->shrink(
(gdb) n
1268          thd->variables.net_buffer_length);
(gdb) n
1270      my_net_set_read_timeout(net, thd->variables.net_read_timeout);
(gdb) n
1272      DEBUG_SYNC(thd, "before_command_dispatch");
(gdb) n
1274      return_value = dispatch_command(thd, &com_data, command);
(gdb) n
1275      thd->get_protocol_classic()->get_output_packet()->shrink(
(gdb) n
1276          thd->variables.net_buffer_length);
(gdb) n
1280      DBUG_ASSERT(thd->m_digest == nullptr);
(gdb) n
1281      DBUG_ASSERT(thd->m_statement_psi == nullptr);
(gdb) n
1282      return return_value;
(gdb) n
1164      DBUG_TRACE;
(gdb) n
1282      return return_value;
(gdb) n
1283    }
(gdb) n
handle_connection (arg=0xa739020)
    at /u01/mysql/source/mysql-8.0.20/sql/conn_handler/connection_handler_per_thread.cc:301
301          while (thd_connection_alive(thd)) {

在1275 1276 行的函数执行完后,客户端就收到了数据


Breakpoint 1, do_command (thd=0x7f1f6c006790)
    at /u01/mysql/source/mysql-8.0.20/sql/sql_parse.cc:1161
1161      NET *net = nullptr;
(gdb) n
1164      DBUG_TRACE;
(gdb) n
1165      DBUG_ASSERT(thd->is_classic_protocol());
(gdb) n
1171      thd->lex->set_current_select(nullptr);
(gdb) n
1179      thd->clear_error();  // Clear error message
(gdb) n
1180      thd->get_stmt_da()->reset_diagnostics_area();
(gdb) n
1188      net = thd->get_protocol_classic()->get_net();
(gdb) n
1189      my_net_set_read_timeout(net, thd->variables.net_wait_timeout);
(gdb) n
1190      net_new_transaction(net);
(gdb) n
1206      DEBUG_SYNC(thd, "before_do_command_net_read");
(gdb) n
1219      thd->m_server_idle = true;
(gdb) n
1220      rc = thd->get_protocol()->get_command(&com_data, &command);
(gdb) n
1221      thd->m_server_idle = false;
(gdb) n
1223      if (rc) {
(gdb) n
1256      vio_description(net->vio, desc);
(gdb) n
1257      DBUG_PRINT("info", ("Command on %s = %d (%s)", desc, command,
(gdb) n
1260      DBUG_PRINT("info", ("packet: '%*.s'; command: %d",
(gdb) n
1263      if (thd->get_protocol_classic()->bad_packet)
(gdb) n
1267      thd->get_protocol_classic()->get_output_packet()->shrink(
(gdb) n
1268          thd->variables.net_buffer_length);
(gdb) n
1270      my_net_set_read_timeout(net, thd->variables.net_read_timeout);
(gdb) n
1272      DEBUG_SYNC(thd, "before_command_dispatch");
(gdb) n
1274      return_value = dispatch_command(thd, &com_data, command);
(gdb) p thd
$17 = (THD *) 0x7f1f6c006790
(gdb) p com_data
$18 = {com_init_db = {db_name = 0x7f1f6c00bbe1 "select * from books", length = 19}, 
  com_refresh = {options = 225 '\341'}, com_kill = {id = 139772932701153}, 
  com_set_option = {opt_command = 1811987425}, com_stmt_execute = {
    stmt_id = 139772932701153, open_cursor = 19, parameters = 0x7f1f6c00bbe8, 
    parameter_count = 139772932688388, has_new_types = 32 ' '}, com_stmt_fetch = {
    stmt_id = 139772932701153, num_rows = 19}, com_stmt_send_long_data = {
    stmt_id = 139772932701153, param_number = 19, longdata = 0x7f1f6c00bbe8 "* from books", 
    length = 139772932688388}, com_stmt_prepare = {
    query = 0x7f1f6c00bbe1 "select * from books", length = 19}, com_stmt_close = {
    stmt_id = 1811987425}, com_stmt_reset = {stmt_id = 1811987425}, com_query = {
    query = 0x7f1f6c00bbe1 "select * from books", length = 19}, com_field_list = {
    table_name = 0x7f1f6c00bbe1 "select * from books", table_name_length = 19, 
    query = 0x7f1f6c00bbe8 "* from books", query_length = 1811974660}}
(gdb) p command
$19 = COM_QUERY
(gdb) n
1275      thd->get_protocol_classic()->get_output_packet()->shrink(
(gdb) n
1276          thd->variables.net_buffer_length);

1274行的dispatch_command(thd, &com_data, command); 这个是命令分配器吗?

THD 估计是线程, COM_DATA是命令数据包, COMMAND 是命令类型

标签：get,gdb,源码,command,阅读,MYSQL,net,thd,com
来源： https://blog.51cto.com/15080028/2643041

MYSQL 源码阅读 六

MYSQL 源码阅读六