码版本：5.7.29
简单记录信号如何生效的。poll收到信号后如何中断后如何处理的，需要确认。
---

###一
初始化信号处理方式，设置信号的处理的处理方式，屏蔽某些信号，并且继承到子线程(pthread_sigmask)，主要屏蔽的为SIGTERM、SIGQUIT、SIGHUP、SIGTSTP四个。其他信号按照默认处理方式，但是处理前打印crash栈，handle_fatal_signal

对于siguser1 进行捕获，处理函数为空empty_signal_handler
```
#0  my_init_signals () at /home/mysql/soft/percona-server-5.7.29-32/sql/mysqld.cc:2225
#1  0x00000000015fefc9 in mysqld_main (argc=79, argv=0x2e22368) at /home/mysql/soft/percona-server-5.7.29-32/sql/mysqld.cc:4846
#2  0x0000000000e8795d in main (argc=4, argv=0x7fffffffe538) at /home/mysql/soft/percona-server-5.7.29-32/sql/main.cc:32
```

###二
启动信号处理线程start_signal_handler线程调用函数为 signal_hand,
使用sigwait获取未决信号集合，如果为SIGTERM和SIGQUIT，设置abort_loop= true;  
并且发送sigusr1个主线程，准备正常退出pthread_kill(main_thread_id, SIGUSR1)) //给主线程发送sigusr1信号
如果为SIGHUP则重载一下reload_acl_and_cache相关缓存。

start_signal_handler线程调用函数为 signal_hand
```
#0  start_signal_handler () at /home/mysql/soft/percona-server-5.7.29-32/sql/mysqld.cc:2301
#1  0x00000000015fff70 in mysqld_main (argc=79, argv=0x2e22368) at /home/mysql/soft/percona-server-5.7.29-32/sql/mysqld.cc:5407
#2  0x0000000000e8795d in main (argc=4, argv=0x7fffffffe538) at /home/mysql/soft/percona-server-5.7.29-32/sql/main.cc:32
```

###三、
如果为SIGTERM和SIGQUIT，主线程（main）正在poll接受连接，收到了sigusr1 信号后进行中断处理，接下来进行判断
全局变量abort_loop，然后进入正常退出流程。
```
#0  empty_signal_handler (sig=32767) at /home/mysql/soft/percona-server-5.7.29-32/sql/mysqld.cc:2219
#1  <signal handler called>
#2  0x00007ffff5f20c3d in poll () from /lib64/libc.so.6
#3  0x00000000015ec077 in Mysqld_socket_listener::listen_for_connection_event (this=0x35a76f0) at /home/mysql/soft/percona-server-5.7.29-32/sql/conn_handler/socket_connection.cc:883
#4  0x0000000001607542 in Connection_acceptor<Mysqld_socket_listener>::connection_event_loop (this=0x35a70e0)
    at /home/mysql/soft/percona-server-5.7.29-32/sql/conn_handler/connection_acceptor.h:73
#5  0x00000000016001f2 in mysqld_main (argc=79, argv=0x2e22368) at /home/mysql/soft/percona-server-5.7.29-32/sql/mysqld.cc:5513
#6  0x0000000000e8795d in main (argc=4, argv=0x7fffffffe538) at /home/mysql/soft/percona-server-5.7.29-32/sql/main.cc:32
```
main thread
```
  mysqld_socket_acceptor->connection_event_loop(); //监听3306 端口
#endif /* _WIN32 */
  server_operational_state= SERVER_SHUTTING_DOWN; //这里开始进行关闭处理

DBUG_PRINT("info", ("No longer listening for incoming connections"));

mysql_audit_notify(MYSQL_AUDIT_SERVER_SHUTDOWN_SHUTDOWN, Mysqld_socket_listener::listen_for_connection_event
                     MYSQL_AUDIT_SERVER_SHUTDOWN_REASON_SHUTDOWN,
                     MYSQLD_SUCCESS_EXIT);

terminate_compress_gtid_table_thread();

```

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MySQL：简单记录信号处理

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