碼版本：5.7.29
簡單記錄訊號如何生效的。poll收到訊號後如何中斷後如何處理的，需要確認。
---
公眾號 MySQL學習

###一
初始化訊號處理方式，設定訊號的處理的處理方式，遮蔽某些訊號，並且繼承到子執行緒(pthread_sigmask)，主要遮蔽的為SIGTERM、SIGQUIT、SIGHUP、SIGTSTP四個。其他訊號按照預設處理方式，但是處理前列印crash棧，handle_fatal_signal

對於siguser1 進行捕獲，處理函式為空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
啟動訊號處理執行緒start_signal_handler執行緒呼叫函式為 signal_hand,
使用sigwait獲取未決訊號集合，如果為SIGTERM和SIGQUIT，設定abort_loop= true;  
並且傳送sigusr1個主執行緒，準備正常退出pthread_kill(main_thread_id, SIGUSR1)) //給主執行緒傳送sigusr1訊號
如果為SIGHUP則過載一下reload_acl_and_cache相關快取。

start_signal_handler執行緒呼叫函式為 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、
如果為SIGTERM和SIGQUIT，主執行緒（main）正在poll接受連線，收到了sigusr1 訊號後進行中斷處理，接下來進行判斷
全域性變數abort_loop，然後進入正常退出流程。
```
#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(); //監聽3306 埠
#endif /* _WIN32 */
  server_operational_state= SERVER_SHUTTING_DOWN; //這裡開始進行關閉處理

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();

```</mysqld_socket_listener></signal>

MySQL：簡單記錄訊號處理

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