原創水平有限

今天在看運維內參第三章原始碼的時候，發現書上很多程式碼入口函式和我用的5.7.14的原始碼對不上，然後再看了一下5.6的原始碼，
書上寫的基本都是5.6的原始碼(書是2015年開始寫的)5.7.14在連線這一塊做了相當大的改動，加了很多抽象類，變得更加複雜了。
當然這也是軟體模組劃分的更加清晰的必然的結果，順便說一下MYSQL在UNIX上使用的select/poll 這種多路I/O轉接模型來監聽
客戶端的連線,監聽的只是讀操作，當客戶端發起連線的時候select/poll透過監聽LISTEN scoket上的是否有讀取資訊來判斷，是否
有新的連線到來,然後呼叫執行緒回撥函式建立新的執行緒來處理這個新的accpet的socket通道(還會判斷是否有空閒的執行緒佇列而不需要
新建立新的執行緒來處理)，執行緒的建立我們說是比程式代價小很多，他沒有獨立的記憶體三區(共享區,程式碼段和堆記憶體)，但是他也有PCB
有獨立的棧空間所以能節約一點是一點吧。
關於MYSQL的原始碼各種多型太多了，真是眼花繚亂

下面是我使用到一些斷點。

(gdb) info b
Num Type Disp Enb Address What
1 breakpoint keep y 0x0000000000ebd333 in main(int, char**) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/main.cc:25
breakpoint already hit 1 time
2 breakpoint keep y 0x00000000016e350d in inline_mysql_socket_accept(char const*, uint, PSI_socket_key, MYSQL_SOCKET, sockaddr*, socklen_t*) at /root/mysql5.7.14/percona-server-5.7.14-7/include/mysql/psi/mysql_socket.h:1110
breakpoint already hit 18 times
6 breakpoint keep y 0x0000000000f1b8d9 in Connection_handler_manager::process_new_connection(Channel_info*) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_manager.cc:289
breakpoint already hit 15 times
8 breakpoint keep y 0x00000000016e197f in handle_connection(void*) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_per_thread.cc:239
breakpoint already hit 6 times
9 breakpoint keep y 0x0000000000f1b1d5 in Connection_handler_manager::check_and_incr_conn_count(bool) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_manager.cc:105
breakpoint already hit 1 time
12 breakpoint keep y 0x00000000016e1f41 in Per_thread_connection_handler::add_connection(Channel_info*) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_per_thread.cc:399

下面按照呼叫流程解釋一下這幾個函式作用，希望幫到在5.7.14下使用GDB除錯MYSQL連線的同學一點幫助
1、不解釋主函式main

2、 inline_mysql_socket_accept
當然也就是sokcet accept返回新的socket檔案描述符的函式，因為找不到多路I/O轉接 select和poll的入口用了這招去找
找到多路I/O轉接介面變為了Channel_info* Mysqld_socket_listener::listen_for_connection_event()
Channel_info就是一個抽象類，這個函式會返回連線socket的相關資訊給 Channel_info這個基類指標。
又有多太發生了擦。

這裡貼出一點和運維內參同樣的程式碼，但是入口函式全部變了，而且除了邏輯相似，很多實現都變了
大量的用多型

點選(此處)摺疊或開啟

#ifdef HAVE_POLL //多路I/O轉接判斷是否有寫連線到來
int retval= poll(&m_poll_info.m_fds[0], m_socket_map.size(), -1);
#else
m_select_info.m_read_fds= m_select_info.m_client_fds;
int retval= select((int) m_select_info.m_max_used_connection,
&m_select_info.m_read_fds, 0, 0, 0);
#endif

點選(此處)摺疊或開啟

#ifdef HAVE_POLL //多路I/O轉接判斷是否有寫連線到來
int retval= poll(&m_poll_info.m_fds[0], m_socket_map.size(), -1);
#else
m_select_info.m_read_fds= m_select_info.m_client_fds;
int retval= select((int) m_select_info.m_max_used_connection,
&m_select_info.m_read_fds, 0, 0, 0);
#endif

點選(此處)摺疊或開啟

for (uint retry= 0; retry < MAX_ACCEPT_RETRY; retry++) //重試次數MAX_ACCEPT_RETRY好像是10
{
socket_len_t length= sizeof(struct sockaddr_storage);
connect_sock= mysql_socket_accept(key_socket_client_connection, listen_sock,
(struct sockaddr *)(&cAddr), &length);//這裡沒什麼好說的返回新的sokcet檔案描述符了
if (mysql_socket_getfd(connect_sock) != INVALID_SOCKET ||//返回的新的socket檔案描述符無效(-1)
(socket_errno != SOCKET_EINTR && socket_errno != SOCKET_EAGAIN)) //如果是訊號中斷或者eagain重試沒關係再來
break;
}

點選(此處)摺疊或開啟

Channel_info* channel_info= NULL;
//注意下面有多型發生這裡和5.6很多不同了
if (is_unix_socket) //是否是本地UNIX_DOMAIN SOCKET
channel_info= new (std::nothrow) Channel_info_local_socket(connect_sock);
else//不是就是遠端TCP/IP SCOKET
channel_info= new (std::nothrow)
Channel_info_tcpip_socket(connect_sock,
(mysql_socket_getfd(listen_sock)
== m_extra_tcp_port_fd)); //根據connect_scok 建立channel_info
if (channel_info == NULL) //分配記憶體是否正常不正常搞下面的。
{
(void) mysql_socket_shutdown(connect_sock, SHUT_RDWR);
(void) mysql_socket_close(connect_sock);
connection_errors_internal++;
return NULL;
}
return channel_info;

棧幀
#0 inline_mysql_socket_accept (src_file=0x21c5f30 "/root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/socket_connection.cc", src_line=928, key=3, socket_listen=..., addr=0x7fffffffdc50, addr_len=0x7fffffffdcdc)
at /root/mysql5.7.14/percona-server-5.7.14-7/include/mysql/psi/mysql_socket.h:1110
#1 0x00000000016e4981 in Mysqld_socket_listener::listen_for_connection_event (this=0x2fca760) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/socket_connection.cc:928
#2 0x0000000000ecf016 in Connection_acceptor::connection_event_loop (this=0x2fc9b30) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_acceptor.h:66
#3 0x0000000000ec6a06 in mysqld_main (argc=52, argv=0x2e97438) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/mysqld.cc:5287
#4 0x0000000000ebd344 in main (argc=9, argv=0x7fffffffe418) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/main.cc:25

6、Connection_handler_manager::process_new_connection，透過Channel_info指標指向的SCOKET資訊建立新執行緒

棧幀
#1 0x0000000000f1b9c6 in Connection_handler_manager::process_new_connection (this=0x2e98fc0, channel_info=0x34c4a30) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_manager.cc:301
#2 0x0000000000ecf034 in Connection_acceptor::connection_event_loop (this=0x2fc9b30) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_acceptor.h:68
#3 0x0000000000ec6a06 in mysqld_main (argc=52, argv=0x2e97438) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/mysqld.cc:5287
#4 0x0000000000ebd344 in main (argc=9, argv=0x7fffffffe418) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/main.cc:25

9、 Connection_handler_manager::check_and_incr_conn_count(bool)
這裡判斷MAX_CONNECTIONS是否合適這裡也和運維內參說的5.6的程式碼有了
很大的不同，但是邏輯一樣如下：

點選(此處)摺疊或開啟

if (extra_port_connection)
{
if (extra_connection_count > extra_max_connections)
{
connection_accepted= false;
m_connection_errors_max_connection++;
}
else
{
++extra_connection_count;
}
}
else if (connection_count > max_connections)
{
connection_accepted= false;
m_connection_errors_max_connection++;
}

GDB如下：

(gdb) n
106 mysql_mutex_lock(&LOCK_connection_count);
(gdb) n
115 if (extra_port_connection)
(gdb) n
127 else if (connection_count > max_connections)
(gdb) P max_connections
$4 = 6
(gdb) p connection_count
$5 = 7
(gdb) n
129 connection_accepted= false;
(gdb) n
130 m_connection_errors_max_connection++;
(gdb) n
141 mysql_mutex_unlock(&LOCK_connection_count);
(gdb) n
142 return connection_accepted;

棧幀
#0 Connection_handler_manager::check_and_incr_conn_count (this=0x2e98fc0, extra_port_connection=false) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_manager.cc:105
#1 0x0000000000f1b904 in Connection_handler_manager::process_new_connection (this=0x2e98fc0, channel_info=0x34c4a30) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_manager.cc:289
#2 0x0000000000ecf034 in Connection_acceptor::connection_event_loop (this=0x2fc9b30) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_acceptor.h:68
#3 0x0000000000ec6a06 in mysqld_main (argc=52, argv=0x2e97438) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/mysqld.cc:5287
#4 0x0000000000ebd344 in main (argc=9, argv=0x7fffffffe418) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/main.cc:25

12、 Per_thread_connection_handler::add_connection(Channel_info*) 這個函式會判斷連線佇列是否有空閒的，如果沒有呼叫mysql_thread_create建立新的執行緒
擷取如下：

點選(此處)摺疊或開啟

if (!check_idle_thread_and_enqueue_connection(channel_info)) //判斷是否有空閒的連線隊裡
DBUG_RETURN(false);
/*
There are no idle threads avaliable to take up the new
connection. Create a new thread to handle the connection
*/
error= mysql_thread_create(key_thread_one_connection, &id, //建立新的執行緒對接
&connection_attrib,
handle_connection,
(void*) channel_info);

棧幀
#0 Per_thread_connection_handler::add_connection (this=0x2f67b10, channel_info=0x34c4a30) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_per_thread.cc:399
#1 0x0000000000f1b9c6 in Connection_handler_manager::process_new_connection (this=0x2e98fc0, channel_info=0x34c4a30) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_handler_manager.cc:301
#2 0x0000000000ecf034 in Connection_acceptor::connection_event_loop (this=0x2fc9b30) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/conn_handler/connection_acceptor.h:68
#3 0x0000000000ec6a06 in mysqld_main (argc=52, argv=0x2e97438) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/mysqld.cc:5287
#4 0x0000000000ebd344 in main (argc=9, argv=0x7fffffffe418) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/main.cc:25

8、 handle_connection(void*) 這裡就是5.7.16真正的回撥函式用於建立執行緒，這裡就不解釋了，因為這裡只是分析流程，以後在詳細研究

這裡終於找到了建立執行緒的回撥函式，除錯有時候不知道入口函式名字也是一種悲劇

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關於MYSQL 5.7 新連線建立流程原始碼介面(和5.6不同)

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