mysqld_main (MySQL Server Entry Point)
	|-network_init (初始化網(wǎng)絡)
		/* 建立tcp套接字 */
		|-create_socket (AF_INET)
		|-mysql_socket_bind (AF_INET)
		|-mysql_socket_listen (AF_INET)
		/* 建立UNIX套接字*/
		|-mysql_socket_socket (AF_UNIX)
		|-mysql_socket_bind (AF_UNIX)
		|-mysql_socket_listen (AF_UNIX)

// 首先創(chuàng)建ipv4連接
ip_sock= create_socket(ai, AF_INET, &a);
// 如果無法創(chuàng)建ipv4連接，則嘗試創(chuàng)建ipv6連接
if(mysql_socket_getfd(ip_sock) == INVALID_SOCKET)
 	ip_sock= create_socket(ai, AF_INET6, &a);

handle_connections_sockets
	|->poll/select
	|->new_sock=mysql_socket_accept(...sock...) /*從listen socket中獲取新連接*/
	|->new THD 連接線程上下文 /* 如果獲取不到足夠內(nèi)存，則shutdown new_sock*/
	|->mysql_socket_getfd(sock) 從socket中獲取
		/** 設置為NONBLOCK和環(huán)境有關 **/
	|->fcntl(mysql_socket_getfd(sock), F_SETFL, flags | O_NONBLOCK);
	|->mysql_socket_vio_new
		|->vio_init (VIO_TYPE_TCPIP)
			|->(vio->write = vio_write)
			/* 默認用的是vio_read */
			|->(vio->read=(flags & VIO_BUFFERED_READ) ?vio_read_buff :vio_read;)
			|->(vio->viokeepalive = vio_keepalive) /*tcp層面的keepalive*/
			|->.....
	|->mysql_net_init
		|->設置超時時間，最大packet等參數(shù)
	|->create_new_thread(thd) /* 實際是從線程池拿，不夠再新建pthread線程 */
		|->最大連接數(shù)限制
		|->create_thread_to_handle_connection
			|->首先看下線程池是否有空閑線程
				|->mysql_cond_signal(&COND_thread_cache) /* 有則發(fā)送信號 */
			/** 這邊的hanlde_one_connection是mysql連接的主要處理函數(shù) */
			|->mysql_thread_create(...handle_one_connection...)

size_t vio_read(Vio *vio, uchar *buf, size_t size)
{
  while ((ret= mysql_socket_recv(vio->mysql_socket, (SOCKBUF_T *)buf, size, flags)) == -1)
  {
    ......
    // 如果上面獲取的數(shù)據(jù)為空，則通過select的方式去獲取讀取事件，并設置超時timeout時間
    if ((ret= vio_socket_io_wait(vio, VIO_IO_EVENT_READ)))
        break;
  }
}

vio_socket_io_wait
	|->vio_io_wait
		|-> (ret= select(fd + 1, &readfds, &writefds, &exceptfds, 
              (timeout >= 0) ? &tm : NULL))

size_t vio_write(Vio *vio, const uchar* buf, size_t size)
{
  while ((ret= mysql_socket_send(vio->mysql_socket, (SOCKBUF_T *)buf, size, flags)) == -1)
  {
    int error= socket_errno;
    /* The operation would block? */
    // 處理EAGAIN和EWOULDBLOCK返回，NON_BLOCK模式都必須處理
    if (error != SOCKET_EAGAIN && error != SOCKET_EWOULDBLOCK)
      break;
    /* Wait for the output buffer to become writable.*/
    if ((ret= vio_socket_io_wait(vio, VIO_IO_EVENT_WRITE)))
      break;
  }
}

mysql_thread_create(...handle_one_connection...)

for(;;){
	// 這邊做了連接的handshake和auth的工作
	rc= thd_prepare_connection(thd);
	// 和通常的線程處理一樣，一個無限循環(huán)獲取連接請求
	while(thd_is_connection_alive(thd))
	{
		if(do_command(thd))
			break;
	}
	// 出循環(huán)之后，連接已經(jīng)被clientdu端關閉或者出現(xiàn)異常
	// 這邊做了連接的銷毀動作
	end_connection(thd);
end_thread:
	...
	// 這邊調(diào)用end_thread做清理動作，并將當前線程返還給線程池重用
	// end_thread對應為one_thread_per_connection_end
	if (MYSQL_CALLBACK_ELSE(thread_scheduler, end_thread, (thd, 1), 0))
		return;	
	...
	// 這邊current_thd是個宏定義，其實是current_thd();
	// 主要是從線程上下文中獲取新塞進去的thd
	// my_pthread_getspecific_ptr(THD*,THR_THD);
	thd= current_thd;
	...
}

MYSQL_CALLBACK_ELSE(...end_thread)
	one_thread_per_connection_end
		|->thd->release_resources()
		|->......
		|->block_until_new_connection

static bool block_until_new_connection()
{	
	mysql_mutex_lock(&LOCK_thread_count);
	......
    while (!abort_loop && !wake_pthread && !kill_blocked_pthreads_flag)
      mysql_cond_wait(&x1, &LOCK_thread_count);
   ......
   // 從等待列表中獲取需要處理的THD
   thd= waiting_thd_list->front();
   waiting_thd_list->pop_front();
   ......
   // 將thd放入到當前線程上下文中
   // my_pthread_setspecific_ptr(THR_THD,  this)    
   thd->store_globals();
   ......
   mysql_mutex_unlock(&LOCK_thread_count);
   .....
}