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在mysqld_main函数的最后，通过下面的code来建立来自客户端的连接mysqld_socket_acceptor->connection_event_loop();  void connection_event_loop() {    Connection_handler_manager *mgr =        Connection_handler_manager::get_instance();    while (!connection_events_loop_aborted()) {      Channel_info *channel_info = m_listener->listen_for_connection_event();      if (channel_info != NULL) mgr->process_new_connection(channel_info);    }  }上次我们讲过listen_for_connection_event，这里我们看看process_new_connection的实现void Connection_handler_manager::process_new_connection(    Channel_info *channel_info) {  if (connection_events_loop_aborted() || !check_and_incr_conn_count()) {    channel_info->send_error_and_close_channel(ER_CON_COUNT_ERROR, 0, true);    delete channel_info;    return;  }#这里通过add_connection 来添加connection，这里有两种选择，一种是每个connection 对应一个thread，一个是所有connection统一用一个thread，这里以每个connection 对应一个thread 为例，这也是目前默认的选择  if (m_connection_handler->add_connection(channel_info)) {    inc_aborted_connects();    delete channel_info;  }}add_connection的实现如下：bool Per_thread_connection_handler::add_connection(Channel_info *channel_info) {  int error = 0;  my_thread_handle id;  DBUG_ENTER("Per_thread_connection_handler::add_connection");  // Simulate thread creation for test case before we check thread cache  DBUG_EXECUTE_IF("fail_thread_create", error = 1; goto handle_error;);  if (!check_idle_thread_and_enqueue_connection(channel_info))    DBUG_RETURN(false);  /*    There are no idle threads avaliable to take up the new    connection. Create a new thread to handle the connection  */  channel_info->set_prior_thr_create_utime();#通过mysql_thread_create 来创建thread，这里的mysql_thread_create 就是pthread_create的包装，注意这里就新建一个#thread来处理client的connetion，注意这里的回调函数是handle_connection  error =      mysql_thread_create(key_thread_one_connection, &id, &connection_attrib,                          handle_connection, (void *)channel_info);}static void *handle_connection(void *arg) {    for (;;) {    THD *thd = init_new_thd(channel_info);    thd_manager->add_thd(thd);    if (thd_prepare_connection(thd))      handler_manager->inc_aborted_connects();    else {      while (thd_connection_alive(thd)) {#这里的核心就是调用do_command 来处理client发送过来的命令        if (do_command(thd)) break;      }      end_connection(thd);    }  }}

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