分析一下下面这段代码while(1) { revents = 0; #ifndef DISABLE_LIBSSH if (session->ssh_chan != NULL) { /* we are getting data from libssh's channel */ status = ssh_channel_poll_timeout(session->ssh_chan, timeout, 0); if (status > 0) { revents = POLLIN; } } else #endif #ifdef ENABLE_TLS if (session->tls != NULL) { /* we are getting data from TLS session using OpenSSL */ fds.fd = SSL_get_fd(session->tls); fds.events = POLLIN; fds.revents = 0; status = poll(&fds, 1, timeout); revents = (unsigned long int) fds.revents; } else #endif if (session->fd_input != -1) { /* we are getting data from standard file descriptor */ fds.fd = session->fd_input; fds.events = POLLIN; fds.revents = 0; status = poll(&fds, 1, timeout); revents = (unsigned long int) fds.revents; } else { ERROR("Invalid session to receive data."); return (NC_MSG_UNKNOWN); } /* process the result */ if (status == 0) { /* timed out */ DBG_UNLOCK("mut_channel"); pthread_mutex_unlock(session->mut_channel); return (NC_MSG_WOULDBLOCK); } else if (((status == -1) && (errno == EINTR)) #ifndef DISABLE_LIBSSH || (status == SSH_AGAIN) #endif ) { /* poll was interrupted */ continue; } else if (status < 0) { /* poll failed - something wrong happend, close this socket and wait for another request */ DBG_UNLOCK("mut_channel"); pthread_mutex_unlock(session->mut_channel); #ifndef DISABLE_LIBSSH if (status == SSH_EOF) { emsg = "end of file"; } else if (!session->ssh_chan) { emsg = strerror(errno); } else if (session->ssh_sess) { emsg = ssh_get_error(session->ssh_sess); } else { emsg = "description not available"; } #else emsg = strerror(errno); #endif WARN("Input channel error (%s)", emsg); nc_session_close(session, NC_SESSION_TERM_DROPPED); if (nc_info) { pthread_rwlock_wrlock(&(nc_info->lock)); nc_info->stats.sessions_dropped++; pthread_rwlock_unlock(&(nc_info->lock)); } return (NC_MSG_UNKNOWN); } /* status > 0 */ /* check the status of the socket */ /* if nothing to read and POLLHUP (EOF) or POLLERR set */ if ((revents & POLLHUP) || (revents & POLLERR)) { /* close client's socket (it's probably already closed by client */ DBG_UNLOCK("mut_channel"); pthread_mutex_unlock(session->mut_channel); ERROR("Input channel closed"); nc_session_close(session, NC_SESSION_TERM_DROPPED); if (nc_info) { pthread_rwlock_wrlock(&(nc_info->lock)); nc_info->stats.sessions_dropped++; pthread_rwlock_unlock(&(nc_info->lock)); } return (NC_MSG_UNKNOWN); } /* we have something to read */ break; }

这段代码是一个循环，用于从网络通道中读取数据，直到有数据可读或者等待超时。在循环中，代码会根据条件编译选择不同的代码路径，从不同的网络通道中获取数据。当获取到数据后，代码会根据获取到的数据的状态进行处理。如果等待超时，则返回NC_MSG_WOULDBLOCK表示没有数据可读；如果获取到的数据出现错误，则关闭网络通道，并返回NC_MSG_UNKNOWN表示未知错误；如果获取到的数据可用，则退出循环，返回获取到的数据。

循环的目的是保证能够不断地从网络通道中获取数据，直到获取到数据为止。在循环中，代码会不断地进行读取数据的尝试，直到获取到数据或者等待超时为止。如果循环执行完毕后仍未获取到数据，则说明网络通道中没有可读数据。