为什么mpd_connection_clear_error(）在mpd_recv_idle(）的mpd_error_TI
Why does mpd_connection_clear_error () fail after MPD_ERROR_TIMEOUT from mpd_recv_idle ()?
我正试图使用libmpdclient运行一个空闲循环，但在第一次空闲调用时，我已经进入了明显不可恢复的错误状态。
我将false传递给mpd_recv_idle的disable_timeout参数，这样我就可以从外部停止循环(它将在后台线程中运行)，以确保干净的关闭过程。
这是我的测试代码：
#include <string>
#include <stdexcept>
#include <memory>
#include <mpd/client.h>
typedef std::unique_ptr<mpd_connection, decltype(&mpd_connection_free)>
mpd_connection_ptr;
void
check_error (const mpd_connection_ptr &c, const std::string &s)
{
if (mpd_connection_get_error (c.get ()) != MPD_ERROR_SUCCESS)
{
throw std::runtime_error (s);
}
}
int
main (void)
{
const std::string host = "127.0.0.1";
const uint16_t port = 7701;
const int timeout = 1 * 1000;
mpd_connection_ptr c {mpd_connection_new (host.c_str (), port, timeout),
&mpd_connection_free
};
if (c == nullptr)
{
throw std::runtime_error ("connection_new returned nullptr");
}
if (!mpd_send_idle (c.get ()))
{
throw std::runtime_error ("mpd_send_idle returned false");
}
check_error (c, "mpd_send_idle caused error status");
auto idle = mpd_recv_idle (c.get (), false);
if (idle == 0)
{
if (mpd_connection_get_error (c.get ()) == MPD_ERROR_TIMEOUT)
{
if (!mpd_connection_clear_error (c.get ()))
{
throw std::runtime_error ("mpd_connection_clear_error returned false");
}
}
else
{
check_error (c, "mpd_recv_idle caused error status");
}
}
return 0;
}
当我运行这个代码时(mpd在127.0.0.1:7701上运行，我用netstat进行了检查)，我得到了这样的结果：
terminate called after throwing an instance of 'std::runtime_error'
what(): mpd_connection_clear_error returned false
为什么我不能清除这里的超时错误，这似乎是一种可以恢复的情况？
通过学习libmpdclient源代码，我想我可以自己回答这个问题。
超时是库设计中不可恢复的错误。这就是为什么mpd_recv_idle ()的disable_timeout参数首先存在的原因。
同步空闲请求预计将"永远"阻止(直到MPD回答请求为止)。这与我想要的不兼容，我可能不得不使用低级异步接口来实现我想要的。
这是我的解决方案(只需最少的错误检查)。
该程序等待用户按下ENTER，并在后台处理MPD空闲消息，该消息可以每200毫秒中断一次。
缺少什么：
- 返回代码解析
- 空闲消息响应解析
这是代码：
#include <string>
#include <stdexcept>
#include <memory>
#include <iostream>
#include <thread>
#include <chrono>
#include <netinet/in.h>
#include <netdb.h>
#include <strings.h>
#include <unistd.h>
#include <mpd/async.h>
// #include <mpd/client.h>
typedef std::unique_ptr<mpd_async, decltype(&mpd_async_free)>
mpd_async_ptr;
void
check_error (const mpd_async_ptr &c, const std::string &s)
{
if (mpd_async_get_error (c.get ()) != MPD_ERROR_SUCCESS)
{
throw std::runtime_error (s);
}
}
mpd_async_event
async_poll (const mpd_async *async, timeval *tv)
{
int events = mpd_async_events (async);
if (events == 0)
{
throw std::runtime_error ("mpd_async_events failed");
}
int fd = mpd_async_get_fd (async);
fd_set rfds, wfds, efds;
FD_ZERO(&rfds);
FD_ZERO(&wfds);
FD_ZERO(&efds);
if (events & MPD_ASYNC_EVENT_READ)
{
FD_SET(fd, &rfds);
}
if (events & MPD_ASYNC_EVENT_WRITE)
{
FD_SET(fd, &wfds);
}
if (events & (MPD_ASYNC_EVENT_HUP|MPD_ASYNC_EVENT_ERROR))
{
FD_SET(fd, &efds);
}
int ret = select (fd + 1, &rfds, &wfds, &efds, tv);
if (ret > 0)
{
if (!FD_ISSET(fd, &rfds))
{
events &= ~MPD_ASYNC_EVENT_READ;
}
if (!FD_ISSET(fd, &wfds))
{
events &= ~MPD_ASYNC_EVENT_WRITE;
}
if (!FD_ISSET(fd, &efds))
{
events &= ~(MPD_ASYNC_EVENT_HUP| MPD_ASYNC_EVENT_ERROR);
}
return (mpd_async_event) events;
}
return (mpd_async_event) 0;
}
int
socket_connect (const std::string &host, uint16_t port)
{
int sockfd = socket (AF_INET, SOCK_STREAM, 0);
hostent *server = gethostbyname (host.c_str ());
sockaddr_in server_addr;
bzero ((char *) &server_addr, sizeof (server_addr));
server_addr.sin_family = AF_INET;
bcopy ((char *) server->h_addr, (char *) &server_addr.sin_addr.s_addr,
server->h_length);
server_addr.sin_port = htons (port);
if (::connect (sockfd, (struct sockaddr*) &server_addr,
sizeof (server_addr)) < 0)
{
throw std::string ("ERROR connecting");
}
return sockfd;
}
void
mpd_notify_thread_proc (bool &app_is_running)
{
const std::string host = "127.0.0.1";
const uint16_t port = 7701;
auto sockfd = socket_connect (host, port);
mpd_async_ptr async_ptr {mpd_async_new (sockfd), mpd_async_free};
auto async = async_ptr.get ();
if (async == nullptr)
{
throw std::runtime_error ("mpd_async_new failed");
}
while (app_is_running)
{
timeval tv;
tv.tv_sec = 0;
tv.tv_usec = 200 * 1000;
auto events = async_poll (async, &tv);
if (events != 0)
{
if (!mpd_async_io (async, (mpd_async_event) events))
{
throw std::runtime_error ("connection was closed");
}
char* line_ptr;
while ((line_ptr = mpd_async_recv_line (async)) != nullptr)
{
std::cout << "recv: " << line_ptr << "n";
std::string line {line_ptr};
if (line.find ("OK") == 0)
{
if (!mpd_async_send_command (async, "idle", nullptr))
{
throw std::runtime_error ("mpd_async_send_command failed");
}
}
}
}
}
}
int
main(void)
{
bool app_is_running = true;
std::thread mpd_notify_thread =
std::thread (
[&] ()
{
mpd_notify_thread_proc (app_is_running);
});
std::string response;
getline (std::cin, response);
std::cout << "shutting down...n";
app_is_running = false;
mpd_notify_thread.join ();
}
通过在没有timeval的情况下调用select ()并观察关闭pipe ()，可以"尽可能快"地中断改进版本：
#include <string>
#include <stdexcept>
#include <memory>
#include <iostream>
#include <thread>
#include <chrono>
#include <netinet/in.h>
#include <netdb.h>
#include <strings.h>
#include <unistd.h>
#include <mpd/async.h>
// #include <mpd/client.h>
typedef std::unique_ptr<mpd_async, decltype(&mpd_async_free)>
mpd_async_ptr;
void
check_error (const mpd_async_ptr &c, const std::string &s)
{
if (mpd_async_get_error (c.get ()) != MPD_ERROR_SUCCESS)
{
throw std::runtime_error (s);
}
}
mpd_async_event
async_poll (const mpd_async *async, int *shutdown_fd)
{
int events = mpd_async_events (async);
if (events == 0)
{
throw std::runtime_error ("mpd_async_events failed");
}
int fd = mpd_async_get_fd (async);
fd_set rfds, wfds, efds;
FD_ZERO(&rfds);
FD_ZERO(&wfds);
FD_ZERO(&efds);
if (events & MPD_ASYNC_EVENT_READ)
{
FD_SET(fd, &rfds);
}
if (events & MPD_ASYNC_EVENT_WRITE)
{
FD_SET(fd, &wfds);
}
if (events & (MPD_ASYNC_EVENT_HUP|MPD_ASYNC_EVENT_ERROR))
{
FD_SET(fd, &efds);
}
FD_SET(*shutdown_fd, &rfds);
FD_SET(*shutdown_fd, &wfds);
FD_SET(*shutdown_fd, &efds);
int ret = select ((fd > *shutdown_fd ? fd : *shutdown_fd) + 1, &rfds, &wfds, &efds, NULL);
if (ret > 0)
{
if (!FD_ISSET(fd, &rfds))
{
events &= ~MPD_ASYNC_EVENT_READ;
}
if (!FD_ISSET(fd, &wfds))
{
events &= ~MPD_ASYNC_EVENT_WRITE;
}
if (!FD_ISSET(fd, &efds))
{
events &= ~(MPD_ASYNC_EVENT_HUP| MPD_ASYNC_EVENT_ERROR);
}
if (FD_ISSET(*shutdown_fd, &rfds))
{
*shutdown_fd = 0;
}
if (FD_ISSET(*shutdown_fd, &wfds))
{
*shutdown_fd = 0;
}
if (FD_ISSET(*shutdown_fd, &efds))
{
*shutdown_fd = 0;
}
return (mpd_async_event) events;
}
return (mpd_async_event) 0;
}
int
socket_connect (const std::string &host, uint16_t port)
{
int sockfd = socket (AF_INET, SOCK_STREAM, 0);
hostent *server = gethostbyname (host.c_str ());
sockaddr_in server_addr;
bzero ((char *) &server_addr, sizeof (server_addr));
server_addr.sin_family = AF_INET;
bcopy ((char *) server->h_addr, (char *) &server_addr.sin_addr.s_addr,
server->h_length);
server_addr.sin_port = htons (port);
if (::connect (sockfd, (struct sockaddr*) &server_addr,
sizeof (server_addr)) < 0)
{
throw std::string ("ERROR connecting");
}
return sockfd;
}
void
mpd_notify_thread_proc (int shutdown_fd)
{
const std::string host = "127.0.0.1";
const uint16_t port = 7701;
auto sockfd = socket_connect (host, port);
mpd_async_ptr async_ptr {mpd_async_new (sockfd), mpd_async_free};
auto async = async_ptr.get ();
if (async == nullptr)
{
throw std::runtime_error ("mpd_async_new failed");
}
while (shutdown_fd != 0)
{
auto events = async_poll (async, &shutdown_fd);
if (shutdown_fd == 0)
{
break;
}
if (events != 0)
{
if (!mpd_async_io (async, (mpd_async_event) events))
{
throw std::runtime_error ("connection was closed");
}
char* line_ptr;
while ((line_ptr = mpd_async_recv_line (async)) != nullptr)
{
std::cout << "recv: " << line_ptr << "n";
std::string line {line_ptr};
if (line.find ("OK") == 0)
{
if (!mpd_async_send_command (async, "idle", nullptr))
{
throw std::runtime_error ("mpd_async_send_command failed");
}
}
}
}
}
}
int
main(void)
{
int shutdown_pipe[2];
pipe (shutdown_pipe);
std::thread mpd_notify_thread = std::thread ([&] ()
{
mpd_notify_thread_proc (shutdown_pipe[0]);
});
std::string response;
getline (std::cin, response);
std::cout << "shutting down...n";
close (shutdown_pipe[1]);
mpd_notify_thread.join ();
close (shutdown_pipe[0]);
}
如果您愿意以基于事件的风格(使用libuv和uvw)编写代码，那么还有一个更好的解决方案：
#include <string>
#include <stdexcept>
#include <memory>
#include <iostream>
#include <uvw.hpp>
int
main(void)
{
auto loop = uvw::Loop::getDefault ();
if (loop == nullptr)
{
throw std::runtime_error ("loop init failed");
}
auto tcp = loop->resource<uvw::TcpHandle>();
if (tcp == nullptr)
{
throw std::runtime_error ("tcp init failed");
}
tcp->once<uvw::ConnectEvent> ([] (const uvw::ConnectEvent &, uvw::TcpHandle &tcp) mutable
{
tcp.read ();
});
tcp->once<uvw::ErrorEvent> ([] (const uvw::ErrorEvent &, uvw::TcpHandle &) mutable
{
std::cerr << "Connection errorn";
});
std::string buf;
tcp->on<uvw::DataEvent> ([&] (const uvw::DataEvent &event, uvw::TcpHandle &tcp) mutable
{
std::string data {event.data.get (), event.length};
buf += data;
std::string::size_type pos;
while ((pos = buf.find ('n')) != std::string::npos)
{
std::string line = buf.substr (0, pos);
buf.erase (0, pos + 1);
if (!line.compare (0, 2, "OK"))
{
const std::string idle = "idlen";
std::unique_ptr<char[]> ptr {new char[idle.size ()]};
idle.copy (ptr.get (), idle.size ());
tcp.write (std::move (ptr), idle.size ());
}
else
{
std::cout << line << "n";
}
}
});
tcp->connect ("127.0.0.1", 7701);
loop->run<uvw::Loop::Mode::DEFAULT> ();
}
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