Boost Asio 对套接字的写入不完整

Question

我正在使用 Boost Asio 编写一个非常简单的玩具键值存储，并且发生了一些非常奇怪的事情。

基于字符串的协议是这样的：

S <key> <value> // to set a key
G <key>         // to get the key value
L               // to list all key-value pairs

写入是同步的，使用

boost::asio::write(socket_,boost::asio::buffer(resp, len));

其中 socket_ 是 boost::asio::ip::tcp::socket - 显然，异步写入的故事不会改变。

问题在于有时它不会将它应该写入的所有字节都写入套接字，或者输出以某种方式被破坏......

重整列表输出示例（在 localhost 上，使用 nc、echo 和 hexdump）：

> echo S a 12 | nc localhost 5000       
A
> echo S b 23 | nc localhost 5000
A
> echo L | nc localhost 5000 | hexdump -C
00000000  61 3a 20 31 32 3b 20 62  60 00 00 00 00 00        |a: 12; b`.....|
0000000e
> echo L | nc localhost 5000 | hexdump -C
00000000  61 3a 20 31 32 3b 20 62  3a 20 32 33 3b 20        |a: 12; b: 23; |
0000000e

我正在使用 Ubuntu 14.10 存储库中的 Boost 1.55。 遵循服务于客户端的函数的代码。

提前感谢您的任何提示！

void ClientSession::handle_read(const boost::system::error_code& error, size_t bytes_transferred) {
if (!error) {

    std::string cmd(data_, bytes_transferred);
    cmd = trim_str(cmd);

    const char* resp = NULL;
    int len = 0;

    switch(cmd.at(0)) {
    case SET: {
        std::size_t k_pos = cmd.find(" ") + 1;
        std::size_t v_pos = cmd.find(" ", k_pos+1) + 1;

        std::string key = trim_str(cmd.substr(k_pos, v_pos-3));
        std::string value = trim_str(cmd.substr(v_pos, cmd.length()-1));
        cout << "SET key " << key << ", value " << value << "*" <<endl;

        kvs->db[key] = std::atoi(value.c_str());

        resp = "A";
        len = 1;
        break;
    }
    case GET: {
        std::size_t k_pos = cmd.find(" ") + 1;
        std::string key = trim_str(cmd.substr(k_pos, cmd.length()));
        cout << "GET key " << key << "*" << endl;

        int value = kvs->db[key];
        char str[5];
        sprintf(str, "%d", value);
        resp = (const char*) str;
        len = strlen(resp);
        break;
    }
    case LIST: {
        ostringstream os;
        for (std::map<string, int>::iterator iter = kvs->db.begin();
              iter != kvs->db.end(); ++iter )
              os << iter->first << ": " << iter->second << "; ";
        cout << "list: " << os.str().c_str() << endl;

        resp = os.str().c_str();
        len = strlen(resp);
        break;
    }
    case DEL: {
        std::size_t k_pos = cmd.find(" ") + 1;
        std::string key = trim_str(cmd.substr(k_pos, cmd.length()));

        kvs->db.erase(key);
        resp = "A";
        len = 1;
        break;
    }
    default: {
        resp = "NACK.";
        len = 5;
    }
    }

    cout << "resp: " << resp << "*" << endl;
    cout << "len: " << len << "*" << endl;
    std::size_t written = boost::asio::write(socket_,
            boost::asio::buffer(resp, len));
    cout << "written: " << written << endl;

    boost::system::error_code ignored_ec;
    socket_.shutdown(boost::asio::ip::tcp::socket::shutdown_both, ignored_ec);
    socket_.close();

} else
    delete this;

Answer 1

至少由于case GET 中的resp 指针悬空而导致您的行为未定义：

{
    // ...
    char str[5];
    resp = (const char*) str; // WHOOOOOOOOOOOPS
    len = strlen(resp);
}

case LIST下的一模一样：

{
    std::ostringstream os;
    // ....
    resp = os.str().c_str(); // WHOOOOOOOOOOOPS
}

当你有 Undefined Behaviour 时，所有关于程序的推理都不再有用。

修复这些问题（可能还有更多我没有发现的问题），然后重新测试。在 valgrind 下运行。使用静态分析工具。

更新：修复单线程版本：https://gist.github.com/sehe/69379e17350fb718892f#comment-1428235

测试运行输出：

$ for a in S{a..d}\ $RANDOM Gnonexisting L; do echo "$a -> $(netcat 127.0.0.1 5000 <<< "$a")"; done | nl
     1  Sa 15936 -> A
     2  Sb 3671 -> A
     3  Sc 10550 -> A
     4  Sd 7741 -> A
     5  Gnonexisting -> 0
     6  L -> 1: 1; 2: 2; a: 15936; asdasd: 0; b: 3671; c: 10550; d: 7741; nonexisting: 0; 

handle_read 的代码如下：

void ClientSession::handle_read(const boost::system::error_code& error, size_t bytes_transferred) {
    if (!error) {
        std::istringstream request(std::string(data_, bytes_transferred));
        boost::asio::streambuf resp;
        std::ostream os(&resp);

        char cmd_char = 0;
        std::string key;
        int value;
        if (request >> cmd_char) switch(cmd_char) {
            case SET:                          
                if (request >> key >> value)
                    kvs->db[key] = value;

                os << "A";
                break;
            case GET:
                if (request >> key)
                    os << kvs->db[key];
                break;

            case LIST:
                for (auto const& e : kvs->db)
                    os << e.first << ": " << e.second << "; ";
                break;

            case DEL: 
                if (request >> key)
                    kvs->db.erase(key);

                os << "A";
                break;
            default: 
                os << "NACK.";
        }

        cout << "resp: " << &resp << "*" << endl;
        cout << "len: " << resp.size() << "*" << endl;
        std::size_t written = boost::asio::write(socket_, resp);
        cout << "written: " << written << endl;

        boost::system::error_code ignored_ec;
        socket_.shutdown(boost::asio::ip::tcp::socket::shutdown_both, ignored_ec);
        socket_.close();
    } else
        delete this;
}