1 踩过的坑

1.1 io_service

boost::asio::io_service::run()会一直运行到没有任务为止，假设中途调用stop()。则全部等待中的任务会立马运行。要在停止的时候抛弃全部的任务，解决方式是用run_one()。即

while (keep_running)

    io_service_.run_one();

keep_running是个bool值。要stop io_service的时候直接置false就可以。

1.2 deadline_timer

在调用async_wait()后。不管调用deadline_timer::cancel()还是这个deadline_timer都析构掉。handler都会被触发。

当然。这个在文档是有写的。规避野指针的办法有两个，一是传入的handler是shared_ptr，二是再封装一层。后者适用于handler的生命周期无法由自身控制的情况，演示样例代码请看http client一节的TimerHolder类。

1.3 async*

这个事实上和deadline_timer::asyn::wait()差点儿相同。async_read、async_read_until等带async_前缀的函数，仅仅要中途被停止（比如调用ip::tcp::socket::close()），Handler都会被运行并传入一个代表aborted的boost::system::error_code。

1.4 ip::tcp::socket

  1. 官方的example过于简单。反而迷惑人了。HTTP协议说来也算复杂，比如chunked encoding还得自己解析。
  2. 从ip::tcp::resolver得到的可能是多个IP，假设把返回的迭代器交给async_connect，那么非常可能出错，应为IP里可能有不合理的地址。比方可能返回的是全0的地址。解决的方法參考http client代码的DoResolveAndConnect()函数。

  3. socket的read可能会读到额外的数据，这个文档里有写。

2. http client的应用

封装成了C++类。这是单线程的实现（io_service是同一线程下run的）。同步地调用socket函数并用deadline_timer来异步返回数据会更easy控制。

不细说了。请看代码。

注：URL、HttpRequest、HttpResponse等类未列出源代码，请自行实现相应函数。

// header

#include <assert.h>

#include <string>

#include "boost/asio.hpp"

#include "boost/make_shared.hpp"

#include "boost/thread.hpp"

class HttpTransaction {

 public:

  // The Delegate implementation MAY delete HttpTransaction during Delegate

  // method is invoked.

  // Possible flow: ("*" means zero or more times.)

  //   1. OnResponseReceived() -> OnDataReceived()* -> OnFinished().

  //   2. OnError().

  //   3. OnResponseReceived() -> OnError().

  //   4. OnResponseReceived() -> OnDataReceived()* -> OnError().

  class Delegate {

   public:

    virtual ~Delegate() {}

    virtual void OnResponseReceived(HttpTransaction* transaction,

                                    const HttpResponse& response) = 0;

    virtual void OnDataReceived(HttpTransaction* transaction,

                                const char* data,

                                size_t length) = 0;

    virtual void OnFinished(HttpTransaction* transaction) = 0;

    virtual void OnError(HttpTransaction* transaction, int error_code) = 0;

  };

  explicit HttpTransaction(boost::asio::io_service& io);

  ~HttpTransaction();

  void Start();

  void Cancel();

  const HttpRequest* request() const {

    return http_request_;

  }

  void set_request(const HttpRequest* request) {

    http_request_ = request;

  }

  Delegate* delegate() const { return delegate_; }

  void set_delegate(Delegate* delegate) {

    delegate_ = delegate;

  }

 private:

  enum State {

    STATE_NONE,

    STATE_CONNECT,

    STATE_SEND_REQUEST,

    STATE_READ_HEADER,

    STATE_READ_BODY,

    STATE_READ_CHUNK_SIZE,

    STATE_READ_CHUNK_DATA,

    STATE_READ_UNTIL_EOF,

    STATE_CALL_ON_FINISHED,

  };

  void DoLoop();

  bool DoResolveAndConnect();

  bool DoSendRequest();

  bool DoReadHeader();

  bool DoReadChunkSize();

  bool DoReadChunkData();

  bool DoReadBody();

  bool DoReadUntilEof();

  bool DoCallOnFinished();

  void CallOnDataReceived(size_t size, size_t additional_consume_size = 0);

  const HttpRequest* http_request_;

  Delegate* delegate_;

  HttpResponse* http_response_;

  boost::asio::ip::tcp::resolver resolver_;

  boost::asio::ip::tcp::socket socket_;

  boost::shared_ptr<boost::asio::streambuf> stream_buf_;

  size_t pending_read_size_;

  State next_state_;

  bool started_;

  class TimerHolder;

  boost::shared_ptr<TimerHolder> timer_holder_;

};

#####################################################################

// implementation

#include <string.h>

#include <assert.h>

#include <algorithm>

#include "boost/algorithm/hex.hpp"

#include "boost/bind.hpp"

#include "http_transaction.h"

#include "http_request.h"

#include "http_response.h"

#include "util/url.h"

#include "util/logging.h"

// TimerHolder is needed because the callback is invoked even the timer

// cancelled or deleted.

class HttpTransaction::TimerHolder {

 public:

  TimerHolder(HttpTransaction* trans,

              boost::asio::io_service& io)  // NOLINT

      : trans_(trans),

        timer_(io) {}

  void Schedule() {

    timer_.expires_from_now(boost::posix_time::microseconds(0));

    timer_.async_wait(boost::bind(&TimerHolder::OnTimer, trans_->timer_holder_,

                                  boost::asio::placeholders::error));

  }

  void OnTransactionCancelled() {

    trans_ = NULL;

    timer_.cancel();

  }

 private:

  void OnTimer(const boost::system::error_code& err) {

    if (!err && trans_) {

      trans_->DoLoop();

    }

  }

  HttpTransaction* trans_;

  boost::asio::deadline_timer timer_;

};

HttpTransaction::HttpTransaction(boost::asio::io_service& io)

    : http_request_(NULL),

      delegate_(NULL),

      http_response_(NULL),

      resolver_(io),

      socket_(io),

      stream_buf_(new boost::asio::streambuf),

      pending_read_size_(0),

      next_state_(STATE_NONE),

      started_(false),

      timer_holder_(new TimerHolder(this, io)) {

}

HttpTransaction::~HttpTransaction() {

  Cancel();

}

void HttpTransaction::Start() {

  assert(!started_);

  started_ = true;

  next_state_ = STATE_CONNECT;

  timer_holder_->Schedule();

}

void HttpTransaction::Cancel() {

  next_state_ = STATE_NONE;

  timer_holder_->OnTransactionCancelled();

  socket_.close();

  if (http_response_) {

    delete http_response_;

    http_response_ = NULL;

  }

}

void HttpTransaction::DoLoop() {

  bool rv = false;

  do {

    State state = next_state_;

    next_state_ = STATE_NONE;

    switch (state) {

      case STATE_CONNECT:

        rv = DoResolveAndConnect();

        break;

      case STATE_SEND_REQUEST:

        rv = DoSendRequest();

        break;

      case STATE_READ_HEADER:

        rv = DoReadHeader();

        break;

      case STATE_READ_BODY:

        rv = DoReadBody();

        break;

      case STATE_READ_UNTIL_EOF:

        rv = DoReadUntilEof();

        break;

      case STATE_READ_CHUNK_SIZE:

        rv = DoReadChunkSize();

        break;

      case STATE_READ_CHUNK_DATA:

        rv = DoReadChunkData();

        break;

      case STATE_CALL_ON_FINISHED:

        rv = DoCallOnFinished();

        break;

      default:

        assert(0);

        break;

    }

  } while (rv);

}

bool HttpTransaction::DoResolveAndConnect() {

  URL url(http_request_->url());

  // TODO(liuhx): if url is ip address.

  boost::asio::ip::tcp::resolver::query query(

      url.host(), url.port() == 0 ? url.protocol() : url.port_string());

  boost::system::error_code err;

  boost::asio::ip::tcp::resolver::iterator it = resolver_.resolve(query, err);

  boost::asio::ip::tcp::resolver::iterator end;  // Default is end.

  if (err || it == end) {

    LOG_DEBUG(kLogTagHttpTrans, "resolve error:%s",

                      err.message().c_str());

    delegate_->OnError(this, err.value());

    return false;

  }

  do {

    LOG_INFO(kLogTagHttpTrans, "dns result:%s",

                     it->endpoint().address().to_string().c_str());

    // "unspecified" means address is "0.0.0.0". It may appear on some machines

    // running Apache. Please google it for more detail.

    if (!it->endpoint().address().is_unspecified()) {

      socket_.close();

      LOG_INFO(kLogTagHttpTrans, "connecting:%s",

                       it->endpoint().address().to_string().c_str());

      socket_.connect(*it, err);

      if (!err)

        break;

    }

    ++it;

  } while (it != end);

  if (err) {

    LOG_DEBUG(kLogTagHttpTrans, "connect error:%s",

                      err.message().c_str());

    delegate_->OnError(this, err.value());

    return false;

  }

  next_state_ = STATE_SEND_REQUEST;

  return true;

}

bool HttpTransaction::DoSendRequest() {

  URL url(http_request_->url());

  std::ostream request_stream(stream_buf_.get());

  request_stream << http_request_->method() << " " << url.path()

                 << " HTTP/1.1\r\n";

  if (!http_request_->HasHeader("Host")) {

    request_stream << "Host: " << url.host() << "\r\n";

  }

  if (!http_request_->HasHeader("Connection")) {

    request_stream << "Connection: keep-alive\r\n";

  }

  const char* name;

  const char* value;

  void* iter = NULL;

  while (http_request_->EnumerateHeaderLines(&iter, &name, &value))

    request_stream << name << ": " << value << "\r\n";

  size_t size = 0;

  if (http_request_->body(&value, &size)) {

    if (!http_request_->HasHeader("Content-Length")) {

      request_stream << "Content-Length: " << size << "\r\n";

    }

    request_stream << "\r\n";

    request_stream.write(value, size);

  } else {

    request_stream << "\r\n";

  }

  boost::system::error_code err;

  // boost::asio::write() consumes |stream_buf_|, no need to do it ourselves.

  boost::asio::write(socket_, *stream_buf_, err);

  if (err) {

    LOG_DEBUG(kLogTagHttpTrans, "send request error:%s",

                      err.message().c_str());

    delegate_->OnError(this, err.value());

  } else {

    next_state_ = STATE_READ_HEADER;

    timer_holder_->Schedule();

  }

  return false;

}

bool HttpTransaction::DoReadHeader() {

  boost::system::error_code err;

  boost::asio::read_until(socket_, *stream_buf_, "\r\n\r\n", err);

  if (err) {

    LOG_DEBUG(kLogTagHttpTrans, "read header error:%s",

        err.message().c_str());

    delegate_->OnError(this, err.value());

    return false;

  }

  size_t size = stream_buf_->size();

  const char* data = boost::asio::buffer_cast<const char*>(stream_buf_->data());

  size_t pos = std::string(data, size).find("\r\n\r\n");

  if (pos == std::string::npos) {

    LOG_DEBUG(kLogTagHttpTrans,

                      "Can not find header end. Maybe TCP data Out-of-Order");

    delegate_->OnError(this, 1234);

    return false;

  }

  http_response_ = new HttpResponse(http_request_->url(), data, pos);

  stream_buf_->consume(pos + 4);  // Skip 4 = "\r\n\r\n".

  if (http_response_->status_code() < 100) {

    LOG_DEBUG(kLogTagHttpTrans, "Header invalid");

    delegate_->OnError(this, 2345);

    return false;

  }

  // TODO(liuhx): Body may be available even status code is not 200.For

  // example, some website may display a web page while replying with 404.

  if (http_response_->status_code() != 200) {

    next_state_ = STATE_CALL_ON_FINISHED;

  } else {

    const char* content_length = http_response_->GetHeader("content-length");

    if (content_length) {

      pending_read_size_ = atoi(content_length);

      next_state_ = STATE_READ_BODY;

    } else {

      const char* encoding = http_response_->GetHeader("Transfer-Encoding");

      bool is_chunk = encoding && (std::string(encoding).find("chunked") !=

                                  std::string::npos);

      if (is_chunk) {

        next_state_ = STATE_READ_CHUNK_SIZE;

      } else {

        next_state_ = STATE_READ_UNTIL_EOF;

      }

    }

  }

  timer_holder_->Schedule();

  delegate_->OnResponseReceived(this, *http_response_);

  return false;

}

bool HttpTransaction::DoReadBody() {

  // If content-length exists, the connection may be keep-alive. We MUST keep

  // counting |pending_read_size_| instead of reading until EOF.

  if (pending_read_size_ == 0) {

    delegate_->OnFinished(this);

    return false;

  }

  while (true) {

    // boost may read addtional data beyond the condition in STATE_READ_HEADER,

    // pass left data first if exists.

    size_t size = stream_buf_->size();

    if (size) {

      next_state_ = STATE_READ_BODY;

      timer_holder_->Schedule();

      // TODO(liuhx): assert -> OnError

      assert(pending_read_size_ >= size);

      pending_read_size_ -= size;

      CallOnDataReceived(size);

      break;

    } else {

      boost::system::error_code err;

      boost::asio::read(socket_, *stream_buf_,

                        boost::asio::transfer_at_least(1), err);

      if (err) {

        LOG_DEBUG(kLogTagHttpTrans, "read body error:%s",

            err.message().c_str());

        delegate_->OnError(this, err.value());

        break;

      }

    }

  }

  return false;

}

// About chunked encoding, refer to http://tools.ietf.org/html/rfc2616#page-25

bool HttpTransaction::DoReadChunkSize() {

  while (true) {

    // boost may read addtional data beyond the condition, find "\r\n" first.

    size_t size = stream_buf_->size();

    if (size) {

      const char* data =

          boost::asio::buffer_cast<const char*>(stream_buf_->data());

      size_t index = std::string(data, size).find("\r\n");

      if (index != std::string::npos) {

        pending_read_size_ = static_cast<size_t>(strtol(data, NULL, 16));

        stream_buf_->consume(index + 2);  // Skip +2 = "\r\n"

        if (pending_read_size_ == 0) {

          delegate_->OnFinished(this);

          return false;

        }

        break;

      }

    }

    boost::system::error_code err;

    boost::asio::read_until(socket_, *stream_buf_, "\r\n", err);

    if (err) {

      LOG_DEBUG(kLogTagHttpTrans, "read chunk size error:%s",

                        err.message().c_str());

      delegate_->OnError(this, err.value());

      return false;

    }

  }

  next_state_ = STATE_READ_CHUNK_DATA;

  return true;

}

bool HttpTransaction::DoReadChunkData() {

  while (true) {

    size_t size = stream_buf_->size();

    if (size) {

      bool reach_end = size >= pending_read_size_;

      size_t data_size = reach_end ?

pending_read_size_ : size;

      pending_read_size_ -= data_size;

      next_state_ = reach_end ?

STATE_READ_CHUNK_SIZE : STATE_READ_CHUNK_DATA;

      timer_holder_->Schedule();

      CallOnDataReceived(data_size, reach_end ?

2 : 0);  // Skip 2 = "\r\n".

      break;

    } else {

      boost::system::error_code err;

      boost::asio::read_until(socket_, *stream_buf_, "\r\n", err);

      if (err) {

        LOG_DEBUG(kLogTagHttpTrans, "read chunk data error:%s",

                          err.message().c_str());

        delegate_->OnError(this, err.value());

        break;

      }

    }

  }

  return false;

}

bool HttpTransaction::DoReadUntilEof() {

  while (true) {

    size_t size = stream_buf_->size();

    if (size) {

      next_state_ = STATE_READ_UNTIL_EOF;

      timer_holder_->Schedule();

      const char* data =

          boost::asio::buffer_cast<const char*>(stream_buf_->data());

      boost::shared_ptr<boost::asio::streambuf> buf = stream_buf_;

      delegate_->OnDataReceived(this, data, size);

      buf->consume(size);

      break;

    } else {

      boost::system::error_code err;

      boost::asio::read(socket_, *stream_buf_,

                        boost::asio::transfer_at_least(1), err);

      if (err) {

        if (err == boost::asio::error::eof) {

          delegate_->OnFinished(this);

        } else {

          LOG_DEBUG(kLogTagHttpTrans, "%s", err.message().c_str());

          delegate_->OnError(this, err.value());

        }

        break;

      }

    }

  }

  return false;

}

bool HttpTransaction::DoCallOnFinished() {

  delegate_->OnFinished(this);

  return false;

}

void HttpTransaction::CallOnDataReceived(size_t size,

                                             size_t additional_consume_size) {

  const char* data = boost::asio::buffer_cast<const char*>(stream_buf_->data());

  // Because Delegate may delete HttpTransaction during callback, we MUST NOT

  // access member variable after callback method, instead, use the |buf|.

  boost::shared_ptr<boost::asio::streambuf> buf = stream_buf_;

  delegate_->OnDataReceived(this, data, size);

  buf->consume(size + additional_consume_size);

}

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