C++ httplib 解读2

因为这个库确实好用，而且也没什么难度，所以就想写写，结果这两天网站挂了，耽误了好久，但愿今天晚上能够写完……
不过这几天工作需要也看了下，应该会比较快
这次从put content provider
先看一下官方的用法

Send content with Content provider

const uint64_t DATA_CHUNK_SIZE = 4;

svr.Get("stream", [&](const Request &req, Response &res) {
  auto data = new std::string("abcdefg");

  res.set_content_provider(
    data->size(), // Content length
    [data](uint64_t offset, uint64_t length, DataSink &sink) {
      const auto &d = *data;
      sink.write(&d[offset], std::min(length, DATA_CHUNK_SIZE));
    },
    [data] { delete data; });
});

这里的lambda表达式，是一个ContentProvider, 看了一个它的定义是一个functional

using ContentProvider =
    std::function<void(size_t offset, size_t length, DataSink &sink)>;

用法是很明显的，它会从offset 写入length的数据，到sink里面，这里的sink有点类似一个数据流的概念，流的承接对象，不过说实话它这个lambda写得挺绕的…… data = new string ,不过这个是为了容易解释后面那个lambda 如何释放的对象。写入数据就是每次从offset 写入min(length, DATA_CHUNK_SIZE) 也就是每次写入不大于DATA_CHUNK_SIZE。至于offset 和length 会在下面的代码里面进行更新。这个的Server代码因为涉及到线程池不太直观，在Client找到了类似的用法

std::shared_ptr<Response>
Client::Put(const char *path, const Headers &headers, size_t content_length,
            ContentProvider content_provider, const char *content_type) {
  return send_with_content_provider("PUT", path, headers, std::string(),
                                    content_length, content_provider,
                                    content_type);
}

std::shared_ptr<Response> Client::send_with_content_provider(
    const char *method, const char *path, const Headers &headers,
    const std::string &body, size_t content_length,
    ContentProvider content_provider, const char *content_type) {
  Request req;
  req.method = method;
  req.headers = headers;
  req.path = path;

  req.headers.emplace("Content-Type", content_type);

  {
    if (content_provider) {
      req.content_length = content_length;
      req.content_provider = content_provider;
    } else {
      req.body = body;
    }
  }

  auto res = std::make_shared<Response>();

  return send(req, *res) ? res : nullptr;
}

provider 已经赋值给了req 下面就是最后的调用了

bool Client::send(const Request &req, Response &res) {
  auto sock = create_client_socket();
  if (sock == INVALID_SOCKET) { return false; }

#ifdef CPPHTTPLIB_OPENSSL_SUPPORT
  if (is_ssl() && !proxy_host_.empty()) {
    bool error;
    if (!connect(sock, res, error)) { return error; }
  }
#endif

  return process_and_close_socket(
      sock, 1, [&](Stream &strm, bool last_connection, bool &connection_close) {
        return handle_request(strm, req, res, last_connection,
                              connection_close);
      });
}



bool Client::handle_request(Stream &strm, const Request &req,
                                   Response &res, bool last_connection,
                                   bool &connection_close) {
  if (req.path.empty()) { return false; }

  bool ret;

  if (!is_ssl() && !proxy_host_.empty()) {
    auto req2 = req;
    req2.path = "http://" + host_and_port_ + req.path;
    ret = process_request(strm, req2, res, last_connection, connection_close);
  } else {
    ret = process_request(strm, req, res, last_connection, connection_close);
  }

  if (!ret) { return false; }

  if (300 < res.status && res.status < 400 && follow_location_) {
    ret = redirect(req, res);
  }

  return ret;
}
bool Client::process_request(Stream &strm, const Request &req,
                                    Response &res, bool last_connection,
                                    bool &connection_close) {
  // Send request
  if (!write_request(strm, req, last_connection)) { return false; }

  // Receive response and headers
  if (!read_response_line(strm, res) ||
      !detail::read_headers(strm, res.headers)) {
    return false;
  }

  if (res.get_header_value("Connection") == "close" ||
      res.version == "HTTP/1.0") {
    connection_close = true;
  }

  if (req.response_handler) {
    if (!req.response_handler(res)) { return false; }
  }

  // Body
  if (req.method != "HEAD" && req.method != "CONNECT") {
    ContentReceiver out = [&](const char *buf, size_t n) {
      if (res.body.size() + n > res.body.max_size()) { return false; }
      res.body.append(buf, n);
      return true;
    };

    if (req.content_receiver) {
      out = [&](const char *buf, size_t n) {
        return req.content_receiver(buf, n);
      };
    }

    int dummy_status;
    if (!detail::read_content(strm, res, (std::numeric_limits<size_t>::max)(),
                              dummy_status, req.progress, out)) {
      return false;
    }
  }

  // Log
  if (logger_) { logger_(req, res); }

  return true;
}

最后一层的调用链

bool Client::write_request(Stream &strm, const Request &req,
                                  bool last_connection) {
  detail::BufferStream bstrm;

  // Request line
  const auto &path = detail::encode_url(req.path);

  bstrm.write_format("%s %s HTTP/1.1\r\n", req.method.c_str(), path.c_str());

  // Additonal headers
  Headers headers;
  if (last_connection) { headers.emplace("Connection", "close"); }

  if (!req.has_header("Host")) {
    if (is_ssl()) {
      if (port_ == 443) {
        headers.emplace("Host", host_);
      } else {
        headers.emplace("Host", host_and_port_);
      }
    } else {
      if (port_ == 80) {
        headers.emplace("Host", host_);
      } else {
        headers.emplace("Host", host_and_port_);
      }
    }
  }

  if (!req.has_header("Accept")) { headers.emplace("Accept", "*/*"); }

  if (!req.has_header("User-Agent")) {
    headers.emplace("User-Agent", "cpp-httplib/0.5");
  }

  if (req.body.empty()) {
    if (req.content_provider) {
      auto length = std::to_string(req.content_length);
      headers.emplace("Content-Length", length);
    } else {
      headers.emplace("Content-Length", "0");
    }
  } else {
    if (!req.has_header("Content-Type")) {
      headers.emplace("Content-Type", "text/plain");
    }

    if (!req.has_header("Content-Length")) {
      auto length = std::to_string(req.body.size());
      headers.emplace("Content-Length", length);
    }
  }

  if (!basic_auth_username_.empty() && !basic_auth_password_.empty()) {
    headers.insert(make_basic_authentication_header(
        basic_auth_username_, basic_auth_password_, false));
  }

  if (!proxy_basic_auth_username_.empty() &&
      !proxy_basic_auth_password_.empty()) {
    headers.insert(make_basic_authentication_header(
        proxy_basic_auth_username_, proxy_basic_auth_password_, true));
  }

  detail::write_headers(bstrm, req, headers);

  // Flush buffer
  auto &data = bstrm.get_buffer();
  strm.write(data.data(), data.size());

  // Body
  if (req.body.empty()) {
    if (req.content_provider) {
      size_t offset = 0;
      size_t end_offset = req.content_length;

      DataSink data_sink;
      data_sink.write = [&](const char *d, size_t l) {
        auto written_length = strm.write(d, l);
        offset += static_cast<size_t>(written_length);
      };
      data_sink.is_writable = [&](void) { return strm.is_writable(); };

      while (offset < end_offset) {
        req.content_provider(offset, end_offset - offset, data_sink);
      }
    }
  } else {
    strm.write(req.body);
  }

  return true;
}

这里构造了一个DataSink 它的write 是直接向socket stream中写入数据，然后会更新offset,然后在while 循环中，会更新length，也就是end_offset - offset(因为offset 更新了，length也更新了……)

至此使用content provider的分析完成了……
这里呢我有一点不懂，就是为啥不直接把Stream 暴露出来，非要整个DataSink呢? 接口就直接是Stream岂不更好。毕竟DataSink的write 也是直接调用的Stream 后来想了下，DataSink只有写的接口，而没有读，而Stream是读写接口，这样的话相当于一道封印，安全啊。唯一的难点就是对于DataSink理解，怎么理解呢，就是iostream的istream，只要这么理解就好理解了。

如果工作中碰到了，我就继续写。现在打算开始看看Qt的源码了，毕竟每次面试提问一问三不知还是不好的……