/* -*- Mode: C++; tab-width: 4; c-basic-offset: 4; indent-tabs-mode: nil -*- */
/*
 *   Copyright 2020-Present Couchbase, Inc.
 *
 *   Licensed under the Apache License, Version 2.0 (the "License");
 *   you may not use this file except in compliance with the License.
 *   You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *   Unless required by applicable law or agreed to in writing, software
 *   distributed under the License is distributed on an "AS IS" BASIS,
 *   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *   See the License for the specific language governing permissions and
 *   limitations under the License.
 */

#include "dns_client.hxx"

#include "core/logger/logger.hxx"
#include "core/utils/join_strings.hxx"
#include "dns_codec.hxx"
#include "dns_config.hxx"

#include <couchbase/error_codes.hxx>

#include <asio/ip/tcp.hpp>
#include <asio/read.hpp>
#include <asio/write.hpp>

#include <memory>
#include <sstream>

namespace couchbase::core::io::dns
{
class dns_srv_command : public std::enable_shared_from_this<dns_srv_command>
{
  public:
    dns_srv_command(asio::io_context& ctx,
                    const std::string& name,
                    const std::string& service,
                    const asio::ip::address& address,
                    std::uint16_t port,
                    utils::movable_function<void(couchbase::core::io::dns::dns_srv_response&& resp)> handler)
      : deadline_(ctx)
      , udp_deadline_(ctx)
      , udp_(ctx)
      , tcp_(ctx)
      , address_(address)
      , port_(port)
      , handler_(std::move(handler))
    {
        static std::string protocol{ "_tcp" };
        dns_message request{};
        question_record qr;
        qr.klass = resource_class::in;
        qr.type = resource_type::srv;
        qr.name.labels.push_back(service);
        qr.name.labels.push_back(protocol);
        std::string label;
        std::istringstream name_stream(name);
        while (std::getline(name_stream, label, '.')) {
            qr.name.labels.push_back(label);
        }
        request.questions.emplace_back(qr);
        send_buf_ = dns_codec::encode(request);
    }

    void execute(std::chrono::milliseconds total_timeout, std::chrono::milliseconds udp_timeout)
    {
        asio::ip::udp::endpoint endpoint(address_, port_);
        udp_.open(endpoint.protocol());
        udp_.async_send_to(
          asio::buffer(send_buf_), endpoint, [self = shared_from_this()](std::error_code ec1, std::size_t /* bytes_transferred */) mutable {
              if (ec1) {
                  self->udp_deadline_.cancel();
                  CB_LOG_DEBUG("DNS UDP write operation has got error {}, retrying with TCP", ec1.message());
                  return self->retry_with_tcp();
              }

              self->recv_buf_.resize(512);
              self->udp_.async_receive_from(
                asio::buffer(self->recv_buf_), self->udp_sender_, [self](std::error_code ec2, std::size_t bytes_transferred) mutable {
                    self->udp_deadline_.cancel();
                    if (ec2) {
                        CB_LOG_DEBUG("DNS UDP read operation has got error {}, retrying with TCP", ec2.message());
                        return self->retry_with_tcp();
                    }
                    self->recv_buf_.resize(bytes_transferred);
                    const dns_message message = dns_codec::decode(self->recv_buf_);
                    if (message.header.flags.tc == truncation::yes) {
                        self->udp_.close();
                        CB_LOG_DEBUG("DNS UDP read operation returned truncated response, retrying with TCP");
                        return self->retry_with_tcp();
                    }
                    self->deadline_.cancel();
                    dns_srv_response resp{ ec2 };
                    resp.targets.reserve(message.answers.size());
                    for (const auto& answer : message.answers) {
                        resp.targets.emplace_back(dns_srv_response::address{ utils::join_strings(answer.target.labels, "."), answer.port });
                    }
                    CB_LOG_DEBUG("DNS UDP returned {} records", resp.targets.size());
                    return self->handler_(std::move(resp));
                });
          });
        udp_deadline_.expires_after(udp_timeout);
        deadline_.async_wait([self = shared_from_this()](std::error_code ec) {
            if (ec == asio::error::operation_aborted) {
                return;
            }
            CB_LOG_DEBUG("DNS UDP deadline has been reached, cancelling UDP operation and fall back to TCP");
            self->udp_.cancel();
            return self->retry_with_tcp();
        });

        deadline_.expires_after(total_timeout);
        deadline_.async_wait([self = shared_from_this()](std::error_code ec) {
            if (ec == asio::error::operation_aborted) {
                return;
            }
            CB_LOG_DEBUG("DNS deadline has been reached, cancelling in-flight operations (tcp.is_open={})", self->tcp_.is_open());
            self->udp_.cancel();
            if (self->tcp_.is_open()) {
                self->tcp_.cancel();
            }
        });
    }

  private:
    void retry_with_tcp()
    {
        if (bool expected_state{ false }; !retrying_with_tcp_.compare_exchange_strong(expected_state, true)) {
            return;
        }

        const asio::ip::tcp::no_delay no_delay(true);
        std::error_code ignore_ec;
        tcp_.set_option(no_delay, ignore_ec);
        const asio::ip::tcp::endpoint endpoint(address_, port_);
        tcp_.async_connect(endpoint, [self = shared_from_this()](std::error_code ec1) mutable {
            if (ec1) {
                self->deadline_.cancel();
                CB_LOG_DEBUG("DNS TCP connection has been aborted, {}", ec1.message());
                return self->handler_({ ec1 });
            }
            auto send_size = static_cast<std::uint16_t>(self->send_buf_.size());
            self->send_buf_.insert(self->send_buf_.begin(), static_cast<std::uint8_t>(send_size & 0xffU));
            self->send_buf_.insert(self->send_buf_.begin(), static_cast<std::uint8_t>(send_size >> 8U));
            asio::async_write(
              self->tcp_, asio::buffer(self->send_buf_), [self](std::error_code ec2, std::size_t /* bytes_transferred */) mutable {
                  if (ec2) {
                      CB_LOG_DEBUG("DNS TCP write operation has been aborted, {}", ec2.message());
                      self->deadline_.cancel();
                      if (ec2 == asio::error::operation_aborted) {
                          ec2 = errc::common::unambiguous_timeout;
                      }
                      return self->handler_({ ec2 });
                  }
                  asio::async_read(self->tcp_,
                                   asio::buffer(&self->recv_buf_size_, sizeof(std::uint16_t)),
                                   [self](std::error_code ec3, std::size_t /* bytes_transferred */) mutable {
                                       if (ec3) {
                                           CB_LOG_DEBUG("DNS TCP buf size read operation has been aborted, {}", ec3.message());
                                           self->deadline_.cancel();
                                           return self->handler_({ ec3 });
                                       }
                                       self->recv_buf_size_ = utils::byte_swap(self->recv_buf_size_);
                                       self->recv_buf_.resize(self->recv_buf_size_);
                                       CB_LOG_DEBUG("DNS TCP schedule read of {} bytes", self->recv_buf_size_);
                                       asio::async_read(self->tcp_,
                                                        asio::buffer(self->recv_buf_),
                                                        [self](std::error_code ec4, std::size_t bytes_transferred) mutable {
                                                            self->deadline_.cancel();
                                                            if (ec4) {
                                                                CB_LOG_DEBUG("DNS TCP read operation has been aborted, {}", ec4.message());
                                                                return self->handler_({ ec4 });
                                                            }
                                                            self->recv_buf_.resize(bytes_transferred);
                                                            const dns_message message = dns_codec::decode(self->recv_buf_);
                                                            dns_srv_response resp{ ec4 };
                                                            resp.targets.reserve(message.answers.size());
                                                            for (const auto& answer : message.answers) {
                                                                resp.targets.emplace_back(dns_srv_response::address{
                                                                  utils::join_strings(answer.target.labels, "."), answer.port });
                                                            }
                                                            CB_LOG_DEBUG("DNS TCP returned {} records", resp.targets.size());
                                                            return self->handler_(std::move(resp));
                                                        });
                                   });
              });
        });
    }

    asio::steady_timer deadline_;
    asio::steady_timer udp_deadline_;
    asio::ip::udp::socket udp_;
    asio::ip::udp::endpoint udp_sender_{};
    asio::ip::tcp::socket tcp_;

    asio::ip::address address_;
    std::uint16_t port_;
    utils::movable_function<void(couchbase::core::io::dns::dns_srv_response&& resp)> handler_;

    std::vector<std::uint8_t> send_buf_{};
    std::uint16_t recv_buf_size_{ 0 };
    std::vector<std::uint8_t> recv_buf_{};

    std::atomic_bool retrying_with_tcp_{ false };
};

void
dns_client::query_srv(const std::string& name,
                      const std::string& service,
                      const dns_config& config,
                      utils::movable_function<void(dns_srv_response&&)>&& handler)
{
    std::error_code ec;
    auto address = asio::ip::address::from_string(config.nameserver(), ec);
    if (ec) {
        return handler({ ec });
    }
    auto cmd = std::make_shared<dns_srv_command>(ctx_, name, service, address, config.port(), std::move(handler));
    return cmd->execute(config.timeout(), config.timeout() / 2);
}
} // namespace couchbase::core::io::dns