/*
* Helpers for TLS ASIO Stream
* (C) 2018-2020 Jack Lloyd
*     2018-2020 Hannes Rantzsch, Tim Oesterreich, Rene Meusel
*
* Botan is released under the Simplified BSD License (see license.txt)
*/

#ifndef BOTAN_ASIO_ASYNC_OPS_H_
#define BOTAN_ASIO_ASYNC_OPS_H_

#include <botan_all.h>
#include <asio.hpp>
#include <asio/yield.hpp>
#include "asio_error.h"

namespace Botan::TLS::detail {

/**
 * Base class for asynchronous stream operations.
 *
 * Asynchronous operations, used for example to implement an interface for boost::asio::async_read_some and
 * boost::asio::async_write_some, are based on boost::asio::coroutines.
 * Derived operations should implement a call operator and invoke it with the correct parameters upon construction. The
 * call operator needs to make sure that the user-provided handler is not called directly. Typically, yield / reenter is
 * used for this in the following fashion:
 *
 * ```
 * void operator()(boost::system::error_code ec, std::size_t bytes_transferred, bool isContinuation = true)
 *    {
 *    reenter(this)
 *       {
 *       // operation specific logic, repeatedly interacting with the stream_core and the next_layer (socket)
 *
 *       // make sure intermediate initiating function is called
 *       if(!isContinuation)
 *          {
 *          yield next_layer.async_operation(empty_buffer, this);
 *          }
 *
 *       // call the completion handler
 *       complete_now(error_code, bytes_transferred);
 *       }
 *    }
 * ```
 *
 * Once the operation is completed and ready to call the completion handler it checks if an intermediate initiating
 * function has been called using the `isContinuation` parameter. If not, it will call an asynchronous operation, such
 * as `async_read_some`, with and empty buffer, set the object itself as the handler, and `yield`. As a result, the call
 * operator will be invoked again, this time as a continuation, and will jump to the location where it yielded before
 * using `reenter`. It is now safe to call the handler function via `complete_now`.
 *
 * \tparam Handler Type of the completion handler
 * \tparam Executor1 Type of the asio executor (usually derived from the lower layer)
 * \tparam Allocator Type of the allocator to be used
 */
template <class Handler, class Executor1, class Allocator>
class AsyncBase : public boost::asio::coroutine {
   public:
      using allocator_type = boost::asio::associated_allocator_t<Handler, Allocator>;
      using executor_type = boost::asio::associated_executor_t<Handler, Executor1>;

      allocator_type get_allocator() const noexcept { return boost::asio::get_associated_allocator(m_handler); }

      executor_type get_executor() const noexcept {
         return boost::asio::get_associated_executor(m_handler, m_work_guard_1.get_executor());
      }

   protected:
      template <class HandlerT>
      AsyncBase(HandlerT&& handler, const Executor1& executor) :
            m_handler(std::forward<HandlerT>(handler)), m_work_guard_1(executor) {}

      /**
       * Call the completion handler.
       *
       * This function should only be called after an intermediate initiating function has been called.
       *
       * @param args Arguments forwarded to the completion handler function.
       */
      template <class... Args>
      void complete_now(Args&&... args) {
         m_work_guard_1.reset();
         m_handler(std::forward<Args>(args)...);
      }

      Handler m_handler;
      boost::asio::executor_work_guard<Executor1> m_work_guard_1;
};

template <class Handler, class Stream, class MutableBufferSequence, class Allocator = std::allocator<void>>
class AsyncReadOperation : public AsyncBase<Handler, typename Stream::executor_type, Allocator> {
   public:
      /**
       * Construct and invoke an AsyncReadOperation.
       *
       * @param handler Handler function to be called upon completion.
       * @param stream The stream from which the data will be read
       * @param buffers The buffers into which the data will be read.
       * @param ec Optional error code; used to report an error to the handler function.
       */
      template <class HandlerT>
      AsyncReadOperation(HandlerT&& handler,
                         Stream& stream,
                         const MutableBufferSequence& buffers,
                         const boost::system::error_code& ec = {}) :
            AsyncBase<Handler, typename Stream::executor_type, Allocator>(std::forward<HandlerT>(handler),
                                                                          stream.get_executor()),
            m_stream(stream),
            m_buffers(buffers),
            m_decodedBytes(0) {
         this->operator()(ec, std::size_t(0), false);
      }

      AsyncReadOperation(AsyncReadOperation&&) = default;

      void operator()(boost::system::error_code ec, std::size_t bytes_transferred, bool isContinuation = true) {
         reenter(this) {
            if(bytes_transferred > 0 && !ec) {
               // We have received encrypted data from the network, now hand it to TLS::Channel for decryption.
               boost::asio::const_buffer read_buffer{m_stream.input_buffer().data(), bytes_transferred};
               m_stream.process_encrypted_data(read_buffer, ec);
            }

            if(m_stream.shutdown_received()) {
               // we just received a 'close_notify' from the peer and don't expect any more data
               ec = boost::asio::error::eof;
            } else if(ec == boost::asio::error::eof) {
               // we did not expect this disconnection from the peer
               ec.assign(StreamError::StreamTruncated, std::generic_category());
            }

            if(!m_stream.has_received_data() && !ec && boost::asio::buffer_size(m_buffers) > 0) {
               // The channel did not decrypt a complete record yet, we need more data from the socket.
               m_stream.next_layer().async_read_some(m_stream.input_buffer(), std::move(*this));
               return;
            }

            if(m_stream.has_received_data() && !ec) {
               // The channel has decrypted a TLS record, now copy it to the output buffers.
               m_decodedBytes = m_stream.copy_received_data(m_buffers);
            }

            if(!isContinuation) {
               // Make sure the handler is not called without an intermediate initiating function.
               // "Reading" into a zero-byte buffer will complete immediately.
               m_ec = ec;
               yield m_stream.next_layer().async_read_some(boost::asio::mutable_buffer(), std::move(*this));
               ec = m_ec;
            }

            this->complete_now(ec, m_decodedBytes);
         }
      }

   private:
      Stream& m_stream;
      MutableBufferSequence m_buffers;
      std::size_t m_decodedBytes;
      boost::system::error_code m_ec;
};

template <typename Handler, class Stream, class Allocator = std::allocator<void>>
class AsyncWriteOperation : public AsyncBase<Handler, typename Stream::executor_type, Allocator> {
   public:
      /**
       * Construct and invoke an AsyncWriteOperation.
       *
       * @param handler Handler function to be called upon completion.
       * @param stream The stream from which the data will be read
       * @param plainBytesTransferred Number of bytes to be reported to the user-provided handler function as
       *                              bytes_transferred. This needs to be provided since the amount of plaintext data
       *                              consumed from the input buffer can differ from the amount of encrypted data written
       *                              to the next layer.
       * @param ec Optional error code; used to report an error to the handler function.
       */
      template <class HandlerT>
      AsyncWriteOperation(HandlerT&& handler,
                          Stream& stream,
                          std::size_t plainBytesTransferred,
                          const boost::system::error_code& ec = {}) :
            AsyncBase<Handler, typename Stream::executor_type, Allocator>(std::forward<HandlerT>(handler),
                                                                          stream.get_executor()),
            m_stream(stream),
            m_plainBytesTransferred(plainBytesTransferred) {
         this->operator()(ec, std::size_t(0), false);
      }

      AsyncWriteOperation(AsyncWriteOperation&&) = default;

      void operator()(boost::system::error_code ec, std::size_t bytes_transferred, bool isContinuation = true) {
         reenter(this) {
            // mark the number of encrypted bytes sent to the network as "consumed"
            // Note: bytes_transferred will be zero on first call
            m_stream.consume_send_buffer(bytes_transferred);

            if(m_stream.has_data_to_send() && !ec) {
               m_stream.next_layer().async_write_some(m_stream.send_buffer(), std::move(*this));
               return;
            }

            if(ec == boost::asio::error::eof && !m_stream.shutdown_received()) {
               // transport layer was closed by peer without receiving 'close_notify'
               ec.assign(StreamError::StreamTruncated, std::generic_category());
            }

            if(!isContinuation) {
               // Make sure the handler is not called without an intermediate initiating function.
               // "Writing" to a zero-byte buffer will complete immediately.
               m_ec = ec;
               yield m_stream.next_layer().async_write_some(boost::asio::const_buffer(), std::move(*this));
               ec = m_ec;
            }

            // The size of the sent TLS record can differ from the size of the payload due to TLS encryption. We need to
            // tell the handler how many bytes of the original data we already processed.
            this->complete_now(ec, m_plainBytesTransferred);
         }
      }

   private:
      Stream& m_stream;
      std::size_t m_plainBytesTransferred;
      boost::system::error_code m_ec;
};

template <class Handler, class Stream, class Allocator = std::allocator<void>>
class AsyncHandshakeOperation : public AsyncBase<Handler, typename Stream::executor_type, Allocator> {
   public:
      /**
       * Construct and invoke an AsyncHandshakeOperation.
       *
       * @param handler Handler function to be called upon completion.
       * @param stream The stream from which the data will be read
       * @param ec Optional error code; used to report an error to the handler function.
       */
      template <class HandlerT>
      AsyncHandshakeOperation(HandlerT&& handler, Stream& stream, const boost::system::error_code& ec = {}) :
            AsyncBase<Handler, typename Stream::executor_type, Allocator>(std::forward<HandlerT>(handler),
                                                                          stream.get_executor()),
            m_stream(stream) {
         this->operator()(ec, std::size_t(0), false);
      }

      AsyncHandshakeOperation(AsyncHandshakeOperation&&) = default;

      void operator()(boost::system::error_code ec, std::size_t bytesTransferred, bool isContinuation = true) {
         reenter(this) {
            if(ec == boost::asio::error::eof) {
               ec.assign(StreamError::StreamTruncated, std::generic_category());
            }

            if(bytesTransferred > 0 && !ec) {
               // Provide encrypted TLS data received from the network to TLS::Channel for decryption
               boost::asio::const_buffer read_buffer{m_stream.input_buffer().data(), bytesTransferred};
               m_stream.process_encrypted_data(read_buffer, ec);
            }

            if(m_stream.has_data_to_send() && !ec) {
               // Write encrypted TLS data provided by the TLS::Channel on the wire

               // Note: we construct `AsyncWriteOperation` with 0 as its last parameter (`plainBytesTransferred`). This
               // operation will eventually call `*this` as its own handler, passing the 0 back to this call operator.
               // This is necessary because the check of `bytesTransferred > 0` assumes that `bytesTransferred` bytes
               // were just read and are available in input_buffer for further processing.
               AsyncWriteOperation<AsyncHandshakeOperation<typename std::decay<Handler>::type, Stream, Allocator>,
                                   Stream,
                                   Allocator>
                  op{std::move(*this), m_stream, 0};
               return;
            }

            if(!m_stream.native_handle()->is_active() && !ec) {
               // Read more encrypted TLS data from the network
               m_stream.next_layer().async_read_some(m_stream.input_buffer(), std::move(*this));
               return;
            }

            if(!isContinuation) {
               // Make sure the handler is not called without an intermediate initiating function.
               // "Reading" into a zero-byte buffer will complete immediately.
               m_ec = ec;
               yield m_stream.next_layer().async_read_some(boost::asio::mutable_buffer(), std::move(*this));
               ec = m_ec;
            }

            this->complete_now(ec);
         }
      }

   private:
      Stream& m_stream;
      boost::system::error_code m_ec;
};

}  // namespace Botan::TLS::detail

   #include <asio/unyield.hpp>

#endif  // BOTAN_ASIO_ASYNC_OPS_H_