Channel.hpp

//------------------------------------------------------------------------------
/*
    This file is part of clio: https://github.com/XRPLF/clio
    Copyright (c) 2025, the clio developers.
 
    Permission to use, copy, modify, and distribute this software for any
    purpose with or without fee is hereby granted, provided that the above
    copyright notice and this permission notice appear in all copies.
 
    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
//==============================================================================
 
#pragma once
 
#include <boost/asio/any_io_executor.hpp>
#include <boost/asio/experimental/channel.hpp>
#include <boost/asio/experimental/concurrent_channel.hpp>
#include <boost/asio/spawn.hpp>
#include <boost/system/detail/error_code.hpp>
 
#include <concepts>
#include <cstddef>
#include <memory>
#include <optional>
#include <type_traits>
#include <utility>
 
namespace util {
 
#ifdef __clang__
namespace detail {
// Forward declaration for compile-time check
template <typename T>
struct ChannelInstantiated;
}  // namespace detail
#endif

template <typename T>
class Channel {
private:
    class ControlBlock {
        using InternalChannelType = boost::asio::experimental::concurrent_channel<void(boost::system::error_code, T)>;
        boost::asio::any_io_executor executor_;
        InternalChannelType ch_;
 
    public:
        ControlBlock(auto&& context, std::size_t capacity) : executor_(context.get_executor()), ch_(context, capacity)
        {
        }
 
        [[nodiscard]] InternalChannelType&
        channel()
        {
            return ch_;
        }
 
        void
        close()
        {
            if (not isClosed()) {
                ch_.close();
                // Workaround for Boost bug: close() alone doesn't cancel pending async operations.
                // We must call cancel() to unblock them. The bug also causes cancel() to return
                // error_code 0 instead of channel_cancelled, so async operations must check
                // isClosed() to detect this case.
                // https://github.com/chriskohlhoff/asio/issues/1575
                ch_.cancel();
            }
        }
 
        [[nodiscard]] bool
        isClosed() const
        {
            return not ch_.is_open();
        }
    };

    struct Guard {
        std::shared_ptr<ControlBlock> shared;
 
        ~Guard()
        {
            shared->close();
        }
    };

    class Sender {
        std::shared_ptr<ControlBlock> shared_;
        std::shared_ptr<Guard> guard_;
 
    public:
        Sender(std::shared_ptr<ControlBlock> shared)
            : shared_(std::move(shared)), guard_(std::make_shared<Guard>(shared_)) {};
 
        Sender(Sender&&) = default;
        Sender(Sender const&) = default;
        Sender&
        operator=(Sender&&) = default;
        Sender&
        operator=(Sender const&) = default;

        template <typename D>
        bool
        asyncSend(D&& data, boost::asio::yield_context yield)
            requires(std::convertible_to<std::remove_cvref_t<D>, std::remove_cvref_t<T>>)
        {
            boost::system::error_code ecIn, ecOut;
            shared_->channel().async_send(ecIn, std::forward<D>(data), yield[ecOut]);
 
            // Workaround: asio channels bug returns ec=0 on cancel, check isClosed() instead
            if (not ecOut and shared_->isClosed())
                return false;
 
            return not ecOut;
        }

        template <typename D>
        void
        asyncSend(D&& data, std::invocable<bool> auto&& fn)
            requires(std::convertible_to<std::remove_cvref_t<D>, std::remove_cvref_t<T>>)
        {
            boost::system::error_code const ecIn;
            shared_->channel().async_send(
                ecIn,
                std::forward<D>(data),
                [fn = std::forward<decltype(fn)>(fn), shared = shared_](boost::system::error_code ec) mutable {
                    // Workaround: asio channels bug returns ec=0 on cancel, check isClosed() instead
                    if (not ec and shared->isClosed()) {
                        fn(false);
                        return;
                    }
 
                    fn(not ec);
                }
            );
        }

        template <typename D>
        bool
        trySend(D&& data)
            requires(std::convertible_to<std::remove_cvref_t<D>, std::remove_cvref_t<T>>)
        {
            boost::system::error_code ec;
            return shared_->channel().try_send(ec, std::forward<D>(data));
        }
    };

    class Receiver {
        std::shared_ptr<ControlBlock> shared_;
        std::shared_ptr<Guard> guard_;
 
    public:
        Receiver(std::shared_ptr<ControlBlock> shared)
            : shared_(std::move(shared)), guard_(std::make_shared<Guard>(shared_)) {};
 
        Receiver(Receiver&&) = default;
        Receiver(Receiver const&) = default;
        Receiver&
        operator=(Receiver&&) = default;
        Receiver&
        operator=(Receiver const&) = default;

        std::optional<T>
        tryReceive()
        {
            std::optional<T> result;
            shared_->channel().try_receive([&result](boost::system::error_code ec, auto&& value) {
                if (not ec)
                    result = std::forward<decltype(value)>(value);
            });
 
            return result;
        }

        [[nodiscard]] std::optional<T>
        asyncReceive(boost::asio::yield_context yield)
        {
            boost::system::error_code ec;
            auto value = shared_->channel().async_receive(yield[ec]);
 
            if (ec)
                return std::nullopt;
 
            return value;
        }

        void
        asyncReceive(std::invocable<std::optional<std::remove_cvref_t<T>>> auto&& fn)
        {
            shared_->channel().async_receive(
                [fn = std::forward<decltype(fn)>(fn)](boost::system::error_code ec, T&& value) mutable {
                    if (ec) {
                        fn(std::optional<T>(std::nullopt));
                        return;
                    }
 
                    fn(std::make_optional<T>(std::move(value)));
                }
            );
        }

        [[nodiscard]] bool
        isClosed() const
        {
            return shared_->isClosed();
        }
    };
 
public:
    static std::pair<Sender, Receiver>
    create(auto&& context, std::size_t capacity)
    {
#ifdef __clang__
        static_assert(
            util::detail::ChannelInstantiated<T>::value,
            "When using Channel<T> with Clang, you must add INSTANTIATE_CHANNEL_FOR_CLANG(T) "
            "to one .cpp file. See documentation at the bottom of Channel.hpp for details."
        );
#endif
        auto shared = std::make_shared<ControlBlock>(std::forward<decltype(context)>(context), capacity);
        auto sender = Sender{shared};
        auto receiver = Receiver{std::move(shared)};
 
        return {std::move(sender), std::move(receiver)};
    }
};
 
}  // namespace util
 
// ================================================================================================
// Clang/Apple Clang Workaround for Boost.Asio Experimental Channels
// ================================================================================================
//
// IMPORTANT: When using Channel<T> with Clang or Apple Clang, you MUST add the following line
// to ONE .cpp file that uses Channel<T>:
//
//     INSTANTIATE_CHANNEL_FOR_CLANG(YourType)
//
// Example:
//     // In ChannelTests.cpp or any .cpp file that uses Channel<int>:
//     #include "util/Channel.hpp"
//     INSTANTIATE_CHANNEL_FOR_CLANG(int)
//
// Why this is needed:
// Boost.Asio's experimental concurrent_channel has a bug where close() doesn't properly cancel
// pending async operations. When using cancellation signals (which we do in our workaround),
// Clang generates vtable references for internal cancellation_handler types but Boost.Asio
// doesn't provide the definitions, causing linker errors:
//
//   Undefined symbols for architecture arm64:
//     "boost::asio::detail::cancellation_handler<...>::call(boost::asio::cancellation_type)"
//     "boost::asio::detail::cancellation_handler<...>::destroy()"
//
// This macro explicitly instantiates the required template specializations.
//
// See: https://github.com/chriskohlhoff/asio/issues/1575
//
#ifdef __clang__
 
#include <boost/asio/cancellation_signal.hpp>
#include <boost/asio/experimental/channel_traits.hpp>
#include <boost/asio/experimental/detail/channel_service.hpp>
 
namespace util::detail {
// Tag type used to verify that INSTANTIATE_CHANNEL_FOR_CLANG was called for a given type
template <typename T>
struct ChannelInstantiated : std::false_type {};
}  // namespace util::detail
 
#define INSTANTIATE_CHANNEL_FOR_CLANG(T)                                                                       \
    /* NOLINTNEXTLINE(cppcoreguidelines-virtual-class-destructor) */                                           \
    template class boost::asio::detail::cancellation_handler<                                                  \
        boost::asio::experimental::detail::channel_service<boost::asio::detail::posix_mutex>::                 \
            op_cancellation<boost::asio::experimental::channel_traits<>, void(boost::system::error_code, T)>>; \
    namespace util::detail {                                                                                   \
    template <>                                                                                                \
    struct ChannelInstantiated<T> : std::true_type {};                                                         \
    }
 
#else
 
// No workaround needed for non-Clang compilers
#define INSTANTIATE_CHANNEL_FOR_CLANG(T)
 
#endif