paced_sender.cpp

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// SPDX-License-Identifier: GPL-2.0-only
//
// Copyright (c) 2026-present FERS Contributors (see AUTHORS.md).
//
// See the GNU GPLv2 LICENSE file in the FERS project root for more information.
 
#include "serial/vita49/paced_sender.h"
 
#include <algorithm>
#include <atomic>
#include <chrono>
#include <stdexcept>
 
#if defined(__i386__) || defined(__x86_64__)
#include <immintrin.h>
#endif
 
namespace serial::vita49
{
    namespace
    {
        constexpr auto kCoarsePacingSleep = std::chrono::milliseconds(1);
        constexpr auto kFinePacingSpin = std::chrono::microseconds(200);
 
        void cpuPause() noexcept
        {
#if defined(__i386__) || defined(__x86_64__)
            _mm_pause();
#elif defined(__aarch64__) && (defined(__GNUC__) || defined(__clang__))
            __asm__ __volatile__("yield" ::: "memory");
#else
            std::atomic_signal_fence(std::memory_order_seq_cst);
#endif
        }
    }
 
    PacedSender::PacedSender(std::unique_ptr<DatagramSender> sender, const std::size_t queue_depth) :
        _sender(std::move(sender)), _queue_depth(queue_depth)
    {
        if (!_sender)
        {
            throw std::invalid_argument("PacedSender requires a datagram sender");
        }
        if (queue_depth == 0)
        {
            throw std::invalid_argument("PacedSender queue depth must be positive");
        }
    }
 
    PacedSender::~PacedSender() { stop(); }
 
    void PacedSender::open(const std::string& host, const std::uint16_t port) { _sender->open(host, port); }
 
    void PacedSender::start(const RealType simulation_epoch_time)
    {
        std::scoped_lock const lock(_mutex);
        if (_started)
        {
            return;
        }
        _simulation_epoch_time = simulation_epoch_time;
        _steady_epoch = std::chrono::steady_clock::now();
        _stopping = false;
        _started = true;
        _thread = std::thread([this] { run(); });
    }
 
    EnqueueResult PacedSender::enqueue(SerializedPacket packet)
    {
        std::unique_lock lock(_mutex);
        if (!_started)
        {
            throw std::logic_error("PacedSender must be started before enqueue");
        }
        if (_stopping)
        {
            return EnqueueResult{
                .enqueued = false,
                .dropped = std::nullopt,
            };
        }
 
        _cv.wait(lock, [this] { return _stopping || queuedOrSendingCount() < _queue_depth; });
        if (_stopping)
        {
            return EnqueueResult{
                .enqueued = false,
                .dropped = std::nullopt,
            };
        }
 
        _queue.push_back(std::move(packet));
        _cv.notify_one();
        return EnqueueResult{.enqueued = true, .dropped = std::nullopt};
    }
 
    void PacedSender::flush()
    {
        std::unique_lock lock(_mutex);
        _cv.wait(lock, [this] { return _queue.empty() && !_send_in_progress; });
    }
 
    void PacedSender::stop()
    {
        {
            std::scoped_lock const lock(_mutex);
            if (!_started && !_thread.joinable())
            {
                _sender->close();
                return;
            }
            _stopping = true;
            _cv.notify_all();
        }
 
        if (_thread.joinable())
        {
            _thread.join();
        }
 
        {
            std::scoped_lock const lock(_mutex);
            _started = false;
            _stopping = false;
            _send_in_progress = false;
            _cv.notify_all();
        }
        _sender->close();
    }
 
    std::uint64_t PacedSender::latePacketCount(const std::uint32_t stream_id) const
    {
        std::scoped_lock const lock(_mutex);
        const auto found = _late_packets.find(stream_id);
        return found == _late_packets.end() ? 0 : found->second;
    }
 
    std::uint64_t PacedSender::sentPacketCount(const std::uint32_t stream_id) const
    {
        std::scoped_lock const lock(_mutex);
        const auto found = _sent_packets.find(stream_id);
        return found == _sent_packets.end() ? 0 : found->second;
    }
 
    std::uint64_t PacedSender::sendFailureCount(const std::uint32_t stream_id) const
    {
        std::scoped_lock const lock(_mutex);
        const auto found = _send_failures.find(stream_id);
        return found == _send_failures.end() ? 0 : found->second;
    }
 
    std::uint64_t PacedSender::droppedDataPacketCount(const std::uint32_t stream_id) const
    {
        std::scoped_lock const lock(_mutex);
        const auto found = _dropped_data_packets.find(stream_id);
        return found == _dropped_data_packets.end() ? 0 : found->second;
    }
 
    std::uint64_t PacedSender::droppedContextPacketCount(const std::uint32_t stream_id) const
    {
        std::scoped_lock const lock(_mutex);
        const auto found = _dropped_context_packets.find(stream_id);
        return found == _dropped_context_packets.end() ? 0 : found->second;
    }
 
    std::uint64_t PacedSender::droppedSampleCount(const std::uint32_t stream_id) const
    {
        std::scoped_lock const lock(_mutex);
        const auto found = _dropped_samples.find(stream_id);
        return found == _dropped_samples.end() ? 0 : found->second;
    }
 
    std::vector<DroppedDatagram> PacedSender::consumeDroppedDatagrams()
    {
        std::scoped_lock const lock(_mutex);
        auto result = std::move(_pending_dropped_datagrams);
        _pending_dropped_datagrams.clear();
        return result;
    }
 
    void PacedSender::run()
    {
        std::unique_lock lock(_mutex);
        while (true)
        {
            if (_queue.empty())
            {
                if (_stopping)
                {
                    return;
                }
                _cv.wait(lock, [this] { return _stopping || !_queue.empty(); });
                continue;
            }
 
            const auto due = dueTime(_queue.front());
            waitUntilDue(lock, due);
 
            auto packet = std::move(_queue.front());
            _queue.pop_front();
            _send_in_progress = true;
            const auto now = std::chrono::steady_clock::now();
            lock.unlock();
            sendOneUnlocked(std::move(packet), now);
            lock.lock();
            _send_in_progress = false;
            _cv.notify_all();
        }
    }
 
    void PacedSender::waitUntilDue(std::unique_lock<std::mutex>& lock, const std::chrono::steady_clock::time_point due)
    {
        while (true)
        {
            const auto now = std::chrono::steady_clock::now();
            if (now >= due)
            {
                return;
            }
 
            const auto remaining = due - now;
            if (remaining > kCoarsePacingSleep)
            {
                const auto coarse_sleep =
                    std::chrono::duration_cast<std::chrono::steady_clock::duration>(kCoarsePacingSleep);
                const auto fine_spin = std::chrono::duration_cast<std::chrono::steady_clock::duration>(kFinePacingSpin);
                const auto wait_duration = std::min(remaining - fine_spin, coarse_sleep);
                _cv.wait_for(lock, wait_duration);
                continue;
            }
 
            lock.unlock();
            while (std::chrono::steady_clock::now() < due)
            {
                cpuPause();
            }
            lock.lock();
        }
    }
 
    void PacedSender::sendOneUnlocked(SerializedPacket packet, const std::chrono::steady_clock::time_point now)
    {
        const auto due = dueTime(packet);
        try
        {
            _sender->send(packet.bytes);
        }
        catch (...)
        {
            std::scoped_lock const lock(_mutex);
            ++_send_failures[packet.stream_id];
            recordDroppedUnlocked(packet);
            _pending_dropped_datagrams.push_back(makeDroppedDatagram(packet));
            return;
        }
        std::scoped_lock const lock(_mutex);
        ++_sent_packets[packet.stream_id];
        if (now > due + std::chrono::milliseconds(1))
        {
            ++_late_packets[packet.stream_id];
        }
    }
 
    void PacedSender::recordDroppedUnlocked(const SerializedPacket& packet)
    {
        if (packet.data_packet || (!packet.context_packet && packet.sample_count > 0))
        {
            ++_dropped_data_packets[packet.stream_id];
            _dropped_samples[packet.stream_id] += packet.sample_count;
            return;
        }
        if (packet.context_packet)
        {
            ++_dropped_context_packets[packet.stream_id];
        }
    }
 
    DroppedDatagram PacedSender::makeDroppedDatagram(const SerializedPacket& packet) const noexcept
    {
        return DroppedDatagram{.stream_id = packet.stream_id,
                               .sample_count = packet.sample_count,
                               .data_packet = packet.data_packet,
                               .context_packet = packet.context_packet};
    }
 
    std::size_t PacedSender::queuedOrSendingCount() const noexcept
    {
        return _queue.size() + (_send_in_progress ? 1u : 0u);
    }
 
    std::chrono::steady_clock::time_point PacedSender::dueTime(const SerializedPacket& packet) const
    {
        const auto seconds = packet.first_sample_time - _simulation_epoch_time;
        const auto nanos = static_cast<std::int64_t>(seconds * 1'000'000'000.0);
        return _steady_epoch + std::chrono::nanoseconds(nanos);
    }
 
}