scheduler.cpp

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// Copyright (c) 2015-2016 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
#include <scheduler.h>
 
#include <sync.h>
#include <util/time.h>
 
#include <cassert>
#include <chrono>
#include <functional>
#include <utility>
 
CScheduler::CScheduler() = default;
 
CScheduler::~CScheduler() {
    assert(nThreadsServicingQueue == 0);
    if (stopWhenEmpty) {
        assert(taskQueue.empty());
    }
}
 
void CScheduler::serviceQueue() {
    WAIT_LOCK(newTaskMutex, lock);
    ++nThreadsServicingQueue;
 
    // newTaskMutex is locked throughout this loop EXCEPT when the thread is
    // waiting or when the user's function is called.
    while (!shouldStop()) {
        try {
            while (!shouldStop() && taskQueue.empty()) {
                // Wait until there is something to do.
                newTaskScheduled.wait(lock);
            }
 
            // Wait until either there is a new task, or until
            // the time of the first item on the queue:
 
            while (!shouldStop() && !taskQueue.empty()) {
                std::chrono::steady_clock::time_point timeToWaitFor =
                    taskQueue.begin()->first;
                if (newTaskScheduled.wait_until(lock, timeToWaitFor) ==
                    std::cv_status::timeout) {
                    // Exit loop after timeout, it means we reached the time of
                    // the event
                    break;
                }
            }
 
            // If there are multiple threads, the queue can empty while we're
            // waiting (another thread may service the task we were waiting on).
            if (shouldStop() || taskQueue.empty()) {
                continue;
            }
 
            Function f = taskQueue.begin()->second;
            taskQueue.erase(taskQueue.begin());
 
            {
                // Unlock before calling f, so it can reschedule itself or
                // another task without deadlocking:
                REVERSE_LOCK(lock);
                f();
            }
        } catch (...) {
            --nThreadsServicingQueue;
            throw;
        }
    }
    --nThreadsServicingQueue;
    newTaskScheduled.notify_one();
}
 
void CScheduler::schedule(CScheduler::Function f,
                          std::chrono::steady_clock::time_point t) {
    {
        LOCK(newTaskMutex);
        taskQueue.insert(std::make_pair(t, f));
    }
    newTaskScheduled.notify_one();
}
 
void CScheduler::MockForward(std::chrono::seconds delta_seconds) {
    assert(delta_seconds > 0s && delta_seconds <= 1h);
 
    {
        LOCK(newTaskMutex);
 
        // use temp_queue to maintain updated schedule
        std::multimap<std::chrono::steady_clock::time_point, Function>
            temp_queue;
 
        for (const auto &element : taskQueue) {
            temp_queue.emplace_hint(temp_queue.cend(),
                                    element.first - delta_seconds,
                                    element.second);
        }
 
        // point taskQueue to temp_queue
        taskQueue = std::move(temp_queue);
    }
 
    // notify that the taskQueue needs to be processed
    newTaskScheduled.notify_one();
}
 
static void Repeat(CScheduler &s, CScheduler::Predicate p,
                   std::chrono::milliseconds delta) {
    if (p()) {
        s.scheduleFromNow([=, &s] { Repeat(s, p, delta); }, delta);
    }
}
 
void CScheduler::scheduleEvery(CScheduler::Predicate p,
                               std::chrono::milliseconds delta) {
    scheduleFromNow([this, p, delta] { Repeat(*this, p, delta); }, delta);
}
 
size_t
CScheduler::getQueueInfo(std::chrono::steady_clock::time_point &first,
                         std::chrono::steady_clock::time_point &last) const {
    LOCK(newTaskMutex);
    size_t result = taskQueue.size();
    if (!taskQueue.empty()) {
        first = taskQueue.begin()->first;
        last = taskQueue.rbegin()->first;
    }
    return result;
}
 
bool CScheduler::AreThreadsServicingQueue() const {
    LOCK(newTaskMutex);
    return nThreadsServicingQueue;
}
 
void SingleThreadedSchedulerClient::MaybeScheduleProcessQueue() {
    {
        LOCK(m_callbacks_mutex);
        // Try to avoid scheduling too many copies here, but if we
        // accidentally have two ProcessQueue's scheduled at once its
        // not a big deal.
        if (m_are_callbacks_running) {
            return;
        }
        if (m_callbacks_pending.empty()) {
            return;
        }
    }
    m_scheduler.schedule([this] { this->ProcessQueue(); },
                         std::chrono::steady_clock::now());
}
 
void SingleThreadedSchedulerClient::ProcessQueue() {
    std::function<void()> callback;
    {
        LOCK(m_callbacks_mutex);
        if (m_are_callbacks_running) {
            return;
        }
        if (m_callbacks_pending.empty()) {
            return;
        }
        m_are_callbacks_running = true;
 
        callback = std::move(m_callbacks_pending.front());
        m_callbacks_pending.pop_front();
    }
 
    // RAII the setting of fCallbacksRunning and calling
    // MaybeScheduleProcessQueue to ensure both happen safely even if callback()
    // throws.
    struct RAIICallbacksRunning {
        SingleThreadedSchedulerClient *instance;
        explicit RAIICallbacksRunning(SingleThreadedSchedulerClient *_instance)
            : instance(_instance) {}
        ~RAIICallbacksRunning() {
            {
                LOCK(instance->m_callbacks_mutex);
                instance->m_are_callbacks_running = false;
            }
            instance->MaybeScheduleProcessQueue();
        }
    } raiicallbacksrunning(this);
 
    callback();
}
 
void SingleThreadedSchedulerClient::AddToProcessQueue(
    std::function<void()> func) {
    {
        LOCK(m_callbacks_mutex);
        m_callbacks_pending.emplace_back(std::move(func));
    }
    MaybeScheduleProcessQueue();
}
 
void SingleThreadedSchedulerClient::EmptyQueue() {
    assert(!m_scheduler.AreThreadsServicingQueue());
    bool should_continue = true;
    while (should_continue) {
        ProcessQueue();
        LOCK(m_callbacks_mutex);
        should_continue = !m_callbacks_pending.empty();
    }
}
 
size_t SingleThreadedSchedulerClient::CallbacksPending() {
    LOCK(m_callbacks_mutex);
    return m_callbacks_pending.size();
}