path: root/indra/llcommon/workqueue.h

/**
 * @file   workqueue.h
 * @author Nat Goodspeed
 * @date   2021-09-30
 * @brief  Queue used for inter-thread work passing.
 * 
 * $LicenseInfo:firstyear=2021&license=viewerlgpl$
 * Copyright (c) 2021, Linden Research, Inc.
 * $/LicenseInfo$
 */

#if ! defined(LL_WORKQUEUE_H)
#define LL_WORKQUEUE_H

#include "llinstancetracker.h"
#include "threadsafeschedule.h"
#include <chrono>
#include <functional>               // std::function
#include <queue>
#include <string>
#include <utility>                  // std::pair
#include <vector>

namespace LL
{
    /**
     * A typical WorkQueue has a string name that can be used to find it.
     */
    class WorkQueue: public LLInstanceTracker<WorkQueue, std::string>
    {
    private:
        using super = LLInstanceTracker<WorkQueue, std::string>;

    public:
        using Work = std::function<void()>;

    private:
        using Queue = ThreadSafeSchedule<Work>;
        // helper for postEvery()
        template <typename Rep, typename Period, typename CALLABLE>
        class BackJack;

    public:
        using TimePoint = Queue::TimePoint;
        using TimedWork = Queue::TimeTuple;
        using Closed    = Queue::Closed;

        /**
         * You may omit the WorkQueue name, in which case a unique name is
         * synthesized; for practical purposes that makes it anonymous.
         */
        WorkQueue(const std::string& name = std::string());

        /**
         * Since the point of WorkQueue is to pass work to some other worker
         * thread(s) asynchronously, it's important that the WorkQueue continue
         * to exist until the worker thread(s) have drained it. To communicate
         * that it's time for them to quit, close() the queue.
         */
        void close();

        /*---------------------- fire and forget API -----------------------*/

        /// fire-and-forget, but at a particular (future?) time
        template <typename CALLABLE>
        void post(const TimePoint& time, CALLABLE&& callable)
        {
            // Defer reifying an arbitrary CALLABLE until we hit this method.
            // All other methods should accept CALLABLEs of arbitrary type to
            // avoid multiple levels of std::function indirection.
            mQueue.push(TimedWork(time, std::move(callable)));
        }

        /// fire-and-forget
        template <typename CALLABLE>
        void post(CALLABLE&& callable)
        {
            // We use TimePoint::clock::now() instead of TimePoint's
            // representation of the epoch because this WorkQueue may contain
            // a mix of past-due TimedWork items and TimedWork items scheduled
            // for the future. Sift this new item into the correct place.
            post(TimePoint::clock::now(), std::move(callable));
        }

        /**
         * Launch a callable returning bool that will trigger repeatedly at
         * specified interval, until the callable returns false.
         *
         * If you need to signal that callable from outside, DO NOT bind a
         * reference to a simple bool! That's not thread-safe. Instead, bind
         * an LLCond variant, e.g. LLOneShotCond or LLBoolCond.
         */
        template <typename Rep, typename Period, typename CALLABLE>
        void postEvery(const std::chrono::duration<Rep, Period>& interval,
                       CALLABLE&& callable);

        /*------------------------- handshake API --------------------------*/

        /**
         * Post work to another WorkQueue to be run at a specified time,
         * requesting a specific callback to be run on this WorkQueue on
         * completion.
         *
         * Returns true if able to post, false if the other WorkQueue is
         * inaccessible.
         */
        // Apparently some Microsoft header file defines a macro CALLBACK? The
        // natural template argument name CALLBACK produces very weird Visual
        // Studio compile errors that seem utterly unrelated to this source
        // code.
        template <typename CALLABLE, typename FOLLOWUP>
        bool postTo(WorkQueue::weak_t target,
                    const TimePoint& time, CALLABLE&& callable, FOLLOWUP&& callback)
        {
            // We're being asked to post to the WorkQueue at target.
            // target is a weak_ptr: have to lock it to check it.
            auto tptr = target.lock();
            if (! tptr)
                // can't post() if the target WorkQueue has been destroyed
                return false;

            // Here we believe target WorkQueue still exists. Post to it a
            // lambda that packages our callable, our callback and a weak_ptr
            // to this originating WorkQueue.
            tptr->post(
                time,
                [reply = super::getWeak(),
                 callable = std::move(callable),
                 callback = std::move(callback)]
                ()
                {
                    // Call the callable in any case -- but to minimize
                    // copying the result, immediately bind it into a reply
                    // lambda. The reply lambda also binds the original
                    // callback, so that when we, the originating WorkQueue,
                    // finally receive and process the reply lambda, we'll
                    // call the bound callback with the bound result -- on the
                    // same thread that originally called postTo().
                    auto rlambda =
                        [result = callable(),
                         callback = std::move(callback)]
                        ()
                        { callback(std::move(result)); };
                    // Check if this originating WorkQueue still exists.
                    // Remember, the outer lambda is now running on a thread
                    // servicing the target WorkQueue, and real time has
                    // elapsed since postTo()'s tptr->post() call.
                    // reply is a weak_ptr: have to lock it to check it.
                    auto rptr = reply.lock();
                    if (rptr)
                    {
                        // Only post reply lambda if the originating WorkQueue
                        // still exists. If not -- who would we tell? Log it?
                        try
                        {
                            rptr->post(std::move(rlambda));
                        }
                        catch (const Closed&)
                        {
                            // Originating WorkQueue might still exist, but
                            // might be Closed. Same thing: just discard the
                            // callback.
                        }
                    }
                });
            // looks like we were able to post()
            return true;
        }

        /**
         * Post work to another WorkQueue, requesting a specific callback to
         * be run on this WorkQueue on completion.
         *
         * Returns true if able to post, false if the other WorkQueue is
         * inaccessible.
         */
        template <typename CALLABLE, typename FOLLOWUP>
        bool postTo(WorkQueue::weak_t target,
                    CALLABLE&& callable, FOLLOWUP&& callback)
        {
            return postTo(target, TimePoint::clock::now(), std::move(callable), std::move(callback));
        }

        /*--------------------------- worker API ---------------------------*/

        /**
         * runUntilClose() pulls TimedWork items off this WorkQueue until the
         * queue is closed, at which point it returns. This would be the
         * typical entry point for a simple worker thread.
         */
        void runUntilClose();

        /**
         * runPending() runs all TimedWork items that are ready to run. It
         * returns true if the queue remains open, false if the queue has been
         * closed. This could be used by a thread whose primary purpose is to
         * serve the queue, but also wants to do other things with its idle time.
         */
        bool runPending();

        /**
         * runOne() runs at most one ready TimedWork item -- zero if none are
         * ready. It returns true if the queue remains open, false if the
         * queue has been closed.
         */
        bool runOne();

        /**
         * runFor() runs a subset of ready TimedWork items, until the
         * timeslice has been exceeded. It returns true if the queue remains
         * open, false if the queue has been closed. This could be used by a
         * busy main thread to lend a bounded few CPU cycles to this WorkQueue
         * without risking the WorkQueue blowing out the length of any one
         * frame.
         */
        template <typename Rep, typename Period>
        bool runFor(const std::chrono::duration<Rep, Period>& timeslice)
        {
            return runUntil(TimePoint::clock::now() + timeslice);
        }

        /**
         * runUntil() is just like runFor(), only with a specific end time
         * instead of a timeslice duration.
         */
        bool runUntil(const TimePoint& until);

    private:
        static void error(const std::string& msg);
        static std::string makeName(const std::string& name);
        void callWork(const Queue::DataTuple& work);
        void callWork(const Work& work);
        Queue mQueue;
    };

    /**
     * BackJack is, in effect, a hand-rolled lambda, binding a WorkQueue, a
     * CALLABLE that returns bool, a TimePoint and an interval at which to
     * relaunch it. As long as the callable continues returning true, BackJack
     * keeps resubmitting it to the target WorkQueue.
     */
    // Why is BackJack a class and not a lambda? Because, unlike a lambda, a
    // class method gets its own 'this' pointer -- which we need to resubmit
    // the whole BackJack callable.
    template <typename Rep, typename Period, typename CALLABLE>
    class WorkQueue::BackJack
    {
    public:
        // bind the desired data
        BackJack(WorkQueue::weak_t target,
                 const WorkQueue::TimePoint& start,
                 const std::chrono::duration<Rep, Period>& interval,
                 CALLABLE&& callable):
            mTarget(target),
            mStart(start),
            mInterval(interval),
            mCallable(std::move(callable))
        {}

        // Call by target WorkQueue -- note that although WE require a
        // callable returning bool, WorkQueue wants a void callable. We
        // consume the bool.
        void operator()()
        {
            // If mCallable() throws an exception, don't catch it here: if it
            // throws once, it's likely to throw every time, so it's a waste
            // of time to arrange to call it again.
            if (mCallable())
            {
                // Modify mStart to the new start time we desire. If we simply
                // added mInterval to now, we'd get actual timings of
                // (mInterval + slop), where 'slop' is the latency between the
                // previous mStart and the WorkQueue actually calling us.
                // Instead, add mInterval to mStart so that at least we
                // register our intent to fire at exact mIntervals.
                mStart += mInterval;

                // We're being called at this moment by the target WorkQueue.
                // Assume it still exists, rather than checking the result of
                // lock().
                // Resubmit the whole *this callable: that's why we're a class
                // rather than a lambda. Allow moving *this so we can carry a
                // move-only callable; but naturally this statement must be
                // the last time we reference this instance, which may become
                // moved-from.
                try
                {
                    mTarget.lock()->post(mStart, std::move(*this));
                }
                catch (const Closed&)
                {
                    // Once this queue is closed, oh well, just stop
                }
            }
        }

    private:
        WorkQueue::weak_t mTarget;
        WorkQueue::TimePoint mStart;
        std::chrono::duration<Rep, Period> mInterval;
        CALLABLE mCallable;
    };

    template <typename Rep, typename Period, typename CALLABLE>
    void WorkQueue::postEvery(const std::chrono::duration<Rep, Period>& interval,
                              CALLABLE&& callable)
    {
        if (interval.count() <= 0)
        {
            // It's essential that postEvery() be called with a positive
            // interval, since each call to BackJack posts another instance of
            // itself at (start + interval) and we order by target time. A
            // zero or negative interval would result in that BackJack
            // instance going to the head of the queue every time, immediately
            // ready to run. Effectively that would produce an infinite loop,
            // a denial of service on this WorkQueue.
            error("postEvery(interval) may not be 0");
        }
        // Instantiate and post a suitable BackJack, binding a weak_ptr to
        // self, the current time, the desired interval and the desired
        // callable.
        post(
            BackJack<Rep, Period, CALLABLE>(
                 getWeak(), TimePoint::clock::now(), interval, std::move(callable)));
    }

} // namespace LL

#endif /* ! defined(LL_WORKQUEUE_H) */