335 lines
13 KiB
C++
335 lines
13 KiB
C++
/**
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* @file workqueue.h
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* @author Nat Goodspeed
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* @date 2021-09-30
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* @brief Queue used for inter-thread work passing.
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*
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* $LicenseInfo:firstyear=2021&license=viewerlgpl$
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* Copyright (c) 2021, Linden Research, Inc.
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* $/LicenseInfo$
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*/
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#if ! defined(LL_WORKQUEUE_H)
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#define LL_WORKQUEUE_H
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#include "llinstancetracker.h"
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#include "threadsafeschedule.h"
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#include <chrono>
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#include <functional> // std::function
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#include <queue>
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#include <string>
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#include <utility> // std::pair
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#include <vector>
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namespace LL
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{
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/**
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* A typical WorkQueue has a string name that can be used to find it.
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*/
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class WorkQueue: public LLInstanceTracker<WorkQueue, std::string>
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{
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private:
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using super = LLInstanceTracker<WorkQueue, std::string>;
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public:
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using Work = std::function<void()>;
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private:
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using Queue = ThreadSafeSchedule<Work>;
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// helper for postEvery()
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template <typename Rep, typename Period, typename CALLABLE>
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class BackJack;
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public:
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using TimePoint = Queue::TimePoint;
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using TimedWork = Queue::TimeTuple;
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using Closed = Queue::Closed;
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/**
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* You may omit the WorkQueue name, in which case a unique name is
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* synthesized; for practical purposes that makes it anonymous.
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*/
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WorkQueue(const std::string& name = std::string());
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/**
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* Since the point of WorkQueue is to pass work to some other worker
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* thread(s) asynchronously, it's important that the WorkQueue continue
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* to exist until the worker thread(s) have drained it. To communicate
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* that it's time for them to quit, close() the queue.
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*/
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void close();
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/// producer end: are we prevented from pushing any additional items?
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bool isClosed();
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/// consumer end: are we done, is the queue entirely drained?
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bool done();
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/*---------------------- fire and forget API -----------------------*/
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/// fire-and-forget, but at a particular (future?) time
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template <typename CALLABLE>
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void post(const TimePoint& time, CALLABLE&& callable)
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{
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// Defer reifying an arbitrary CALLABLE until we hit this method.
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// All other methods should accept CALLABLEs of arbitrary type to
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// avoid multiple levels of std::function indirection.
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mQueue.push(TimedWork(time, std::move(callable)));
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}
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/// fire-and-forget
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template <typename CALLABLE>
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void post(CALLABLE&& callable)
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{
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// We use TimePoint::clock::now() instead of TimePoint's
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// representation of the epoch because this WorkQueue may contain
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// a mix of past-due TimedWork items and TimedWork items scheduled
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// for the future. Sift this new item into the correct place.
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post(TimePoint::clock::now(), std::move(callable));
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}
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/**
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* Launch a callable returning bool that will trigger repeatedly at
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* specified interval, until the callable returns false.
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*
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* If you need to signal that callable from outside, DO NOT bind a
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* reference to a simple bool! That's not thread-safe. Instead, bind
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* an LLCond variant, e.g. LLOneShotCond or LLBoolCond.
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*/
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template <typename Rep, typename Period, typename CALLABLE>
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void postEvery(const std::chrono::duration<Rep, Period>& interval,
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CALLABLE&& callable);
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/*------------------------- handshake API --------------------------*/
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/**
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* Post work to another WorkQueue to be run at a specified time,
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* requesting a specific callback to be run on this WorkQueue on
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* completion.
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*
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* Returns true if able to post, false if the other WorkQueue is
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* inaccessible.
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*/
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// Apparently some Microsoft header file defines a macro CALLBACK? The
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// natural template argument name CALLBACK produces very weird Visual
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// Studio compile errors that seem utterly unrelated to this source
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// code.
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template <typename CALLABLE, typename FOLLOWUP>
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bool postTo(WorkQueue::weak_t target,
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const TimePoint& time, CALLABLE&& callable, FOLLOWUP&& callback)
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{
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// We're being asked to post to the WorkQueue at target.
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// target is a weak_ptr: have to lock it to check it.
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auto tptr = target.lock();
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if (! tptr)
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// can't post() if the target WorkQueue has been destroyed
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return false;
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// Here we believe target WorkQueue still exists. Post to it a
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// lambda that packages our callable, our callback and a weak_ptr
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// to this originating WorkQueue.
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tptr->post(
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time,
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[reply = super::getWeak(),
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callable = std::move(callable),
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callback = std::move(callback)]
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()
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{
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// Call the callable in any case -- but to minimize
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// copying the result, immediately bind it into a reply
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// lambda. The reply lambda also binds the original
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// callback, so that when we, the originating WorkQueue,
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// finally receive and process the reply lambda, we'll
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// call the bound callback with the bound result -- on the
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// same thread that originally called postTo().
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auto rlambda =
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[result = callable(),
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callback = std::move(callback)]
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()
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{ callback(std::move(result)); };
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// Check if this originating WorkQueue still exists.
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// Remember, the outer lambda is now running on a thread
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// servicing the target WorkQueue, and real time has
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// elapsed since postTo()'s tptr->post() call.
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// reply is a weak_ptr: have to lock it to check it.
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auto rptr = reply.lock();
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if (rptr)
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{
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// Only post reply lambda if the originating WorkQueue
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// still exists. If not -- who would we tell? Log it?
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try
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{
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rptr->post(std::move(rlambda));
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}
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catch (const Closed&)
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{
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// Originating WorkQueue might still exist, but
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// might be Closed. Same thing: just discard the
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// callback.
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}
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}
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});
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// looks like we were able to post()
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return true;
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}
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/**
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* Post work to another WorkQueue, requesting a specific callback to
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* be run on this WorkQueue on completion.
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*
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* Returns true if able to post, false if the other WorkQueue is
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* inaccessible.
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*/
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template <typename CALLABLE, typename FOLLOWUP>
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bool postTo(WorkQueue::weak_t target,
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CALLABLE&& callable, FOLLOWUP&& callback)
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{
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return postTo(target, TimePoint::clock::now(), std::move(callable), std::move(callback));
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}
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/*--------------------------- worker API ---------------------------*/
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/**
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* runUntilClose() pulls TimedWork items off this WorkQueue until the
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* queue is closed, at which point it returns. This would be the
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* typical entry point for a simple worker thread.
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*/
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void runUntilClose();
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/**
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* runPending() runs all TimedWork items that are ready to run. It
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* returns true if the queue remains open, false if the queue has been
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* closed. This could be used by a thread whose primary purpose is to
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* serve the queue, but also wants to do other things with its idle time.
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*/
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bool runPending();
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/**
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* runOne() runs at most one ready TimedWork item -- zero if none are
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* ready. It returns true if the queue remains open, false if the
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* queue has been closed.
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*/
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bool runOne();
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/**
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* runFor() runs a subset of ready TimedWork items, until the
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* timeslice has been exceeded. It returns true if the queue remains
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* open, false if the queue has been closed. This could be used by a
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* busy main thread to lend a bounded few CPU cycles to this WorkQueue
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* without risking the WorkQueue blowing out the length of any one
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* frame.
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*/
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template <typename Rep, typename Period>
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bool runFor(const std::chrono::duration<Rep, Period>& timeslice)
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{
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return runUntil(TimePoint::clock::now() + timeslice);
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}
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/**
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* runUntil() is just like runFor(), only with a specific end time
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* instead of a timeslice duration.
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*/
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bool runUntil(const TimePoint& until);
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private:
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static void error(const std::string& msg);
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static std::string makeName(const std::string& name);
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void callWork(const Queue::DataTuple& work);
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void callWork(const Work& work);
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Queue mQueue;
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};
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/**
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* BackJack is, in effect, a hand-rolled lambda, binding a WorkQueue, a
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* CALLABLE that returns bool, a TimePoint and an interval at which to
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* relaunch it. As long as the callable continues returning true, BackJack
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* keeps resubmitting it to the target WorkQueue.
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*/
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// Why is BackJack a class and not a lambda? Because, unlike a lambda, a
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// class method gets its own 'this' pointer -- which we need to resubmit
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// the whole BackJack callable.
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template <typename Rep, typename Period, typename CALLABLE>
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class WorkQueue::BackJack
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{
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public:
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// bind the desired data
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BackJack(WorkQueue::weak_t target,
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const WorkQueue::TimePoint& start,
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const std::chrono::duration<Rep, Period>& interval,
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CALLABLE&& callable):
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mTarget(target),
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mStart(start),
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mInterval(interval),
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mCallable(std::move(callable))
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{}
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// Call by target WorkQueue -- note that although WE require a
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// callable returning bool, WorkQueue wants a void callable. We
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// consume the bool.
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void operator()()
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{
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// If mCallable() throws an exception, don't catch it here: if it
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// throws once, it's likely to throw every time, so it's a waste
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// of time to arrange to call it again.
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if (mCallable())
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{
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// Modify mStart to the new start time we desire. If we simply
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// added mInterval to now, we'd get actual timings of
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// (mInterval + slop), where 'slop' is the latency between the
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// previous mStart and the WorkQueue actually calling us.
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// Instead, add mInterval to mStart so that at least we
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// register our intent to fire at exact mIntervals.
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mStart += mInterval;
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// We're being called at this moment by the target WorkQueue.
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// Assume it still exists, rather than checking the result of
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// lock().
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// Resubmit the whole *this callable: that's why we're a class
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// rather than a lambda. Allow moving *this so we can carry a
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// move-only callable; but naturally this statement must be
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// the last time we reference this instance, which may become
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// moved-from.
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try
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{
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mTarget.lock()->post(mStart, std::move(*this));
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}
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catch (const Closed&)
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{
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// Once this queue is closed, oh well, just stop
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}
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}
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}
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private:
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WorkQueue::weak_t mTarget;
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WorkQueue::TimePoint mStart;
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std::chrono::duration<Rep, Period> mInterval;
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CALLABLE mCallable;
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};
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template <typename Rep, typename Period, typename CALLABLE>
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void WorkQueue::postEvery(const std::chrono::duration<Rep, Period>& interval,
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CALLABLE&& callable)
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{
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if (interval.count() <= 0)
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{
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// It's essential that postEvery() be called with a positive
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// interval, since each call to BackJack posts another instance of
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// itself at (start + interval) and we order by target time. A
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// zero or negative interval would result in that BackJack
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// instance going to the head of the queue every time, immediately
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// ready to run. Effectively that would produce an infinite loop,
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// a denial of service on this WorkQueue.
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error("postEvery(interval) may not be 0");
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}
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// Instantiate and post a suitable BackJack, binding a weak_ptr to
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// self, the current time, the desired interval and the desired
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// callable.
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post(
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BackJack<Rep, Period, CALLABLE>(
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getWeak(), TimePoint::clock::now(), interval, std::move(callable)));
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}
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} // namespace LL
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#endif /* ! defined(LL_WORKQUEUE_H) */
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