201 lines
8.4 KiB
C++
201 lines
8.4 KiB
C++
/*
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* Copyright (C) 2016-2020 Apple Inc. All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
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* EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR
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* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
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* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
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* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
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* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
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* OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/
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#pragma once
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#include <wtf/Box.h>
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#include <wtf/Condition.h>
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#include <wtf/Lock.h>
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#include <wtf/ThreadSafeRefCounted.h>
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#include <wtf/Threading.h>
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#include <wtf/Vector.h>
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namespace WTF {
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// Often, we create threads that have this as their body:
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//
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// for (;;) {
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// {
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// Locker locker { m_lock };
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// for (;;) {
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// [1] stuff that could break, return, or fall through;
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// m_condition.wait(m_lock);
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// }
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// }
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//
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// [2] do work;
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// }
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//
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// When we do this, we don't always do a good job of managing this thread's lifetime, which may lead
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// to this thread sitting around even when it is not needed.
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//
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// AutomaticThread is here to help you in these situations. It encapsulates a lock, a condition
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// variable, and a thread. It will automatically shut the thread down after a timeout of inactivity.
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// You use AutomaticThread by subclassing it, and put any state that is needed between [1] and [2]
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// in the subclass.
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//
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// The terminology we use is:
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//
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// [1] PollResult AutomaticThread::poll()
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// [2] WorkResult AutomaticThread::work()
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//
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// Note that poll() and work() may not be called on the same thread every time, since this will shut
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// down the thread as necessary. This is legal since m_condition.wait(m_lock) can drop the lock, and
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// so there is no reason to keep the thread around.
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class AutomaticThread;
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class AutomaticThreadCondition : public ThreadSafeRefCounted<AutomaticThreadCondition> {
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public:
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static WTF_EXPORT_PRIVATE Ref<AutomaticThreadCondition> create();
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WTF_EXPORT_PRIVATE ~AutomaticThreadCondition();
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WTF_EXPORT_PRIVATE void notifyOne(const AbstractLocker&);
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WTF_EXPORT_PRIVATE void notifyAll(const AbstractLocker&);
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// You can reuse this condition for other things, just as you would any other condition.
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// However, since conflating conditions could lead to thundering herd, it's best to avoid it.
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// One known-good case for one-true-condition is when the communication involves just two
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// threads. In such cases, the thread doing the notifyAll() can wake up at most one thread -
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// its partner.
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WTF_EXPORT_PRIVATE void wait(Lock& lock) WTF_REQUIRES_LOCK(lock);
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WTF_EXPORT_PRIVATE bool waitFor(Lock& lock, Seconds) WTF_REQUIRES_LOCK(lock);
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private:
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friend class AutomaticThread;
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WTF_EXPORT_PRIVATE AutomaticThreadCondition();
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void add(const AbstractLocker&, AutomaticThread*);
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void remove(const AbstractLocker&, AutomaticThread*);
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bool contains(const AbstractLocker&, AutomaticThread*);
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Condition m_condition;
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Vector<AutomaticThread*> m_threads;
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};
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class WTF_EXPORT_PRIVATE AutomaticThread : public ThreadSafeRefCounted<AutomaticThread> {
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public:
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// Note that if you drop all of your references to an AutomaticThread then as soon as there is a
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// timeout during which it doesn't get woken up, it will simply die on its own. This is a
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// permanent kind of death where the AutomaticThread object goes away, rather than the temporary
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// kind of death where AutomaticThread lives but its underlying thread dies. All you have to do
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// to prevent permanent death is keep a ref to AutomaticThread. At time of writing, every user of
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// AutomaticThread keeps a ref to it and does join() as part of the shutdown process, so only the
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// temporary kind of automatic death happens in practice. We keep the permanent death feature
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// because it leads to an easy-to-understand reference counting discipline (AutomaticThread holds
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// strong ref to AutomaticThreadCondition and the underlying thread holds a strong ref to
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// AutomaticThread).
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virtual ~AutomaticThread();
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// Sometimes it's possible to optimize for the case that there is no underlying thread.
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bool hasUnderlyingThread(const AbstractLocker&) const { return m_hasUnderlyingThread; }
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// This attempts to quickly stop the thread. This will succeed if the thread happens to not be
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// running. Returns true if the thread has been stopped. A good idiom for stopping your automatic
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// thread is to first try this, and if that doesn't work, to tell the thread using your own
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// mechanism (set some flag and then notify the condition).
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bool tryStop(const AbstractLocker&);
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bool isWaiting(const AbstractLocker&);
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bool notify(const AbstractLocker&);
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void join();
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virtual const char* name() const { return "WTF::AutomaticThread"; }
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protected:
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// This logically creates the thread, but in reality the thread won't be created until someone
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// calls AutomaticThreadCondition::notifyOne() or notifyAll().
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AutomaticThread(const AbstractLocker&, Box<Lock>, Ref<AutomaticThreadCondition>&&, Seconds timeout = 10_s);
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AutomaticThread(const AbstractLocker&, Box<Lock>, Ref<AutomaticThreadCondition>&&, ThreadType, Seconds timeout = 10_s);
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// To understand PollResult and WorkResult, imagine that poll() and work() are being called like
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// so:
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//
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// void AutomaticThread::runThread()
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// {
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// for (;;) {
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// {
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// Locker locker { m_lock };
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// for (;;) {
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// PollResult result = poll();
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// if (result == PollResult::Work)
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// break;
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// if (result == PollResult::Stop)
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// return;
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// RELEASE_ASSERT(result == PollResult::Wait);
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// m_condition.wait(m_lock);
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// }
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// }
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//
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// WorkResult result = work();
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// if (result == WorkResult::Stop)
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// return;
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// RELEASE_ASSERT(result == WorkResult::Continue);
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// }
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// }
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enum class PollResult { Work, Stop, Wait };
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virtual PollResult poll(const AbstractLocker&) = 0;
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enum class WorkResult { Continue, Stop };
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virtual WorkResult work() = 0;
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// It's sometimes useful to allocate resources while the thread is running, and to destroy them
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// when the thread dies. These methods let you do this. You can override these methods, and you
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// can be sure that the default ones don't do anything (so you don't need a super call).
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virtual void threadDidStart();
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virtual void threadIsStopping(const AbstractLocker&);
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// Control whether this automatic thread should sleep when timeout happens.
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// By overriding this function, we can customize how automatic threads will sleep.
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// For example, when you have thread pool, you can decrease active threads moderately.
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virtual bool shouldSleep(const AbstractLocker&) { return true; }
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private:
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friend class AutomaticThreadCondition;
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void start(const AbstractLocker&);
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Box<Lock> m_lock;
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Ref<AutomaticThreadCondition> m_condition;
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Seconds m_timeout;
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ThreadType m_threadType { ThreadType::Unknown };
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bool m_isRunning { true };
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bool m_isWaiting { false };
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bool m_hasUnderlyingThread { false };
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Condition m_waitCondition;
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Condition m_isRunningCondition;
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};
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} // namespace WTF
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using WTF::AutomaticThread;
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using WTF::AutomaticThreadCondition;
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