125 lines
5.6 KiB
C++
125 lines
5.6 KiB
C++
/**************************************************************************/
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/* semaphore.h */
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/**************************************************************************/
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/* This file is part of: */
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/* GODOT ENGINE */
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/* https://godotengine.org */
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/**************************************************************************/
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/* Copyright (c) 2014-present Godot Engine contributors (see AUTHORS.md). */
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/* Copyright (c) 2007-2014 Juan Linietsky, Ariel Manzur. */
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/* */
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/* Permission is hereby granted, free of charge, to any person obtaining */
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/* a copy of this software and associated documentation files (the */
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/* "Software"), to deal in the Software without restriction, including */
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/* without limitation the rights to use, copy, modify, merge, publish, */
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/* distribute, sublicense, and/or sell copies of the Software, and to */
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/* permit persons to whom the Software is furnished to do so, subject to */
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/* the following conditions: */
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/* */
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/* The above copyright notice and this permission notice shall be */
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/* included in all copies or substantial portions of the Software. */
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/* */
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/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, */
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/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF */
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/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. */
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/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY */
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/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, */
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/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE */
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/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. */
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/**************************************************************************/
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#ifndef SEMAPHORE_H
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#define SEMAPHORE_H
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#include "core/error/error_list.h"
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#include "core/typedefs.h"
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#ifdef DEBUG_ENABLED
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#include "core/error/error_macros.h"
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#endif
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#include <condition_variable>
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#include <mutex>
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class Semaphore {
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private:
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mutable std::mutex mutex;
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mutable std::condition_variable condition;
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mutable uint32_t count = 0; // Initialized as locked.
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#ifdef DEBUG_ENABLED
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mutable uint32_t awaiters = 0;
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#endif
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public:
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_ALWAYS_INLINE_ void post() const {
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std::lock_guard lock(mutex);
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count++;
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condition.notify_one();
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}
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_ALWAYS_INLINE_ void wait() const {
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std::unique_lock lock(mutex);
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#ifdef DEBUG_ENABLED
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++awaiters;
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#endif
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while (!count) { // Handle spurious wake-ups.
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condition.wait(lock);
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}
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--count;
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#ifdef DEBUG_ENABLED
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--awaiters;
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#endif
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}
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_ALWAYS_INLINE_ bool try_wait() const {
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std::lock_guard lock(mutex);
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if (count) {
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count--;
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return true;
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} else {
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return false;
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}
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}
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#ifdef DEBUG_ENABLED
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~Semaphore() {
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// Destroying an std::condition_variable when not all threads waiting on it have been notified
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// invokes undefined behavior (e.g., it may be nicely destroyed or it may be awaited forever.)
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// That means other threads could still be running the body of std::condition_variable::wait()
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// but already past the safety checkpoint. That's the case for instance if that function is already
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// waiting to lock again.
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//
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// We will make the rule a bit more restrictive and simpler to understand at the same time: there
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// should not be any threads at any stage of the waiting by the time the semaphore is destroyed.
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//
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// We do so because of the following reasons:
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// - We have the guideline that threads must be awaited (i.e., completed), so the waiting thread
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// must be completely done by the time the thread controlling it finally destroys the semaphore.
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// Therefore, only a coding mistake could make the program run into such a attempt at premature
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// destruction of the semaphore.
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// - In scripting, given that Semaphores are wrapped by RefCounted classes, in general it can't
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// happen that a thread is trying to destroy a Semaphore while another is still doing whatever with
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// it, so the simplification is mostly transparent to script writers.
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// - The redefined rule can be checked for failure to meet it, which is what this implementation does.
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// This is useful to detect a few cases of potential misuse; namely:
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// a) In scripting:
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// * The coder is naughtily dealing with the reference count causing a semaphore to die prematurely.
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// * The coder is letting the project reach its termination without having cleanly finished threads
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// that await on semaphores (or at least, let the usual semaphore-controlled loop exit).
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// b) In the native side, where Semaphore is not a ref-counted beast and certain coding mistakes can
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// lead to its premature destruction as well.
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//
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// Let's let users know they are doing it wrong, but apply a, somewhat hacky, countermeasure against UB
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// in debug builds.
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std::lock_guard lock(mutex);
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if (awaiters) {
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WARN_PRINT(
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"A Semaphore object is being destroyed while one or more threads are still waiting on it.\n"
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"Please call post() on it as necessary to prevent such a situation and so ensure correct cleanup.");
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// And now, the hacky countermeasure (i.e., leak the condition variable).
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new (&condition) std::condition_variable();
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}
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}
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#endif
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};
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#endif // SEMAPHORE_H
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