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/*
* Copyright (C) 2019 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#pragma once
#include <condition_variable>
#include <functional>
#include <list>
#include <mutex>
#include <optional>
#include "android-base/thread_annotations.h"
namespace android {
/**
* A thread-safe FIFO queue. This list-backed queue stores up to <i>capacity</i> objects if
* a capacity is provided at construction, and is otherwise unbounded.
* Objects can always be added. Objects are added immediately.
* If the queue is full, new objects cannot be added.
*
* The action of retrieving an object will block until an element is available.
*/
template <class T>
class BlockingQueue {
public:
explicit BlockingQueue() = default;
explicit BlockingQueue(size_t capacity) : mCapacity(capacity){};
/**
* Retrieve and remove the oldest object.
* Blocks execution indefinitely while queue is empty.
*/
T pop() {
std::unique_lock lock(mLock);
android::base::ScopedLockAssertion assumeLock(mLock);
mHasElements.wait(lock, [this]() REQUIRES(mLock) { return !this->mQueue.empty(); });
T t = std::move(mQueue.front());
mQueue.erase(mQueue.begin());
return t;
};
/**
* Retrieve and remove the oldest object.
* Blocks execution for the given duration while queue is empty, and returns std::nullopt
* if the queue was empty for the entire duration.
*/
std::optional<T> popWithTimeout(std::chrono::nanoseconds duration) {
std::unique_lock lock(mLock);
android::base::ScopedLockAssertion assumeLock(mLock);
if (!mHasElements.wait_for(lock, duration,
[this]() REQUIRES(mLock) { return !this->mQueue.empty(); })) {
return {};
}
T t = std::move(mQueue.front());
mQueue.erase(mQueue.begin());
return t;
};
/**
* Add a new object to the queue.
* Does not block.
* Return true if an element was successfully added.
* Return false if the queue is full.
*/
bool push(T&& t) {
{ // acquire lock
std::scoped_lock lock(mLock);
if (mCapacity && mQueue.size() == mCapacity) {
return false;
}
mQueue.push_back(std::move(t));
} // release lock
mHasElements.notify_one();
return true;
};
/**
* Construct a new object into the queue.
* Does not block.
* Return true if an element was successfully added.
* Return false if the queue is full.
*/
template <class... Args>
bool emplace(Args&&... args) {
{ // acquire lock
std::scoped_lock lock(mLock);
if (mCapacity && mQueue.size() == mCapacity) {
return false;
}
mQueue.emplace_back(args...);
} // release lock
mHasElements.notify_one();
return true;
};
void erase_if(const std::function<bool(const T&)>& pred) {
std::scoped_lock lock(mLock);
std::erase_if(mQueue, pred);
}
/**
* Remove all elements.
* Does not block.
*/
void clear() {
std::scoped_lock lock(mLock);
mQueue.clear();
};
/**
* How many elements are currently stored in the queue.
* Primary used for debugging.
* Does not block.
*/
size_t size() {
std::scoped_lock lock(mLock);
return mQueue.size();
}
private:
const std::optional<size_t> mCapacity;
/**
* Used to signal that mQueue is non-empty.
*/
std::condition_variable mHasElements;
/**
* Lock for accessing and waiting on elements.
*/
std::mutex mLock;
std::list<T> mQueue GUARDED_BY(mLock);
};
} // namespace android
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