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/*
* Copyright (C) 2021 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 <mutex>
#include <optional>
#include <queue>
#include <thread>
#include "Callable.h"
namespace aidl::android::hardware::biometrics::fingerprint {
// A class that encapsulates a worker thread and a task queue, and provides a convenient interface
// for a Session to schedule its tasks for asynchronous execution.
class WorkerThread final {
public:
// Internally creates a queue that cannot exceed maxQueueSize elements and a new thread that
// polls the queue for tasks until this instance is destructed.
explicit WorkerThread(size_t maxQueueSize);
// Unblocks the internal queue and calls join on the internal thread allowing it to gracefully
// exit.
~WorkerThread();
// Disallow copying this class.
WorkerThread(const WorkerThread&) = delete;
WorkerThread& operator=(const WorkerThread&) = delete;
// Also disable moving this class to simplify implementation.
WorkerThread(WorkerThread&&) = delete;
WorkerThread& operator=(WorkerThread&&) = delete;
// If the internal queue is not full, pushes a task at the end of the queue and returns true.
// Otherwise, returns false. If the queue is busy, blocks until it becomes available.
// This method expects heap-allocated tasks because it's the simplest way to represent function
// objects of any type. Stack-allocated std::function could be used instead, but it cannot
// represent functions with move-only captures because std::function is inherently copyable.
// Not being able to pass move-only lambdas is a major limitation for the HAL implementation,
// so heap-allocated tasks that share a common interface (Callable) were chosen instead.
bool schedule(std::unique_ptr<Callable> task);
private:
// The function that runs on the internal thread. Sequentially runs the available tasks from
// the queue. If the queue is empty, waits until a new task is added. If the worker is being
// destructed, finishes its current task and gracefully exits.
void threadFunc();
// The maximum size that the queue is allowed to expand to.
size_t mMaxSize;
// Whether the destructor was called. If true, tells threadFunc to exit as soon as possible, and
// tells schedule to avoid doing any work.
std::atomic<bool> mIsDestructing;
// Queue that's guarded by mQueueMutex and mQueueCond.
std::deque<std::unique_ptr<Callable>> mQueue;
std::mutex mQueueMutex;
std::condition_variable mQueueCond;
// The internal thread that works on the tasks from the queue.
std::thread mThread;
};
} // namespace aidl::android::hardware::biometrics::fingerprint
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