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/**
* Copyright (C) 2023-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <list>
#include "mongo/platform/atomic_word.h"
#include "mongo/util/concurrency/admission_context.h"
#include "mongo/util/time_support.h"
namespace mongo {
/**
* A ticket waiter represents an operation that queues when no tickets are available.
*/
struct TicketWaiter {
enum State : uint32_t {
// This is the initial state. May transition to only Acquired or TimedOut.
Waiting = 0,
// A releaser will set the waiter to the Acquired state when a ticket is available. This is
// a terminal state.
Acquired,
// The waiter will transition to this state when it times out. Releasers will not give
// tickets to waiters in the TimedOut state. This is a terminal state.
TimedOut,
};
AtomicWord<uint32_t> futexWord{Waiting};
// Only valid to dereference when in the Waiting state and while holding the queue lock.
AdmissionContext* context{nullptr};
};
/**
* A TicketQueue is an interface that represents a queue of waiters whose ordering is
* implementation-defined.
*/
class TicketQueue {
public:
virtual ~TicketQueue(){};
virtual bool empty() const = 0;
virtual void push(std::shared_ptr<TicketWaiter>) = 0;
virtual std::shared_ptr<TicketWaiter> pop() = 0;
};
/**
* The FifoTicketQueue is a simple FIFO queue where new waiters are placed at the end and the oldest
* waiters are removed first.
*/
class FifoTicketQueue : public TicketQueue {
public:
bool empty() const {
return _queue.empty();
}
void push(std::shared_ptr<TicketWaiter> val) {
_queue.push_back(std::move(val));
}
std::shared_ptr<TicketWaiter> pop() {
auto front = std::move(_queue.front());
_queue.pop_front();
return front;
}
private:
std::list<std::shared_ptr<TicketWaiter>> _queue;
};
/**
* A TicketPool holds tickets and queues waiters in the provided TicketQueue. The TicketPool
* attempts to emulate a semaphore with a custom queueing policy.
*
* All public functions are thread-safe except where explicitly stated otherwise.
*/
class TicketPool {
public:
TicketPool(int numTickets, std::unique_ptr<TicketQueue> queue);
/**
* Attempt to acquire a ticket without blocking. Returns true if a ticket was granted.
*/
bool tryAcquire();
/**
* Acquire a ticket until the provided deadline. Returns false on timeout, true otherwise.
*/
bool acquire(AdmissionContext* admCtx, Date_t deadline);
/**
* Releases a ticket to the pool. Will will wake a waiter, if there are any queued operations.
*/
void release();
/**
* If there are queued operations, releases a ticket and returns true. Otherwise, does nothing
* and returns false.
*/
bool releaseIfWaiters();
/**
* Returns the number of tickets available.
*/
int32_t available() const {
return _available.load();
}
/**
* Returns the number of queued waiters.
*/
int32_t queued() const {
return _queued.load();
}
/*
* Provides direct access to the underlying queue. Callers must ensure they only use thread-safe
* functions.
*/
TicketQueue* getQueue() {
return _waiters.get();
}
private:
/**
* Attempt to give the ticket to a waiter, and otherwise the pool.
*/
void _release();
/**
* Removes the next waiter from the queue. If there are no waiters, adds the ticket to the pool.
* Ensures that no new waiters queue while this is happening.
*/
std::shared_ptr<TicketWaiter> _popWaiterOrAddTicketToPool();
AtomicWord<int32_t> _available;
// This counter is redundant with the _waiters queue length, but provides the releaseIfWaiters()
// a fast-path that avoids taking the queue mutex.
AtomicWord<int32_t> _queued;
// This mutex protects the _waiters queue by preventing items from being added and removed, but
// does not protect the elements of the queue.
Mutex _mutex = MONGO_MAKE_LATCH("TicketPool::_mutex");
std::unique_ptr<TicketQueue> _waiters;
};
} // namespace mongo
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