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/**
* Copyright (C) 2018-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
#if defined(__linux__)
#include <semaphore.h>
#endif
#include <queue>
#include "mongo/db/operation_context.h"
#include "mongo/platform/mutex.h"
#include "mongo/stdx/condition_variable.h"
#include "mongo/stdx/future.h"
#include "mongo/util/concurrency/mutex.h"
#include "mongo/util/hierarchical_acquisition.h"
#include "mongo/util/time_support.h"
namespace mongo {
class TicketHolder {
public:
virtual ~TicketHolder() = 0;
/**
* Attempts to acquire a ticket without blocking.
* Returns a boolean indicating whether the operation was successful or not.
*/
virtual bool tryAcquire() = 0;
/**
* Attempts to acquire a ticket. Blocks until a ticket is acquired or the OperationContext
* 'opCtx' is killed, throwing an AssertionException.
* If 'opCtx' is not provided or equal to nullptr, the wait is not interruptible.
*/
virtual void waitForTicket(OperationContext* opCtx) = 0;
void waitForTicket() {
this->waitForTicket(nullptr);
};
/**
* Attempts to acquire a ticket within a deadline, 'until'. Returns 'true' if a ticket is
* acquired and 'false' if the deadline is reached, but the operation is retryable. Throws an
* AssertionException if the OperationContext 'opCtx' is killed and no waits for tickets can
* proceed.
* If 'opCtx' is not provided or equal to nullptr, the wait is not interruptible.
*/
virtual bool waitForTicketUntil(OperationContext* opCtx, Date_t until) = 0;
bool waitForTicketUntil(Date_t until) {
return this->waitForTicketUntil(nullptr, until);
};
virtual void release() = 0;
virtual Status resize(int newSize) = 0;
virtual int available() const = 0;
virtual int used() const = 0;
virtual int outof() const = 0;
};
class SemaphoreTicketHolder final : public TicketHolder {
public:
explicit SemaphoreTicketHolder(int num);
~SemaphoreTicketHolder() override final;
bool tryAcquire() override final;
void waitForTicket(OperationContext* opCtx) override final;
bool waitForTicketUntil(OperationContext* opCtx, Date_t until) override final;
void release() override final;
Status resize(int newSize) override final;
int available() const override final;
int used() const override final;
int outof() const override final;
private:
#if defined(__linux__)
mutable sem_t _sem;
// You can read _outof without a lock, but have to hold _resizeMutex to change.
AtomicWord<int> _outof;
Mutex _resizeMutex =
MONGO_MAKE_LATCH(HierarchicalAcquisitionLevel(0), "SemaphoreTicketHolder::_resizeMutex");
#else
bool _tryAcquire();
AtomicWord<int> _outof;
int _num;
Mutex _mutex =
MONGO_MAKE_LATCH(HierarchicalAcquisitionLevel(0), "SemaphoreTicketHolder::_mutex");
stdx::condition_variable _newTicket;
#endif
};
/**
* A ticketholder implementation that uses a queue for pending operations.
* Any change to the implementation should be paired with a change to the _ticketholder.tla_ file in
* order to formally verify that the changes won't lead to a deadlock.
*/
class FifoTicketHolder final : public TicketHolder {
public:
explicit FifoTicketHolder(int num);
~FifoTicketHolder() override final;
bool tryAcquire() override final;
void waitForTicket(OperationContext* opCtx) override final;
bool waitForTicketUntil(OperationContext* opCtx, Date_t until) override final;
void release() override final;
Status resize(int newSize) override final;
int available() const override final;
int used() const override final;
int outof() const override final;
private:
void _release(WithLock);
Mutex _resizeMutex =
MONGO_MAKE_LATCH(HierarchicalAcquisitionLevel(2), "FifoTicketHolder::_resizeMutex");
AtomicWord<int> _capacity;
enum class WaitingState { Waiting, Cancelled, Assigned };
struct WaitingElement {
stdx::condition_variable signaler;
Mutex modificationMutex = MONGO_MAKE_LATCH(
HierarchicalAcquisitionLevel(0), "FifoTicketHolder::WaitingElement::modificationMutex");
WaitingState state;
};
std::queue<std::shared_ptr<WaitingElement>> _queue;
// _queueMutex protects all modifications made to either the _queue, or the statistics of the
// queue.
Mutex _queueMutex =
MONGO_MAKE_LATCH(HierarchicalAcquisitionLevel(1), "FifoTicketHolder::_queueMutex");
AtomicWord<int> _enqueuedElements;
AtomicWord<int> _ticketsAvailable;
};
class ScopedTicket {
public:
ScopedTicket(TicketHolder* holder) : _holder(holder) {
_holder->waitForTicket();
}
~ScopedTicket() {
_holder->release();
}
private:
TicketHolder* _holder;
};
class TicketHolderReleaser {
TicketHolderReleaser(const TicketHolderReleaser&) = delete;
TicketHolderReleaser& operator=(const TicketHolderReleaser&) = delete;
public:
TicketHolderReleaser() {
_holder = nullptr;
}
explicit TicketHolderReleaser(TicketHolder* holder) {
_holder = holder;
}
~TicketHolderReleaser() {
if (_holder) {
_holder->release();
}
}
bool hasTicket() const {
return _holder != nullptr;
}
void reset(TicketHolder* holder = nullptr) {
if (_holder) {
_holder->release();
}
_holder = holder;
}
private:
TicketHolder* _holder;
};
} // namespace mongo
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