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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
#include "mongo/platform/mutex.h"
#include "mongo/util/hierarchical_acquisition.h"
namespace mongo {
template <int level = 0>
struct LeveledSynchronizedValueMutexPolicy {
using mutex_type = Mutex;
static mutex_type construct() {
return MONGO_MAKE_LATCH(HierarchicalAcquisitionLevel(level), "synchronized_value::_mutex");
}
};
struct RawSynchronizedValueMutexPolicy {
using mutex_type = stdx::mutex; // NOLINT
static mutex_type construct() {
return {};
}
};
/**
* Provides mutex guarded access to an object.
*
* The protected object can be accessed by either:
* 1. auto tmp = sv.synchronize()
* This is useful if you need to do multiple operations on it
* 2. operator* or operator->
* Creates a temporary that holds the lock for the duration of the complete expression.
* 3. get() - makes a copy of the object while the lock is held
*
* Inspired by https://isocpp.org/files/papers/n4033.html and boost::synchronized_value
*/
template <typename T, typename MutexPolicy = LeveledSynchronizedValueMutexPolicy<>>
class synchronized_value {
public:
using value_type = T;
using mutex_policy_type = MutexPolicy;
using mutex_type = typename MutexPolicy::mutex_type;
template <typename SV>
class UpdateGuard {
template <typename SV_>
using RequiresMutable = std::enable_if_t<!std::is_const_v<SV_>>;
public:
explicit UpdateGuard(SV& sv) : _sv(sv), _lock(_sv._mutex) {}
// Accessors, always available.
const value_type& operator*() const {
return _sv._value;
}
const value_type* operator->() const {
return &_sv._value;
}
operator value_type() const {
return **this;
}
// Mutators, only available if non-const.
template <typename SV_ = SV, typename = RequiresMutable<SV_>>
value_type& operator*() {
return _sv._value;
}
template <typename SV_ = SV, typename = RequiresMutable<SV_>>
value_type* operator->() {
return &_sv._value;
}
template <typename SV_ = SV, typename = RequiresMutable<SV_>>
UpdateGuard& operator=(const value_type& v) {
_sv._value = v;
return *this;
}
template <typename SV_ = SV, typename = RequiresMutable<SV_>>
UpdateGuard& operator=(value_type&& v) {
_sv._value = std::move(v);
return *this;
}
private:
SV& _sv;
stdx::unique_lock<mutex_type> _lock;
};
synchronized_value() = default;
explicit synchronized_value(value_type value) : _value(std::move(value)) {}
synchronized_value(const synchronized_value&) = delete;
synchronized_value& operator=(const synchronized_value&) = delete;
auto synchronize() const {
return UpdateGuard<std::remove_reference_t<decltype(*this)>>{*this};
}
auto synchronize() {
return UpdateGuard<std::remove_reference_t<decltype(*this)>>{*this};
}
/** Lock and return a holder to the value and lock. Const or non-const. */
auto operator-> () const {
return synchronize();
}
auto operator*() const {
return synchronize();
}
/** Return a copy of the protected object. */
value_type get() const {
return *synchronize();
}
/** Mutators */
auto operator-> () {
return synchronize();
}
auto operator*() {
return synchronize();
}
/** Support assigning from the contained value */
synchronized_value& operator=(const value_type& v) {
*synchronize() = v;
return *this;
}
synchronized_value& operator=(value_type&& v) {
*synchronize() = std::move(v);
return *this;
}
friend bool operator==(const synchronized_value& a, const synchronized_value& b) {
std::scoped_lock lk(a._mutex, b._mutex);
return a._value == b._value;
}
friend bool operator==(const synchronized_value& a, const value_type& b) {
return **a == b;
}
friend bool operator==(const value_type& a, const synchronized_value& b) {
return b == a;
}
friend bool operator!=(const synchronized_value& a, const synchronized_value& b) {
return !(a == b);
}
friend bool operator!=(const synchronized_value& a, const value_type& b) {
return !(a == b);
}
friend bool operator!=(const value_type& a, const synchronized_value& b) {
return !(a == b);
}
private:
value_type _value; ///< guarded by _mutex
mutable mutex_type _mutex = mutex_policy_type::construct();
};
} // namespace mongo
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