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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 <memory>
#include <utility>
#include <absl/container/flat_hash_map.h>
#include "mongo/platform/mutex.h"
#include "mongo/stdx/mutex.h"
namespace mongo {
/**
* A Read-Copy-Update map where multiple threads can access and modify the map concurrently. All
* Value's of the map will be shared across accessing threads and copies of the
* ConcurrentSharedValuesMap, so thread-safe methods must be used in order to avoid issues.
*
* Whenever the map is modified, it will perform a copy of the current map with the modification and
* redirect new readers to the new map. The old map version will be freed from memory once all
* references to it are gone. This extends to the values returned by a find operation.
*/
template <typename Key, typename Value, typename... ExtraAbslArgs>
class ConcurrentSharedValuesMap {
public:
using Map = absl::flat_hash_map<Key, std::shared_ptr<Value>, ExtraAbslArgs...>;
ConcurrentSharedValuesMap() : _map(std::make_shared<Map>()) {}
/**
* Constructs a copy of the map. The map will point to the same values of the original map.
*/
ConcurrentSharedValuesMap(const ConcurrentSharedValuesMap& other)
: _map(atomic_load(&other._map)) {}
/**
* Operations other than creating a full new copy of the map can be sources of potential
* concurrency issues. Disallow operator= and the move constructor to avoid misuse.
*/
ConcurrentSharedValuesMap(ConcurrentSharedValuesMap&& other) = delete;
ConcurrentSharedValuesMap& operator=(const ConcurrentSharedValuesMap& other) = delete;
ConcurrentSharedValuesMap& operator=(ConcurrentSharedValuesMap&& other) = delete;
/**
* Returns a shared_ptr of the value associated with the given key.
*
* This is a lock-free operation and will not block under any circumstances.
*/
std::shared_ptr<Value> find(const Key& key) const {
auto currentMap = atomic_load(&_map);
if (auto it = currentMap->find(key); it != currentMap->end()) {
return it->second;
}
return nullptr;
}
/**
* Performs an atomic find or insert operation. The returned value will contain the newly
* created element or the previously existing one.
*
* Existing copies of the map or values previously obtained via find() will not be invalidated
* by the insertion.
*
* This is a blocking operation as all writes are serialized.
*/
template <typename... Args>
std::shared_ptr<Value> getOrEmplace(const Key& key, Args&&... args) {
stdx::lock_guard lk(_mapModificationMutex);
auto currentMap = atomic_load(&_map);
// Check in case it already exists.
if (auto it = currentMap->find(key); it != currentMap->end()) {
return it->second;
}
auto mapCopy = std::make_shared<Map>(*currentMap);
auto newValue = std::make_shared<Value>(std::forward<Args>(args)...);
auto [it, _] = mapCopy->emplace(key, std::move(newValue));
atomic_store(&_map, std::move(mapCopy));
return it->second;
}
/**
* Erases the key from the map. Existing copies of the map or values previously obtained via
* find() will not be invalidated.
*
* This is a blocking operation as all writes are serialized.
*/
void erase(const Key& key) {
stdx::lock_guard lk(_mapModificationMutex);
auto currentMap = atomic_load(&_map);
auto mapCopy = std::make_shared<Map>(*currentMap);
mapCopy->erase(key);
atomic_store(&_map, std::move(mapCopy));
}
/**
* Clears the map, erasing all entries. Existing copies of the map or values previously obtained
* via find() will not be invalidated.
*
* This is a blocking operation as all writes are serialized.
*/
void clear() {
stdx::lock_guard lk(_mapModificationMutex);
auto emptyMap = std::make_shared<Map>();
atomic_store(&_map, std::move(emptyMap));
}
/**
* Updates the map atomically, this method accepts a function that will return the new map to
* use. Existing copies of the map or values previously obtained via find() will not be
* invalidated.
*
* This is a blocking operation as all writes are serialized. As the function can be expensive
* to execute, care must be taken in order to minimize its cost. Otherwise it risks blocking
* writes for longer than desired.
*/
template <typename F>
void updateWith(F&& updateFunc) {
static_assert(std::is_invocable_r_v<Map, F, const Map&>,
"Function must be of type Map(const Map&)");
stdx::lock_guard lk(_mapModificationMutex);
auto currentMap = atomic_load(&_map);
auto newMap = std::make_shared<Map>(std::forward<F>(updateFunc)(*currentMap));
atomic_store(&_map, std::move(newMap));
}
/**
* Returns the current map. This map will not observe changes to it performed concurrently. That
* is to say that the map won't see changes to the key-value pairings performed by other
* threads.
*/
std::shared_ptr<const Map> getUnderlyingSnapshot() const {
return atomic_load(&_map);
}
private:
ConcurrentSharedValuesMap(std::shared_ptr<Map> otherMap) : _map(std::move(otherMap)){};
// shared_ptr in order to allow lock-free reads of the values.
std::shared_ptr<Map> _map;
// Locked in order to modify the map by either inserting or removing an element.
Mutex _mapModificationMutex;
};
} // namespace mongo
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