be/src/common/multi_version.h

// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you 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.
// This file is copied from
// https://github.com/ClickHouse/ClickHouse/blob/master/src/Common/MultiVersion.h
// and modified by Doris

#pragma once

#include <atomic>
#include <memory>

/** Allow to store and read-only usage of an object in several threads,
  *  and to atomically replace an object in another thread.
  * The replacement is atomic and reading threads can work with different versions of an object.
  *
  * Usage:
  *  MultiVersion<T> x;
  * - on data update:
  *  x.set(new value);
  * - on read-only usage:
  * {
  *     MultiVersion<T>::Version current_version = x.get();
  *     // use *current_version
  * }   // now we finish own current version; if the version is outdated and no one else is using it - it will be destroyed.
  *
  * All methods are thread-safe.
  */
template <typename T>
class MultiVersion {
public:
    /// Version of object for usage. shared_ptr manage lifetime of version.
    using Version = std::shared_ptr<const T>;

    /// Default initialization - by nullptr.
    MultiVersion() = default;

    explicit MultiVersion(std::unique_ptr<const T>&& value) : current_version(std::move(value)) {}

    /// Obtain current version for read-only usage. Returns shared_ptr, that manages lifetime of version.
    Version get() const { return std::atomic_load(&current_version); }

    /// TODO: replace atomic_load/store() on shared_ptr (which is deprecated as of C++20) by C++20 std::atomic<std::shared_ptr>.
    /// Clang 15 currently does not support it.

    /// Update an object with new version.
    void set(std::unique_ptr<const T>&& value) {
        std::atomic_store(&current_version, Version {std::move(value)});
    }

private:
    Version current_version;
};