ClickHouse/dbms/Common/MultiVersion.h

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#pragma once
#include <mutex>
#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* } // now we finish own current version; if the version is outdated and no one else is using it - it will be destroyed.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;
MultiVersion(std::unique_ptr<const T> && value)
{
set(std::move(value));
}
/// Obtain current version for read-only usage. Returns shared_ptr, that manages lifetime of version.
Version get() const
{
/// NOTE: is it possible to lock-free replace of shared_ptr?
std::lock_guard lock(mutex);
return current_version;
}
/// Update an object with new version.
void set(std::unique_ptr<const T> && value)
{
std::lock_guard lock(mutex);
current_version = std::move(value);
}
private:
Version current_version;
mutable std::mutex mutex;
};