lazy_shared_ptr.hpp

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#pragma once
 
/// @file userver/utils/lazy_shared_ptr.hpp
/// @brief @copybrief utils::LazySharedPtr
 
#include <atomic>
#include <functional>
#include <memory>
#include <utility>
 
#include <userver/engine/sleep.hpp>
#include <userver/utils/assert.hpp>
#include <userver/utils/shared_readable_ptr.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils {
 
/// @ingroup userver_containers
///
/// @brief A lazy wrapper around utils::SharedReadablePtr that fetches the data
/// on first access.
///
/// Provides standard thread-safety guarantee: `const` methods can be called
/// concurrently. Once fetched, the same data is guaranteed to be returned
/// unless the pointer is assigned to.
template <typename T>
class LazySharedPtr final {
public:
    /// @brief The default constructor, initializes with `nullptr`.
    LazySharedPtr() noexcept : value_(nullptr), shared_filled_(true), get_data_() {}
 
    /// @brief The non-lazy constructor
    LazySharedPtr(utils::SharedReadablePtr<T> ptr) noexcept
        : value_(ptr.Get()), shared_filled_(true), shared_(std::move(ptr)) {}
 
    /// @brief The lazy constructor
    LazySharedPtr(std::function<utils::SharedReadablePtr<T>()> function) noexcept : get_data_(std::move(function)) {}
 
    /// @brief The copy constructor.
    /// @note If `other` has not been fetched yet, `this` will not fetch
    /// immediately, so `this` and `other` may end up pointing to different
    /// objects.
    LazySharedPtr(const LazySharedPtr& other)
        : get_data_(other.get_data_)
    {
        if (other.shared_filled_.load()) {
            value_.store(other.shared_.Get());
            shared_ = other.shared_;
            shared_filled_.store(true);
        }
    }
 
    /// @brief The move constructor.
    LazySharedPtr(LazySharedPtr&& other) noexcept
        : value_(other.value_.load()),
          shared_filled_(other.shared_filled_.load()),
          shared_(std::move(other.shared_)),
          get_data_(std::move(other.get_data_)) {}
 
    /// @brief The copy assignment operator.
    /// @note Like copy-constructor, it does not generate pointers if `other` do
    /// not generate them.
    LazySharedPtr& operator=(const LazySharedPtr& other) {
        *this = LazySharedPtr(other);
        return *this;
    }
 
    /// @brief The move assignment operator.
    LazySharedPtr& operator=(LazySharedPtr&& other) noexcept {
        value_.store(other.value_.load(), std::memory_order_relaxed);
        shared_filled_.store(other.shared_filled_.load(), std::memory_order_relaxed);
        shared_ = std::move(other.shared_);
        get_data_ = std::move(other.get_data_);
    }
 
    /// @brief Get a pointer to the data (may be null). Fetches the data on the
    /// first access.
    const T* Get() const& {
        if (const auto* current_value = value_.load(); current_value != kUnfilled) {
            return current_value;
        }
        auto readable = get_data_();
        if (const auto* expected = kUnfilled; value_.compare_exchange_strong(expected, readable.Get())) {
            shared_ = std::move(readable);
            shared_filled_.store(true);
        }
        return value_;
    }
 
    /// @brief Get a smart pointer to the data (may be null). Fetches the data on
    /// the first access.
    const utils::SharedReadablePtr<T>& GetShared() const& {
        if (value_ == kUnfilled) {
            auto readable = get_data_();
            if (const auto* expected = kUnfilled; value_.compare_exchange_strong(expected, readable.Get())) {
                shared_ = std::move(readable);
                shared_filled_.store(true);
            }
        }
        while (!shared_filled_.load()) {
            // Another thread has filled 'value_', but not yet 'shared_'. It will be
            // filled in a few CPU cycles. Discovering LazySharedPtr in such a state
            // is expected to be rare.
            engine::Yield();
        }
        return shared_;
    }
 
    ///@returns `*Get()`
    ///@note `Get()` must not be `nullptr`.
    const T& operator*() const& {
        const auto* res = Get();
        UASSERT(res);
        return *res;
    }
 
    /// @returns `Get()`
    /// @note `Get()` must not be `nullptr`.
    const T* operator->() const& { return &**this; }
 
    /// @returns `Get() != nullptr`
    explicit operator bool() const& { return !!Get(); }
 
    /// @returns `GetShared()`
    operator const utils::SharedReadablePtr<T>&() const& { return GetShared(); }
 
    /// @returns `GetShared()`
    operator std::shared_ptr<const T>() const& { return GetShared(); }
 
private:
    static inline const auto kUnfilled = reinterpret_cast<const T*>(std::uintptr_t{1});
 
    mutable std::atomic<const T*> value_{kUnfilled};
    mutable std::atomic<bool> shared_filled_{false};
    mutable utils::SharedReadablePtr<T> shared_{nullptr};
    std::function<utils::SharedReadablePtr<T>()> get_data_{};
};
 
/// @brief Make a lazy pointer to the data of a cache.
///
/// The cache type must have:
/// - `DataType` member type;
/// - `Get() -> utils::SharedReadablePtr<DataType>` method.
///
/// For example, components::CachingComponentBase satisfies these requirements.
template <typename Cache>
LazySharedPtr<typename Cache::DataType> MakeLazyCachePtr(Cache& cache) {
    return utils::LazySharedPtr<typename Cache::DataType>([&cache] { return cache.Get(); });
}
 
}  // namespace utils
 
USERVER_NAMESPACE_END