task_with_result.hpp

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#pragma once
 
/// @file userver/engine/task/task_with_result.hpp
/// @brief @copybrief engine::TaskWithResult
 
#include <memory>
#include <stdexcept>
#include <utility>
 
#include <userver/engine/exception.hpp>
#include <userver/engine/impl/task_context_holder.hpp>
#include <userver/engine/task/task.hpp>
#include <userver/utils/fast_scope_guard.hpp>
#include <userver/utils/impl/wrapped_call.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace engine {
 
/// Asynchronous task with result
///
/// @warning This class supports only a single concurrent awaiter. Use
/// @ref engine::SharedTaskWithResult to await and retrieve the same result from
/// multiple coroutines.
///
/// ## Example usage:
///
/// @snippet core/src/engine/task/task_with_result_test.cpp  Sample TaskWithResult usage
///
/// @see @ref scripts/docs/en/userver/synchronization.md
template <typename T>
class [[nodiscard]] TaskWithResult : public Task {
public:
    /// @brief Default constructor
    ///
    /// Creates an invalid task.
    TaskWithResult() = default;
 
    TaskWithResult(const TaskWithResult&) = delete;
    TaskWithResult& operator=(const TaskWithResult&) = delete;
 
    /// @brief Moves the other task into this, leaving the other in an invalid
    /// state.
    TaskWithResult(TaskWithResult&& other) noexcept = default;
 
    /// @brief If this Task is still valid and is not finished, cancels it and
    /// waits until it finishes before moving the other. Otherwise just moves the
    /// other task into this, leaving the other in invalid state.
    TaskWithResult& operator=(TaskWithResult&& other) noexcept = default;
 
    /// @brief Returns (or rethrows) the result of task invocation.
    /// After return from this method the task is not valid.
    /// @throws WaitInterruptedException when `current_task::IsCancelRequested()`
    /// and no TaskCancellationBlockers are present.
    /// @throws TaskCancelledException
    ///   if no result is available because the task was cancelled
    T Get() noexcept(false) {
        EnsureValid();
 
        Wait();
        const utils::FastScopeGuard invalidate([this]() noexcept { Invalidate(); });
        if (GetState() == State::kCancelled) {
            throw TaskCancelledException(CancellationReason());
        }
 
        return utils::impl::CastWrappedCall<T>(GetPayload()).Retrieve();
    }
 
    /// Satisfies @ref engine::Awaitable, for use with @ref engine::WaitAnyContext and friends.
    using Task::GetAwaitableToken;
 
    /// @cond
    static constexpr WaitMode kWaitMode = WaitMode::kSingleAwaiter;
 
    // For internal use only.
    explicit TaskWithResult(impl::TaskContextHolder&& context)
        : Task(std::move(context))
    {}
    /// @endcond
 
    Task AsTask() && {
        // NOLINTNEXTLINE(cppcoreguidelines-slicing)
        return std::move(*this);
    }
 
private:
    void EnsureValid() const {
        UINVARIANT(
            IsValid(),
            "TaskWithResult::Get was called on an invalid task. Note that "
            "Get invalidates self, so it must be called at most once "
            "per task"
        );
    }
};
 
}  // namespace engine
 
USERVER_NAMESPACE_END