Creating a task completion source for a C++ coroutine: Failing to produce a result

Raymond Chen

Raymond

So far, we’ve been working on building a result_holder that can hold any type of result. But what about errors?

Because maybe you have code that’s waiting for a result, and the code that’s supposed to produce the result realizes that it messed up and wants to say, “Sorry, no result today.”

We can do that by storing a std::exception_ptr as a possible result. This means that our result is no longer merely an optional value, but rather it’s a variant, since it could be empty, or it could have a value, or it could have an exception. It also means that ready needs to become a discriminant.

template<typename T>
struct result_holder_state :
    async_helpers::awaitable_state<result_holder_state<T>>
{
    enum class result_kind {
        unset,
        value,
        exception,
    };

    std::atomic<result_kind> kind{ result_kind::unset };

    struct wrapper
    {
        T value;
    };

    union variant
    {
        variant() {}
        ~variant() {}

        wrapper wrap;
        std::exception_ptr ex;
    } result;

    result_holder_state() {}
    result_holder_state(result_holder_state const&) = delete;
    void operator=(result_holder_state const&) = delete;

    ~result_holder_state()
    {
        switch (kind.load(std::memory_order_relaxed)) {
        case result_kind::value:
            result.wrap.~wrapper();
            break;
        case result_kind::exception:
            result.ex.~exception_ptr();
            break;
        }
    }

    using typename result_holder_state::extra_await_data;
    using typename result_holder_state::node_list;

    bool fast_claim(extra_await_data const&) noexcept
    {
        return kind.load(std::memory_order_acquire)
            != result_kind::unset;
    }

    bool claim(extra_await_data const&) noexcept
    {
        return kind.load(std::memory_order_relaxed)
            != result_kind::unset;
    }

    void set_result(node_list& list, T v)
    {
        if (kind.load(std::memory_order_relaxed)
            == result_kind::unset) {
            new (std::addressof(result.wrap))
                wrapper{ std::forward<T>(v) };
            kind.store(result_kind::value,
                std::memory_order_release);
            this->resume_all(list);
        }
    }

    void set_exception(
        node_list& list, std::exception_ptr ex)
    {
        if (kind.load(std::memory_order_relaxed)
            == result_kind::unset) {
            new (std::addressof(result.ex))
                std::exception_ptr{ std::move(ex) };
            kind.store(result_kind::exception,
                std::memory_order_release);
            this->resume_all(list);
        }
    }

    T get_result()
    {
        switch (kind.load(std::memory_order_relaxed)) {
        case result_kind::value:
            return result.wrap.value;
        case result_kind::exception:
            std::rethrow_exception(result.ex);
        }
        std::terminate(); // shouldn't get here
    }
};

There isn’t much exciting going on here. It’s just changing the things that need to be changed: Instead of a simple bool tracking what is in the result, we use a discriminant which starts out unset. Cleaning up our variant requires us to call the appropriate destructor for the contents of the result, and setting the result requires us to update the discriminant.

Setting an exception is the same as setting a value, except that we put the result in to the ex member instead of the wrap wrapper.

When it comes time to fetch the result, we check what we have. If we have a value, we return it. If we have an exception, we rethrow it. (Otherwise, something went wrong and we terminate.)

The result_holder itself is basically the same, just with an extra method for storing the exception.

template<typename T>
struct result_holder
    : async_helpers::awaitable_sync_object<
        result_holder_state<T>>
{
    using typename result_holder::state;

    void set_result(T result) const noexcept
    {
        this->action_impl(&state::set_result,
            std::move(result));
    }

    void set_exception(std::exception_ptr ex) const noexcept
    {
        this->action_impl(&state::set_exception,
            std::move(ex));
    }
};

The Windows Runtime IAsync­Operation can be awaited only once, but you can use this result_holder to make it possible to await multiple times.

template<typename T>
result_holder<T> MakeMultiAwaitable(IAsyncOperation<T> async)
{
    result_holder<T> holder;
    async.Completed([holder, async](auto, auto status) {
        try {
            switch (status) {
            case AsyncStatus::Completed:
                holder.set_result(async.GetResults());
                break;
            case AsyncStatus::Canceled:
                throw hresult_canceled();
            case AsyncStatus::Error:
                throw_hresult(async.ErrorCode());
            }
        } catch (...) {
            holder.set_exception(std::current_exception());
        }
    });
    return holder;
}

You would use it something like this:

class MyClass
{
    result_holder<Widget> widget_result;

    MyClass()
    {
        // Kick off initialization but don't wait for it.
        widget_result = MakeMultiAwaitable(InitializeAsync());
    }

    IAsyncOperation<Widget> InitializeAsync();

    IAsyncAction DoSomethingAsync()
    {
        // Wait for the widget that InitializeAsync produced.
        // rethrow any exception that occurred during initialization.
        auto widget = co_await widget_result;
        ... do something interesting ...
    }
};

More generally, you would do something like this:

void CalculateResult(result_holder<Widget>& holder)
{
    try
    {
        /* do a bunch of calculations */
        widget = /* the answer */;
        holder.set_result(widget);
    } catch (...) {
        holder.set_exception(std::current_exception());
    }
}

You know what this looks like?

A coroutine!

result_holder<Widget> CalculateResult()
{
    /* do a bunch of calculations */
    widget = /* the answer */;
    co_return widget;

    /* exception is captured into the result_holder */
}

So I guess it’s time to learn how to create our own coroutines. The dive into the deep end begins next time.

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