Using cppcoro and ASIO's co_spawn together

Question

I've a library is written using cppcoro and wish to use it with ASIO. But whenever I try to co_spawn a coroutine from said library. Boost complain that the awaitable type isn't correct.

For example:

#include <asio/io_context.hpp>
#include <asio/coroutine.hpp>
#include <asio/co_spawn.hpp>
#include <asio/detached.hpp>
#include <asio/awaitable.hpp>
#include <cppcoro/task.hpp>

cppcoro::task<int> foo()
{
        co_return 1;
}

int main()
{
        asio::io_context ctx;
        asio::co_spawn(ctx, foo(), asio::detached);
        ctx.run();
}

Boost complains that cppcoro::task<int> isn't derived from asio::awaitable

asio_cppcoro.cpp: In function ‘int main()’:
asio_cppcoro.cpp:15:43: error: no matching function for call to ‘co_spawn(asio::io_context&, cppcoro::task<int>, const asio::detached_t&)’
   15 |  asio::co_spawn(ctx, foo(), asio::detached);
      |                                           ^
In file included from /usr/include/asio/co_spawn.hpp:467,
                 from asio_cppcoro.cpp:3:
/usr/include/asio/impl/co_spawn.hpp:199:1: note: candidate: ‘template<class Executor, class T, class AwaitableExecutor, class CompletionToken>  requires  completion_token_for<CompletionToken, void()> auto asio::co_spawn(const Executor&, asio::awaitable<T, AwaitableExecutor>, CompletionToken&&, typename std::enable_if<((asio::is_executor<Executor>::value || asio::execution::is_executor<T>::value) && std::is_convertible<Executor, AwaitableExecutor>::value)>::type*)’
  199 | co_spawn(const Executor& ex,
      | ^~~~~~~~
/usr/include/asio/impl/co_spawn.hpp:199:1: note:   template argument deduction/substitution failed:
asio_cppcoro.cpp:15:43: note:   ‘cppcoro::task<int>’ is not derived from ‘asio::awaitable<T, AwaitableExecutor>’
   15 |  asio::co_spawn(ctx, foo(), asio::detached);

I also tried to wrap my coroutine in a asio::awaitable with no success.

asio::awaitable<int> bar()
{
        co_return co_await foo();
}

The compiler complains that it cannot wrap the two type together

❯ c++ asio_cppcoro.cpp -o asio_cppcoro -std=c++20 -fcoroutines
asio_cppcoro.cpp: In function ‘asio::awaitable<int> bar()’:
asio_cppcoro.cpp:15:25: error: no matching function for call to ‘asio::detail::awaitable_frame<int, asio::execution::any_executor<asio::execution::context_as_t<asio::execution_context&>, asio::execution::detail::blocking::never_t<0>, asio::execution::prefer_only<asio::execution::detail::blocking::possibly_t<0> >, asio::execution::prefer_only<asio::execution::detail::outstanding_work::tracked_t<0> >, asio::execution::prefer_only<asio::execution::detail::outstanding_work::untracked_t<0> >, asio::execution::prefer_only<asio::execution::detail::relationship::fork_t<0> >, asio::execution::prefer_only<asio::execution::detail::relationship::continuation_t<0> > > >::await_transform(cppcoro::task<int>)’
   15 |  co_return co_await foo();
      |                         ^
In file included from /usr/include/asio/awaitable.hpp:129,
                 from /usr/include/asio/co_spawn.hpp:22,
                 from asio_cppcoro.cpp:3:
/usr/include/asio/impl/awaitable.hpp:150:8: note: candidate: ‘template<class T> auto asio::detail::awaitable_frame_base<Executor>::await_transform(asio::awaitable<T, Executor>) const [with T = T; Executor = asio::execution::any_executor<asio::execution::context_as_t<asio::execution_context&>, asio::execution::detail::blocking::never_t<0>, asio::execution::prefer_only<asio::execution::detail::blocking::possibly_t<0> >, asio::execution::prefer_only<asio::execution::detail::outstanding_work::tracked_t<0> >, asio::execution::prefer_only<asio::execution::detail::outstanding_work::untracked_t<0> >, asio::execution::prefer_only<asio::execution::detail::relationship::fork_t<0> >, asio::execution::prefer_only<asio::execution::detail::relationship::continuation_t<0> > >]’
  150 |   auto await_transform(awaitable<T, Executor> a) const
      |        ^~~~~~~~~~~~~~~
/usr/include/asio/impl/awaitable.hpp:150:8: note:   template argument deduction/substitution failed:
asio_cppcoro.cpp:15:25: note:   ‘cppcoro::task<int>’ is not derived from ‘asio::awaitable<T, asio::execution::any_executor<asio::execution::context_as_t<asio::execution_context&>, asio::execution::detail::blocking::never_t<0>, asio::execution::prefer_only<asio::execution::detail::blocking::possibly_t<0> >, asio::execution::prefer_only<asio::execution::detail::outstanding_work::tracked_t<0> >, asio::execution::prefer_only<asio::execution::detail::outstanding_work::untracked_t<0> >, asio::execution::prefer_only<asio::execution::detail::relationship::fork_t<0> >, asio::execution::prefer_only<asio::execution::detail::relationship::continuation_t<0> > > >’
   15 |  co_return co_await foo();
      |  

Is there a way to call a cppcoro::task<> based coroutine from ASIO's io_context? Also, why is this happning? AKAIF C++ coroutines are stackless so spawning a new stack frame for any coroutine implementaion should be possible. Why does ASIO stop me from doing it?

Compiler: GCC 10.2 ASIO: 1.18.1 (Not Boost ASIO. This is the standalone version)

Answer 1

Unfortunately, asio awaitables and cppcoro awaitables could not be co_await'ed by each other. This happens because asio awaitables require cooperation between caller and callee to coordinate shutdown/ownership. That's why asio awaitables do not satisfy the cppcoro Awaitable concept. There is an issue on github that explains this problem in detail: https://github.com/lewissbaker/cppcoro/issues/131

Nevertheless, you can use asio with cppcoro by providing coroutine wrapper for asio. For this, you can use callback approach for asio, and provide cppcoro::single_consumer_event to the handler, and call event.set() in there.

Here is an example: https://gist.github.com/isergeyam/ce10bef00abfaee3d0ec7f31a21c4b95