How do I cancel composed asynchronous operations in Asio?

I want to use the composed async operation asio::async_connect, and cancel it, possibly between the individual calls to basic_socket::async_connect which it makes.

In the same manner, I'd like to cancel my own composed async operations (which use async_compose).

I am not asking how to cancel the non-composed member function basic_socket::async_connect, for which I could call basic_socket::cancel.

The free function is composed, so while calling cancel on the socket might stop an ongoing socket::async_connect, it would not stop queued operations.

1. How do I cancel asio::async_connect?

2. In general, how do I cancel composed operations?

3. How do I cancel custom composed operations? (At least with this third option I could hack in some kind of cancellation token system that can check a flag between "building-block" async operations, but I still can't use existing composed operations in this case).

To summarize the problem, see this diagram from Robert Leahy's 2019 CppCon talk:

This shows the issue with async_write, a composed operation which also has this problem.

Robert Leahy solves this by reimplementing async_write in a wrapper object which checks a cancellation flag before calling every atomic async_write_some operation on the underlying stream.

Perhaps this is the solution, but it means that I would have to re-implement every built-in composed asio operation to use this kind of similar wrapper object.

Revisiting my own question later for posterity: I think composed cancellation support is a pretty new feature and lots of this stuff is still experimental:: as of time of writing.

As of Asio 1.19.0, if a composed op supports cancellation (see docs for it), it appears you're able to bind a cancellation_slot to it (with bind_cancellation_slot) and trigger that with an associated cancellation_signal, or by using logical operators with awaitables which will trigger cancellation automatically.

This process is described here in the docs: https://think-async.com/Asio/boost_asio_1_22_1/doc/html/boost_asio/overview/core/cancellation.html with this example given:

class session
  : public std::enable_shared_from_this<proxy>
{
  ...

  void do_read()
  {
    auto self = shared_from_this();
    socket_.async_read_some(
        buffer(data_),
        boost::asio::bind_cancellation_slot(
          cancel_signal_.slot(),
          [self](boost::system::error_code error, std::size_t n)
          {
            ...
          }
        )
      );
  }

  ...

  void request_cancel()
  {
    cancel_signal_.emit(boost::asio::cancellation_type::total);
  }

  ...

  boost::asio::cancellation_signal cancel_signal_;
};

These cancellation_slots/cancellation_signals appear to work like fancy std::stop_tokens. Implementing cancellation with them in custom composed ops appears to be different depending on which method of composition you're using (async_compose, experimental::co_composed, callback chains), but the co_composed docs offers examples for manual and implicit cancellation checking for an echo protocol. co_composed uses some magic auto state object for cancellation checking though, which I'm guessing is a wrapper around a cancellation_slot somewhere.