Using a minimal executor causes compile errors with beast::websocket

[bgemmill]

Beast 351 from boost 1.84

For some background, I'm trying to use boost::beast with my own executor, based on the minimal example here.

However, it seems like something inside beast's websocket is doing something funny with the executor's require interface, and leading to compile errors.

For example, imagine a simple executor that's merely a forwarder to asio::io_context:

  struct executor {
    boost::asio::io_context* context_{nullptr};

    executor& operator=(const executor& other) = default;
    executor& operator=(executor&& other) = default;

    bool operator==(const executor& other) const noexcept {
      return context_ == other.context_;
    }

    bool operator!=(const executor& other) const noexcept {
      return !(*this == other);
    }

    boost::asio::execution_context& query(boost::asio::execution::context_t) const noexcept {
      return *context_;
    }

    static constexpr boost::asio::execution::blocking_t::never_t query(boost::asio::execution::blocking_t) noexcept {
      return boost::asio::execution::blocking_t::never;
    }

    constexpr executor require(boost::asio::execution::blocking_t::never_t) const {
      return *this; 
    }

    template<class F>
    void execute(F&& f) const {
      context_->get_executor().execute(std::forward<F>(f));
    }
  };

Creating sockets with this executor works just fine:

using socket_t = boost::asio::basic_stream_socket<boost::asio::ip::tcp, common::base::executor>;
...
auto p = std::make_shared<socket_t>(executor{&ioc_from_somewhere});

But creating websockets leads to compile errors (for example, in handshake):

using websocket_t = boost::beast::websocket::stream<socket_t, true>;
...
auto p = std::make_shared<websocket_t>(executor{&ioc_from_somewhere});
p->async_handshake("host", "target", [](boost::system::error_code ec){});

...compile....

boost/asio/associated_immediate_executor.hpp:57:12: error: binding reference of type 'executor' to value of type 'const executor' drops 'const' qualifier
   57 |     return boost::asio::require(e, execution::blocking.never);
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The require api is admittedly super clunky, so it's not at all clear what else the executor needs to make beast happy.

[ashtum]

The issue is reproducible with the following too: https://godbolt.org/z/5cosahG6M

asio::io_context ioc;
auto e = asio::get_associated_immediate_executor([] {}, executor{ &ioc });

It seems we need to check for the existence of the associated immediate executor before attempting to use it in a dispatch operation. There is a possibility that this might be a bug in Asio as well, which I am currently investigating.

[bgemmill]

Thanks for having a look at this!

[ashtum]

This seems to be a bug in Asio. I've made a pull request for it: https://github.com/chriskohlhoff/asio/pull/1465 As a workaround you might consider commenting out the static query(asio::execution::blocking_t) member function:

// static constexpr asio::execution::blocking_t::never_t query(asio::execution::blocking_t) noexcept
// {
//   return asio::execution::blocking.never;
// }

[ashtum]

Addressed in https://github.com/chriskohlhoff/asio/commit/e5cbafb261d73060690c5befa4723db11716a02f,