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boost::asio::bind_executor does not execute in strand

Time:03-15

The following example completes with no assertions:

#include <cassert>
#include <functional>
#include <future>
#include <thread>
#include <boost/asio.hpp>

class example1
{
public:
    typedef boost::asio::io_context io_context;
    typedef boost::asio::io_context::executor_type executor_type;
    typedef boost::asio::strand<executor_type> strand;
    typedef boost::asio::executor_work_guard<executor_type> work_guard;
    typedef std::function<void()> handler;

    example1()
      : work_(boost::asio::make_work_guard(context_)),
        thread_([this]() { context_.run(); }),
        strand1_(context_.get_executor()),
        strand2_(context_.get_executor())
    {

    }

    ~example1()
    {
        assert(result_.get_future().get());
        work_.reset();
        thread_.join();
    }

    void invoke()
    {
        handler handle = boost::asio::bind_executor(strand2_,
            std::bind(&example1::strand2_handler, this));

        boost::asio::post(strand1_,
            std::bind(&example1::strand1_handler, this, handle));
    }

    void strand1_handler(handler handle)
    {
        assert(strand1_.running_in_this_thread());
        handle();
    }

    void strand2_handler()
    {
        assert(strand1_.running_in_this_thread());
        ////assert(strand2_.running_in_this_thread());
        result_.set_value(true);
    }

private:
    io_context context_;
    work_guard work_;
    std::thread thread_;
    strand strand1_;
    strand strand2_;
    std::promise<bool> result_;
};

int main()
{
    example test{};
    test.invoke();
}

However my expectation is that the commented-out assertion should succeed, as opposed to the one directly above it. According to strand::running_in_this_thread() the handler handle has been invoked in the caller's strand, not that provided to bind_executor.

I can work around this using intermediate methods, as follows.

class example2
{
public:
    typedef boost::asio::io_context io_context;
    typedef boost::asio::io_context::executor_type executor_type;
    typedef boost::asio::strand<executor_type> strand;
    typedef boost::asio::executor_work_guard<executor_type> work_guard;
    typedef std::function<void()> handler;

    example2()
      : work_(boost::asio::make_work_guard(context_)),
        thread_([this]() { context_.run(); }),
        strand1_(context_.get_executor()),
        strand2_(context_.get_executor())
    {

    }

    ~example2()
    {
        assert(result_.get_future().get());
        work_.reset();
        thread_.join();
    }

    void invoke()
    {
        handler handle =
            std::bind(&example2::do_strand2_handler, this);

        boost::asio::post(strand1_,
            std::bind(&example2::strand1_handler, this, handle));
    }

    void strand1_handler(handler handle)
    {
        assert(strand1_.running_in_this_thread());
        handle();
    }

    // Do the job of bind_executor.
    void do_strand2_handler()
    {
        boost::asio::post(strand2_,
            std::bind(&example2::strand2_handler, this));
    }

    void strand2_handler()
    {
        ////assert(strand1_.running_in_this_thread());
        assert(strand2_.running_in_this_thread());
        result_.set_value(true);
    }

private:
    io_context context_;
    work_guard work_;
    std::thread thread_;
    strand strand1_;
    strand strand2_;
    std::promise<bool> result_;
};

int main()
{
    example2 test2{};
    test2.invoke();
}

But avoiding that is presumably the purpose of bind_executor. Is this a boost bug or am I missing something? I've tried following this through the boost::asio sources but to no avail.

Update

Thanks to @sehe for a lot of help. The above problem can be resolved in a number of ways, for example:

class example3
{
public:
    typedef boost::asio::io_context io_context;
    typedef boost::asio::io_context::executor_type executor_type;
    typedef boost::asio::strand<executor_type> strand;
    typedef boost::asio::executor_work_guard<executor_type> work_guard;
    typedef boost::asio::executor_binder<std::function<void()>,
        boost::asio::any_io_executor> handler;

    example3()
      : work_(boost::asio::make_work_guard(context_)),
        thread_([this]() { context_.run(); }),
        strand1_(context_.get_executor()),
        strand2_(context_.get_executor())
    {
    }

    ~example3()
    {
        assert(result_.get_future().get());
        work_.reset();
        thread_.join();
    }

    void invoke()
    {
        auto handle = boost::asio::bind_executor(strand2_,
            std::bind(&example3::strand2_handler, this));

        boost::asio::post(strand1_,
            std::bind(&example3::strand1_handler, this, handle));
    }

    void strand1_handler(handler handle)
    {
        assert(strand1_.running_in_this_thread());
        boost::asio::dispatch(handle);
    }

    void strand2_handler()
    {
        assert(strand2_.running_in_this_thread());
        result_.set_value(true);
    }

private:
    io_context context_;
    work_guard work_;
    std::thread thread_;
    strand strand1_;
    strand strand2_;
    std::promise<bool> result_;
};

int main
{
    example3 test3{};
    test3.invoke();
}

CodePudding user response:

Yes, you're missing something indeed. Two things, actually.

Type Erasure

Binding an executor doesn't modify the function, it modifies its type.

However, by erasing the callable's type using std::function<> you've hidden the bound executor. You could easily determine this:

erased_handler handle = bind_executor(s2, s2_handler);
assert(asio::get_associated_executor(handle, s1) == s1);

The problem is gone when you preserve the type:

auto handle = bind_executor(s2, s2_handler);
assert(asio::get_associated_executor(handle, s1) == s2);

Dispatch (formerly handler_invoke)

Invoking handle straight up calls it according to the C language semantics, as you have found out.

To ask Asio to honour the potentially bound executor, you could use dispatch (or post):

auto s1_handler = [&](auto chain) {
    assert(s1.running_in_this_thread());
    dispatch(get_associated_executor(chain, s1), chain);
};

In fact, if you're sure that chain will have an associated executor, you could accept the default fallback (which is a system executor):

auto s1_handler = [&](auto chain) {
    assert(s1.running_in_this_thread());
    dispatch(chain);
};

Putting It All Together

Demonstrating the wisdom in a simplified, extended tester:

Live On Coliru

#include <boost/asio.hpp>
#include <functional>
#include <iostream>

namespace asio = boost::asio;

int main() {
    asio::thread_pool io(1);

    auto s1 = make_strand(io), s2 = make_strand(io);
    assert(s1 != s2); // implementation defined! strands may hash equal

    auto s1_handler = [&](auto chain) {
        assert(s1.running_in_this_thread());

        // immediate invocation runs on the current strand:
        chain();

        // dispatch *might* invoke directly if already on the right strand
        dispatch(chain);                                     // 1
        dispatch(get_associated_executor(chain, s1), chain); // 2

        // posting never immediately invokes, even if already on the right
        // strand
        post(chain);                                     // 3
        post(get_associated_executor(chain, s1), chain); // 4
    };

    int count_chain_invocations = 0;
    auto s2_handler = [&] {
        if (s2.running_in_this_thread()) {
            count_chain_invocations  = 1;
        } else {
            std::cout << "(note: direct C   call ends up on wrong strand)\n";
        }
    };

    {
        using erased_handler  = std::function<void()>;
        erased_handler handle = bind_executor(s2, s2_handler);
        assert(asio::get_associated_executor(handle, s1) == s1);
    }
    {
        auto handle = bind_executor(s2, s2_handler);
        assert(asio::get_associated_executor(handle, s1) == s2);
    }

    auto handle = bind_executor(s2, s2_handler);
    post(s1, std::bind(s1_handler, handle));

    io.join();

    std::cout << "count_chain_invocations: " << count_chain_invocations << "\n";
}

All the assertions pass, and the output is as expected:

(note: direct C   call ends up on wrong strand)
count_chain_invocations: 4

BONUS: What If You Need Type-Erased Bound Calleables?

Whatever you do, don't use std::function. You can wrap one, though;

template <typename Sig> struct ErasedHandler {
    using executor_type = asio::any_io_executor;
    std::function<Sig> _erased;
    executor_type      _ex;
    executor_type get_executor() const { return _ex; }

    template <typename F>
    explicit ErasedHandler(F&& f)
        : _erased(std::forward<F>(f))
        , _ex(asio::get_associated_executor(f)) {}

    ErasedHandler() = default;

    template <typename... Args>
    decltype(auto) operator()(Args&&... args) const {
        return _erased(std::forward<Args>(args)...);
    }
    template <typename... Args>
    decltype(auto) operator()(Args&&... args) {
        return _erased(std::forward<Args>(args)...);
    }

    explicit operator bool() const { return _erased; }
};

See it Live On Coliru

Before you do, note that

  • using any_io_executor also type erases the executor, which potentially hurts performance
  • it does not provide a good fallback, just using the system executor for unbound calleables. You could get around this by detecting it and requiring an explicit constructor arugment etc. but...
  • all of this still completely ignores other handler attributes like associated allocator

I would probably avoid generically storing type-erased chainable handlers. You can most often store the actual type of the handler deduced by template type parameter.

PS: Afterthoughts

What you were perhaps expecting was this behaviour:

template <typename... Args>
decltype(auto) operator()(Args&&... args) const {
    // CAUTION: NOT WHAT YOU WANT
    boost::asio::dispatch(_ex,
                          std::bind(_erased, std::forward<Args>(args)...));
}
template <typename... Args>
decltype(auto) operator()(Args&&... args) {
    // CAUTION: NOT WHAT YOU WANT
    boost::asio::dispatch(_ex,
                          std::bind(_erased, std::forward<Args>(args)...));
}

See that Live On Coliru

Under this regimen even direct C calls will "do the right thing".

That seems nice. Until you think about it.

The issue is that handlers cannot be rebound this way. More specifically, if you had a handler that is associated with a "free-threaded" executor, doing bind_executor(strand, f) would have no effect (except slowing down your program), as the f would be obnoxiously dispatching to another executor anyways.

So don't do that :)

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