How to get boost::asio::io_service current action number - c++

Question

Boost::asio::io_service provides "handler tracking" for debugging purposes, it is enabled by defining BOOST_ASIO_ENABLE_HANDLER_TRACKING but logs its data to stderr. I'd like to use this tracking information in my application. My question is what is the best way to get access to the <action> inside my application?
For more context as to why I want to do this; I would like to attach the <action> as a parameter to other async operations so that I can track where the originating request came from.

Answer 1

Asio does not expose its handler tracking data. Attempting to extract the tracking information contained within Asio would be far more of a dirty hack than rolling ones own custom handler.
Here is a snippet from Asio's handler tracking:
namespace boost {
namespace asio {
namespace detail {
class handler_tracking
{
public:
class completion;
// Base class for objects containing tracked handlers.
class tracked_handler
{
private:
// Only the handler_tracking class will have access to the id.
friend class handler_tracking;
friend class completion;
uint64_t id_;
// ...
private:
friend class handler_tracking;
uint64_t id_;
bool invoked_;
completion* next_;
};
// ...
private:
struct tracking_state;
static tracking_state* get_state();
};
} // namespace detail
} // namespace asio
} // namespace boost
As others have mentioned, passing a GUID throughout the handlers would allow one to associate multiple asynchronous operations. One non-intrusive way to accomplish this is to create a custom tracking handler type that wraps existing handlers and manages the tracking data. For an example on custom handlers, see the Boost.Asio Invocation example.
Also, be aware that if a custom handler type is used, one should be very careful when composing handlers. In particular, the custom handler type's invocation hook (asio_handler_invoke()) may need to account for the context of other handlers. For example, if one does not explicitly account for wrapped handler returned from strand::wrap(), then it will prevent intermediate operations from running in the correct context for composed operations. To avoid having to explicitly handle this, one can wrap the custom handler by strand::wrap():
boost::asio::async_read(..., strand.wrap(tracker.wrap(&handle_read))); // Good.
boost::asio::async_read(..., tracker.wrap(strand.wrap(&handle_read))); // Bad.

Answer 2

An example that mimics asio debug handler tracking. Caveats:
Assumes ioService only run from a single thread. I never use any other way so I'm not sure what needs to change to fix this limitation.
Non-thread safe access to std::cerr - fixing this left as an exercise.
Code:
#include <boost/asio.hpp>
#include <boost/atomic.hpp>
#include <iostream>
class HandlerTracking
{
public:
HandlerTracking()
:
mCount(1)
{ }
template <class Handler>
class WrappedHandler
{
public:
WrappedHandler(HandlerTracking& t, Handler h, std::uint64_t id)
:
mHandlerTracking(t),
mHandler(h),
mId(id)
{ }
WrappedHandler(const WrappedHandler& other)
:
mHandlerTracking(other.mHandlerTracking),
mHandler(other.mHandler),
mId(other.mId),
mInvoked(other.mInvoked)
{
other.mInvoked = true;
}
~WrappedHandler()
{
if (!mInvoked)
std::cerr << '~' << mId << std::endl;
}
template <class... Args>
void operator()(Args... args)
{
mHandlerTracking.mCurrHandler = mId;
std::cerr << '>' << mId << std::endl;
try
{
mInvoked = true;
mHandler(args...);
}
catch(...)
{
std::cerr << '!' << mId << std::endl;
throw;
}
std::cerr << '<' << mId << std::endl;
}
const std::uint64_t id() { return mId; }
private:
HandlerTracking& mHandlerTracking;
Handler mHandler;
const std::uint64_t mId;
mutable bool mInvoked = false;
};
template <class Handler>
WrappedHandler<Handler> wrap(Handler handler)
{
auto next = mCount.fetch_add(1);
std::cerr << mCurrHandler << '*' << next << std::endl;
return WrappedHandler<Handler>(*this, handler, next);
}
boost::atomic<std::uint64_t> mCount;
std::uint64_t mCurrHandler = 0; // Note: If ioService run on multiple threads we need a curr handler per thread
};
// Custom invokation hook for wrapped handlers
//template <typename Function, typename Handler>
//void asio_handler_invoke(Function f, HandlerTracking::WrappedHandler<Handler>* h)
//{
// std::cerr << "Context: " << h << ", " << h->id() << ", " << f.id() << std::endl;
// f();
//}
// Class to demonstrate callback with arguments
class MockSocket
{
public:
MockSocket(boost::asio::io_service& ioService) : mIoService(ioService) {}
template <class Handler>
void async_read(Handler h)
{
mIoService.post([h]() mutable { h(42); }); // we always read 42 bytes
}
private:
boost::asio::io_service& mIoService;
};
int main(int argc, char* argv[])
{
boost::asio::io_service ioService;
HandlerTracking tracking;
MockSocket socket(ioService);
std::function<void()> f1 = [&]() { std::cout << "Handler1" << std::endl; };
std::function<void()> f2 = [&]() { std::cout << "Handler2" << std::endl; ioService.post(tracking.wrap(f1)); };
std::function<void()> f3 = [&]() { std::cout << "Handler3" << std::endl; ioService.post(tracking.wrap(f2)); };
std::function<void()> f4 = [&]() { std::cout << "Handler4" << std::endl; ioService.post(tracking.wrap(f3)); };
std::function<void(int)> s1 = [](int s) { std::cout << "Socket read " << s << " bytes" << std::endl; };
socket.async_read(tracking.wrap(s1));
ioService.post(tracking.wrap(f1));
ioService.post(tracking.wrap(f2));
ioService.post(tracking.wrap(f3));
auto tmp = tracking.wrap(f4); // example handler destroyed without invocation
ioService.run();
return 0;
}
Output:
0*1
0*2
0*3
0*4
0*5
>1
Socket read 42 bytes
<1
>2
Handler1
<2
>3
Handler2
3*6
<3
>4
Handler3
4*7
<4
>6
Handler1
<6
>7
Handler2
7*8
<7
>8
Handler1
<8
~5