C ++ Rest SDK卡萨布兰卡Sigtrap

Question

I am using the C++ Rest SDK ("Casablanca") to receive feed from Websocket-Servers. Currently, I have three different connections to three different servers running at the same time using the websocket_callback_client class .

The program runs for an undefined time and then suddenly receives SIGTRAP, Trace/ Breakpoint trap . This is the output of GDB :

#0  0x00007ffff5abec37 in __GI_raise (sig=5) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#1  0x000000000047bb8e in pplx::details::_ExceptionHolder::~_ExceptionHolder() ()
#2  0x000000000044be29 in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release() ()
#3  0x000000000047fa39 in pplx::details::_Task_impl<unsigned char>::~_Task_impl() ()
#4  0x000000000044be29 in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release() ()
#5  0x00007ffff6feb09f in std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count (this=0x7fffc8021420, __in_chrg=<optimized out>) at /usr/include/c++/4.8/bits/shared_ptr_base.h:546
#6  0x00007ffff6fffa38 in std::__shared_ptr<pplx::details::_Task_impl<unsigned char>, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr (this=0x7fffc8021418, __in_chrg=<optimized out>) at /usr/include/c++/4.8/bits/shared_ptr_base.h:781
#7  0x00007ffff6fffa52 in std::shared_ptr<pplx::details::_Task_impl<unsigned char> >::~shared_ptr (this=0x7fffc8021418, __in_chrg=<optimized out>) at /usr/include/c++/4.8/bits/shared_ptr.h:93
#8  0x00007ffff710f766 in pplx::details::_PPLTaskHandle<unsigned char, pplx::task<unsigned char>::_InitialTaskHandle<void, void web::websockets::client::details::wspp_callback_client::shutdown_wspp_impl<websocketpp::config::asio_tls_client>(std::weak_ptr<void> const&, bool)::{lambda()#1}, pplx::details::_TypeSelectorNoAsync>, pplx::details::_TaskProcHandle>::~_PPLTaskHandle() (this=0x7fffc8021410, __in_chrg=<optimized out>)
    at /home/cpprestsdk/Release/include/pplx/pplxtasks.h:1631
#9  0x00007ffff716e6f2 in pplx::task<unsigned char>::_InitialTaskHandle<void, void web::websockets::client::details::wspp_callback_client::shutdown_wspp_impl<websocketpp::config::asio_tls_client>(std::weak_ptr<void> const&, bool)::{lambda()#1}, pplx::details::_TypeSelectorNoAsync>::~_InitialTaskHandle() (this=0x7fffc8021410, __in_chrg=<optimized out>) at /home/cpprestsdk/Release/include/pplx/pplxtasks.h:3710
#10 0x00007ffff716e722 in pplx::task<unsigned char>::_InitialTaskHandle<void, void web::websockets::client::details::wspp_callback_client::shutdown_wspp_impl<websocketpp::config::asio_tls_client>(std::weak_ptr<void> const&, bool)::{lambda()#1}, pplx::details::_TypeSelectorNoAsync>::~_InitialTaskHandle() (this=0x7fffc8021410, __in_chrg=<optimized out>) at /home/cpprestsdk/Release/include/pplx/pplxtasks.h:3710
#11 0x00007ffff71f9cdd in boost::_bi::list1<boost::_bi::value<void*> >::operator()<void (*)(void*), boost::_bi::list0> (this=0x7fffdc7d7d28, f=@0x7fffdc7d7d20: 0x479180 <pplx::details::_TaskProcHandle::_RunChoreBridge(void*)>, a=...)
    at /usr/local/include/boost/bind/bind.hpp:259
#12 0x00007ffff71f9c8f in boost::_bi::bind_t<void, void (*)(void*), boost::_bi::list1<boost::_bi::value<void*> > >::operator() (this=0x7fffdc7d7d20) at /usr/local/include/boost/bind/bind.hpp:1222
#13 0x00007ffff71f9c54 in boost::asio::asio_handler_invoke<boost::_bi::bind_t<void, void (*)(void*), boost::_bi::list1<boost::_bi::value<void*> > > > (function=...) at /usr/local/include/boost/asio/handler_invoke_hook.hpp:69
#14 0x00007ffff71f9bea in boost_asio_handler_invoke_helpers::invoke<boost::_bi::bind_t<void, void (*)(void*), boost::_bi::list1<boost::_bi::value<void*> > >, boost::_bi::bind_t<void, void (*)(void*), boost::_bi::list1<boost::_bi::value<void*> > > > (function=..., context=...) at /usr/local/include/boost/asio/detail/handler_invoke_helpers.hpp:37
#15 0x00007ffff71f9b2e in boost::asio::detail::completion_handler<boost::_bi::bind_t<void, void (*)(void*), boost::_bi::list1<boost::_bi::value<void*> > > >::do_complete (owner=0x7488d0, base=0x7fffc801ecd0)
    at /usr/local/include/boost/asio/detail/completion_handler.hpp:68
#16 0x00000000004c34c1 in boost::asio::detail::task_io_service::run(boost::system::error_code&) ()
#17 0x00007ffff709fb27 in boost::asio::io_service::run (this=0x7ffff759ab78 <crossplat::threadpool::shared_instance()::s_shared+24>) at /usr/local/include/boost/asio/impl/io_service.ipp:59
#18 0x00007ffff7185a81 in crossplat::threadpool::thread_start (arg=0x7ffff759ab60 <crossplat::threadpool::shared_instance()::s_shared>) at /home/cpprestsdk/Release/include/pplx/threadpool.h:133
#19 0x00007ffff566e184 in start_thread (arg=0x7fffdc7d8700) at pthread_create.c:312
#20 0x00007ffff5b8237d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

At line #18 the soruce /pplx/threadpool.h:133 is given. This is the source-code around these lines:

  123     static void* thread_start(void *arg)
  124     {
  125 #if (defined(ANDROID) || defined(__ANDROID__))
  126         // Calling get_jvm_env() here forces the thread to be attached.
  127         get_jvm_env();
  128         pthread_cleanup_push(detach_from_java, nullptr);
  129 #endif
  130         threadpool* _this = reinterpret_cast<threadpool*>(arg);
  131         try
  132         {
  133             _this->m_service.run();
  134         }
  135         catch (const _cancel_thread&)
  136         {
  137             // thread was cancelled
  138         }
  139         catch (...)
  140         {
  141             // Something bad happened
  142 #if (defined(ANDROID) || defined(__ANDROID__))
  143             // Reach into the depths of the 'droid!
  144             // NOTE: Uses internals of the bionic library
  145             // Written against android ndk r9d, 7/26/2014
  146             __pthread_cleanup_pop(&__cleanup, true);
  147             throw;
  148 #endif
  149         }
  150 #if (defined(ANDROID) || defined(__ANDROID__))
  151         pthread_cleanup_pop(true);
  152 #endif
  153         return arg;
  154     }

For clarification, m_service is a boost::asio::io_service . To me it looks like line #133 throws an exception, it gets caught at line #139 and then rethrown. At this point, I have to catch it personally, because if I don't and the pplx -object gets destroyed with an uncaught exception, it will raise SIGTRAP .

This is how far I got with my research. The problem is I do not have a clue where this is happening. I have surrounded every position where data is sent through or received from websocket_callback_client with try {} catch(...){} and it is still happening.

Maybe someone who has used this library before can help me out.

Answer 1

In my experience this happens due to a separate issue.
When the websocket_callback_client's close handler gets called, most people try to delete the websocket_callback_client. This internally calls the close function.
When this happens the websocket_callback_client will wait for the close to finish. If another thread realizes the connection is dead and tries to clean up you will have the same object being deleted from 2 different locations, which will cause major issues.
Howto reconnect to a server which does not answer to close() has a fairly thorough review of what happens when cpprestsdk calls close.

Hope this helps :)

Edit: As it turns out (the response I gave in the linked question has this), if you try to close or delete the websocket_callback_client from the close handler it will itself call the close handler, which will lock the thread.
The solution I have found that works best for me is to set a flag in the close handler and handle the cleanup in the main thread or at the very least an alternate thread.

Answer 2

Revisiting this. I've found a work around, which I've posted on the cpprestsdk github ( https://github.com/Microsoft/cpprestsdk/issues/427 ).

The SDK does a poor job in surfacing exceptions and in the issue I've indicated they need to improve the documentation around this as well as provide a clean public interface to do this (you'll see the solution has a code smell to it).

What one needs to do is to rethrow the user exception.

This is in the context of making an http_client request call, but should be applicable for any usage of pplx.

client->request(request).then([=] (web::http::http_response response) mutable {
    // Your code here
}).then([=] (pplx::task<void> previous_task) mutable {
    if (previous_task._GetImpl()->_HasUserException()) {
        auto holder = previous_task._GetImpl()->_GetExceptionHolder(); // Probably should put in try

        try {
            // Need to make sure you try/catch here, as _RethrowUserException can throw
            holder->_RethrowUserException();
        } catch (std::exception& e) {
            // Do what you need to do here
        }
    }
});

The handling to catch that "unobserved exception" is done in the second then() .