Hi PyTango Experts! I would like to report the following strange issue. I have a PyTango client which subscribes in the stateless mode to an attribute. This subscription however constantly fails due to `API_EventPropertiesNotSet` - event properties (abs_change or rel_change) for this attribute are not set. Meanwhile, this client reads one attribute using a disposable `AttributeProxy`. I have observed that the `Tango::AttributeProxy::~AttributeProxy` collides with the `Tango::EventConsumerKeepAliveThread` and a deadlock occurs. Below tracaback of the affected threads (all threads are in this [file](https://github.com/reszelaz/test-tango-hang-stateless-sub/blob/master/backtrace.txt): ``` Thread 7 (Thread 0x7f76e6fba700 (LWP 16514)): #0 pthread_cond_wait@@GLIBC_2.3.2 () at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185 #1 0x00007f76f2253c60 in omni_condition::wait() () from /usr/lib/libomnithread.so.3 #2 0x00007f76f3c22758 in ReadersWritersLock::readerIn (this=<optimized out>) at ../../../lib/cpp/server/readers_writers_lock.h:66 #3 ReaderLock::ReaderLock (l=..., this=<synthetic pointer>) at ../../../lib/cpp/server/readers_writers_lock.h:176 #4 Tango::ZmqEventConsumer::get_subscribed_event_ids (this=<optimized out>, _dev=_dev@entry=0x7f76dc000d40, _ids=std::vector of length 0, capacity 0) at zmqeventconsumer.cpp:3341 #5 0x00007f76f3b5f73c in Tango::DeviceProxy::~DeviceProxy (this=0x7f76dc000d40, __in_chrg=<optimized out>) at devapi_base.cpp:2493 #6 0x00007f76f3b5f8f9 in Tango::DeviceProxy::~DeviceProxy (this=0x7f76dc000d40, __in_chrg=<optimized out>) at devapi_base.cpp:2531 #7 0x00007f76f3bc275d in Tango::AttributeProxy::~AttributeProxy (this=0x7f76dc0102c0, __in_chrg=<optimized out>) at attr_proxy.cpp:801 #8 0x00007f76f3bc2829 in Tango::AttributeProxy::~AttributeProxy (this=0x7f76dc0102c0, __in_chrg=<optimized out>) at attr_proxy.cpp:806 #9 0x00007f76f45e77d9 in boost::detail::sp_counted_base::release (this=0x7f76dc085b50) at /usr/include/boost/smart_ptr/detail/sp_counted_base_gcc_x86.hpp:146 #10 boost::detail::shared_count::~shared_count (this=0x7f76dc00f998, __in_chrg=<optimized out>) at /usr/include/boost/smart_ptr/detail/shared_count.hpp:473 #11 boost::shared_ptr<Tango::AttributeProxy>::~shared_ptr (this=0x7f76dc00f990, __in_chrg=<optimized out>) at /usr/include/boost/smart_ptr/shared_ptr.hpp:336 #12 boost::python::objects::pointer_holder<boost::shared_ptr<Tango::AttributeProxy>, Tango::AttributeProxy>::~pointer_holder (this=0x7f76dc00f980, __in_chrg=<optimized out>) at /usr/include/boost/python/object/pointer_holder.hpp:52 #13 0x00007f76f3732c7c in ?? () from /usr/lib/x86_64-linux-gnu/libboost_python-py27.so.1.62.0 #14 0x00005567f2d955ca in subtype_dealloc.lto_priv () at ../Objects/typeobject.c:1050 #15 0x00007f76f373c30c in ?? () from /usr/lib/x86_64-linux-gnu/libboost_python-py27.so.1.62.0 #16 0x00005567f2d480c3 in PyObject_Call () at ../Objects/abstract.c:2547 #17 0x00005567f2d50799 in PyObject_CallFunctionObjArgs () at ../Objects/abstract.c:2774 #18 0x00005567f2dc0b63 in handle_callback.lto_priv () at ../Objects/weakrefobject.c:894 #19 0x00005567f2d9583b in PyObject_ClearWeakRefs () at ../Objects/weakrefobject.c:939 #20 0x00007f76f3732ca6 in ?? () from /usr/lib/x86_64-linux-gnu/libboost_python-py27.so.1.62.0 #21 0x00005567f2d955ca in subtype_dealloc.lto_priv () at ../Objects/typeobject.c:1050 #22 0x00005567f2d951b6 in dict_dealloc.lto_priv.61 (mp=0x7f76ea80b5c8) at ../Objects/dictobject.c:1040 #23 subtype_dealloc.lto_priv () at ../Objects/typeobject.c:1035 #24 0x00005567f2d70c67 in frame_dealloc.lto_priv () at ../Objects/frameobject.c:458 #25 0x00005567f2d61170 in fast_function (nk=<optimized out>, na=<optimized out>, n=<optimized out>, pp_stack=0x7f76e6fb9938, func=<function at remote 0x7f76ed382578>) at ../Python/ceval.c:4439 #26 call_function (oparg=<optimized out>, pp_stack=0x7f76e6fb9938) at ../Python/ceval.c:4372 #27 PyEval_EvalFrameEx () at ../Python/ceval.c:2989 #28 0x00005567f2d6114f in fast_function (nk=<optimized out>, na=<optimized out>, n=<optimized out>, pp_stack=0x7f76e6fb9a88, func=<function at remote 0x7f76f5cc7230>) at ../Python/ceval.c:4437 #29 call_function (oparg=<optimized out>, pp_stack=0x7f76e6fb9a88) at ../Python/ceval.c:4372 #30 PyEval_EvalFrameEx () at ../Python/ceval.c:2989 #31 0x00005567f2d599f5 in PyEval_EvalCodeEx () at ../Python/ceval.c:3584 #32 0x00005567f2d765be in function_call.lto_priv () at ../Objects/funcobject.c:523 #33 0x00005567f2d480c3 in PyObject_Call () at ../Objects/abstract.c:2547 #34 0x00005567f2d8ce1e in instancemethod_call.lto_priv () at ../Objects/classobject.c:2602 #35 0x00005567f2d480c3 in PyObject_Call () at ../Objects/abstract.c:2547 #36 0x00005567f2d65a10 in PyEval_CallObjectWithKeywords () at ../Python/ceval.c:4221 #37 0x00005567f2e2e722 in t_bootstrap () at ../Modules/threadmodule.c:620 #38 0x00007f76f5a49494 in start_thread (arg=0x7f76e6fba700) at pthread_create.c:333 #39 0x00007f76f4e66acf in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:97 ``` ``` Thread 3 (Thread 0x7f76e8fbe700 (LWP 21640)): #0 0x00007f76f5a51536 in futex_abstimed_wait_cancelable (private=0, abstime=0x0, expected=0, futex_word=0x5567f4177590) at ../sysdeps/unix/sysv/linux/futex-internal.h:205 #1 do_futex_wait (sem=sem@entry=0x5567f4177590, abstime=0x0) at sem_waitcommon.c:111 #2 0x00007f76f5a515e4 in __new_sem_wait_slow (sem=0x5567f4177590, abstime=0x0) at sem_waitcommon.c:181 #3 0x00005567f2d36a04 in PyThread_acquire_lock () at ../Python/thread_pthread.h:324 #4 0x00005567f2d8283a in PyEval_RestoreThread () at ../Python/ceval.c:359 #5 0x00005567f2e3af2f in PyGILState_Ensure () at ../Python/pystate.c:611 #6 0x00007f76f4671dba in AutoPythonGIL::AutoPythonGIL (safe=true, this=<synthetic pointer>) at /home/zreszela/workspace/pytango/ext/pytgutils.h:48 #7 (anonymous namespace)::_push_event<Tango::EventData> (ev=0x7f76d00089f0, self=0x5567f46d8ff0) at /home/zreszela/workspace/pytango/ext/callback.cpp:228 #8 PyCallBackPushEvent::push_event (this=0x5567f46d8ff0, ev=0x7f76d00089f0) at /home/zreszela/workspace/pytango/ext/callback.cpp:327 #9 0x00007f76f3c0a88d in Tango::EventConsumerKeepAliveThread::stateless_subscription_failed (this=this@entry=0x5567f487d590, vpos= {device = 0x5567f4648f50, attribute = "attr2", event_type = Tango::CHANGE_EVENT, event_name = "change", event_id = 1, callback = 0x5567f46d8ff0, ev_queue = 0x0, filters = std::vector of length 0, capacity 0, last_heartbeat = 1567697671, prefix = "tango://pc255.cells.es:10000/"}, e=..., now=@0x7f76e8fbdc58: 1567697671) at eventkeepalive.cpp:1885 #10 0x00007f76f3c0b3eb in Tango::EventConsumerKeepAliveThread::not_conected_event (this=this@entry=0x5567f487d590, event_consumer=event_consumer@entry=0x5567f4ccaf50, now=<optimized out>, now@entry=1567697671, notifd_event_consumer=notifd_event_consumer@entry=0x0) at eventkeepalive.cpp:745 #11 0x00007f76f3c12a1b in Tango::EventConsumerKeepAliveThread::run_undetached (this=0x5567f487d590, arg=<optimized out>) at eventkeepalive.cpp:581 #12 0x00007f76f22546c1 in omni_thread_wrapper () from /usr/lib/libomnithread.so.3 #13 0x00007f76f5a49494 in start_thread (arg=0x7f76e8fbe700) at pthread_create.c:333 #14 0x00007f76f4e66acf in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:97 ``` To reproduce the problem one can use the following example [test-tango-hang-stateless-sub](https://github.com/reszelaz/test-tango-hang-stateless-sub).