Pure virtual function called when Session object is being destructed

After the destructor of a Session object is called, it will go to the destructor of its parent class's destructor (SessionBase). Sometimes, when the SessionBase's destructor is calling event_loop->join(), some other thread will try to call the Session object's on_host_up() method. Since the Session object is destructed, on_host_up() becomes a pure virtual method, hence the crash.

Stack trace:
[worker]
#0 0x00007ffff67c84f5 in raise () from /lib64/libc.so.6
#1 0x00007ffff67c9cd5 in abort () from /lib64/libc.so.6
#2 0x00007fffef22d61d in _gnu_cxx::_verbose_terminate_handler () at ../../../../src/gcc-6.2.0/libstdc+-v3/libsupc+/vterminate.cc:95
#3 0x00007fffef22b5e6 in _cxxabiv1::_terminate (handler=<optimized out>) at ../../../../src/gcc-6.2.0/libstdc+-v3/libsupc+/eh_terminate.cc:47
#4 0x00007fffef22b631 in std::terminate () at ../../../../src/gcc-6.2.0/libstdc+-v3/libsupc+/eh_terminate.cc:57
#5 0x00007fffef22c10f in _cxxabiv1::_cxa_pure_virtual () at ../../../../src/gcc-6.2.0/libstdc+-v3/libsupc+/pure.cc:50
#6 0x00007fffefb752cd in cass::ClusterEvent::process_event (event=..., listener=0x7e2b70)
at cpp-driver-2.12.0/src/cluster.cpp:139
#7 0x00007fffefb786dc in cass::Cluster::notify_or_record (this=0x7fffdc068f70, event=...)
at cpp-driver-2.12.0/src/cluster.cpp:687
#8 0x00007fffefb77645 in cass::Cluster::notify_host_up_after_prepare (this=0x7fffdc068f70, host=...)
at cpp-driver-2.12.0/src/cluster.cpp:545
#9 0x00007fffefb78901 in cass::Cluster::on_prepare_host_up (this=0x7fffdc068f70, handler=0x7fffdc005700)
at cpp-driver-2.12.0/src/cluster.cpp:713
#10 0x00007fffefb89593 in cass::Callback<void, cass::PrepareHostHandler const*>::MemberInvoker<void (cass::Cluster::)(cass::PrepareHostHandler const), cass::Cluster>::invoke (this=0x7fffdc005728, arg=@0x7fffea8e6d48: 0x7fffdc005700)
at cpp-driver-2.12.0/src/callback.hpp:94
#11 0x00007fffefbfc408 in cass::Callback<void, cass::PrepareHostHandler const*>::operator() (this=0x7fffdc005720, arg=@0x7fffea8e6d48: 0x7fffdc005700)
at cpp-driver-2.12.0/src/callback.hpp:71
#12 0x00007fffefbfb3e9 in cass::PrepareHostHandler::on_close (this=0x7fffdc005700, connection=0x7fffdc000ad0)
at cpp-driver-2.12.0/src/prepare_host_handler.cpp:77
#13 0x00007fffefc93e44 in cass::Connection::on_close (this=0x7fffdc000ad0)
at cpp-driver-2.12.0/src/connection.cpp:351
#14 0x00007fffefc929de in cass::ConnectionHandler::on_close (this=0x7fffdc000960)
at cpp-driver-2.12.0/src/connection.cpp:84
#15 0x00007fffefcbeb8c in cass::Socket::handle_close (this=0x7fffdc8047f0)
at cpp-driver-2.12.0/src/socket.cpp:392
#16 0x00007fffefcbea8d in cass::Socket::on_close (handle=0x7fffdc8047f8)
at cpp-driver-2.12.0/src/socket.cpp:377
#17 0x00007fffefce6bd5 in uv__finish_close (handle=0x7fffdc8047f8) at src/unix/core.c:282
#18 uv__run_closing_handles (loop=0x7e4358) at src/unix/core.c:296
#19 uv_run (loop=0x7e4358, mode=UV_RUN_DEFAULT) at src/unix/core.c:366
#20 0x00007fffefbbd1ec in cass::EventLoop::handle_run (this=0x7e4350)
at cpp-driver-2.12.0/src/event_loop.cpp:172
#21 0x00007fffefbbd1ae in cass::EventLoop::internal_on_run (arg=0x7e4350)
at cpp-driver-2.12.0/src/event_loop.cpp:167
#22 0x00007ffff73c4aa1 in start_thread () from /lib64/libpthread.so.0
#23 0x00007ffff687ec4d in clone () from /lib64/libc.so.6

[main]
#0 0x00007ffff73c52fd in pthread_join () from /lib64/libpthread.so.0
#1 0x00007fffefcee95e in uv_thread_join (tid=<optimized out>) at src/unix/thread.c:252
#2 0x00007fffefbbce4e in cass::EventLoop::join (this=0x7e4350)
at cpp-driver-2.12.0/src/event_loop.cpp:105
#3 0x00007fffefc1b125 in cass::SessionBase::~SessionBase (this=0x7e2b70, __in_chrg=<optimized out>)
at cpp-driver-2.12.0/src/session_base.cpp:49
#4 0x00007fffefc14b5c in cass::Session::~Session (this=0x7e2b70, __in_chrg=<optimized out>)
at cpp-driver-2.12.0/src/session.cpp:300
#5 0x00007fffefc14c1c in cass::Session::~Session (this=0x7e2b70, __in_chrg=<optimized out>)
at cpp-driver-2.12.0/src/session.cpp:303
#6 0x00007fffefc13f3c in cass_session_free (session=0x7e2b70)
at cpp-driver-2.12.0/src/session.cpp:46
[outside of cpp-driver-2.12.0 library]