Deadlock in bucket logger code [ETA 2019/7/11]

Thread 32 (Thread 0x7f319c97d700 (LWP 15422)):

#0  0x00007f319eb4951d in __lll_lock_wait () from /lib64/libpthread.so.0

#1  0x00007f319eb44e1b in _L_lock_812 () from /lib64/libpthread.so.0

#2  0x00007f319eb44ce8 in pthread_mutex_lock () from /lib64/libpthread.so.0

#3  0x00007f31a2199d41 in cb::logger::unregisterSpdLogger(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) () from /opt/couchbase/bin/../lib/libmemcached_logger.so.1.0.0

#4  0x00007f3199fd634b in std::_Sp_counted_ptr<BucketLogger*, (__gnu_cxx::_Lock_policy)2>::_M_dispose() () from /opt/couchbase/bin/../lib/../lib/ep.so

#5  0x00007f31a219fb81 in spdlog::details::registry::flush_all() () from /opt/couchbase/bin/../lib/libmemcached_logger.so.1.0.0

#6  0x00007f31a21a0405 in _ZNSt6thread11_State_implINS_8_InvokerISt5tupleIJZN6spdlog7details15periodic_workerC4ERKSt8functionIFvvEENSt6chrono8durationIlSt5ratioILl1ELl1EEEEEUlvE_EEEEE6_M_runEv () from /opt/couchbase/bin/../lib/libmemcached_logger.so.1.0.0

#7  0x00007f319f329dcf in execute_native_thread_routine () from /opt/couchbase/bin/../lib/libstdc++.so.6

#8  0x00007f319eb42e25 in start_thread () from /lib64/libpthread.so.0

#9  0x00007f319e86cbad in clone () from /lib64/libc.so.6

    void flush_all()

    {

        std::lock_guard<std::mutex> lock(logger_map_mutex_);

        for (auto &l : loggers_)

        {

            auto shared = l.second.lock();

            if (shared)

            {

                shared->flush();

            }

        }

    }

    void drop(const std::string &logger_name)

    {

        std::lock_guard<std::mutex> lock(logger_map_mutex_);

        loggers_.erase(logger_name);

    }

Thread 25 (Thread 0x7f3194717700 (LWP 15429)):

#0  0x00007f319eb4951d in __lll_lock_wait () from /lib64/libpthread.so.0

#1  0x00007f319eb44e1b in _L_lock_812 () from /lib64/libpthread.so.0

#2  0x00007f319eb44ce8 in pthread_mutex_lock () from /lib64/libpthread.so.0

#3  0x00007f31a21a4e74 in spdlog::register_logger(std::shared_ptr<spdlog::logger>) () from /opt/couchbase/bin/../lib/libmemcached_logger.so.1.0.0

#4  0x00007f31a219a615 in cb::logger::registerSpdLogger(std::shared_ptr<spdlog::logger>) () from /opt/couchbase/bin/../lib/libmemcached_logger.so.1.0.0

#5  0x0000000000437840 in ServerLogApi::register_spdlogger(std::shared_ptr<spdlog::logger>) ()

#6  0x00007f3199fd601d in BucketLogger::createBucketLogger(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) () from /opt/couchbase/bin/../lib/../lib/ep.so

#7  0x00007f3199feb75a in ConnHandler::ConnHandler(EventuallyPersistentEngine&, void const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) () from /opt/couchbase/bin/../lib/../lib/ep.so

#8  0x00007f319a037918 in DcpProducer::DcpProducer(EventuallyPersistentEngine&, void const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, unsigned int, bool) () from /opt/couchbase/bin/../lib/../lib/ep.so

#9  0x00007f319a019bc1 in DcpConnMap::newProducer(void const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, unsigned int) () from /opt/couchbase/bin/../lib/../lib/ep.so

#10 0x00007f319a080af1 in EventuallyPersistentEngine::dcpOpen(void const*, unsigned int, unsigned int, unsigned int, cb::const_char_buffer, cb::const_char_buffer) () from /opt/couchbase/bin/../lib/../lib/ep.so

#11 0x00007f319a080e80 in non-virtual thunk to EventuallyPersistentEngine::open(gsl::not_null<void const*>, unsigned int, unsigned int, unsigned int, cb::const_char_buffer, cb::const_char_buffer) () from /opt/couchbase/bin/../lib/../lib/ep.so

#12 0x000000000045ed0a in dcpOpen(Cookie&, unsigned int, unsigned int, unsigned int, cb::const_char_buffer, cb::const_char_buffer) ()

#13 0x00000000004b8806 in dcp_open_executor(Cookie&) ()

#14 0x00000000004ad2ef in execute_client_request_packet(Cookie&, cb::mcbp::Request const&) ()

#15 0x000000000049b8dc in Cookie::execute() ()

#16 0x00000000004ccf48 in StateMachine::conn_execute() ()

#17 0x000000000048db7c in Connection::runStateMachinery() ()

#18 0x000000000048dc1b in Connection::runEventLoop(short) ()

#19 0x000000000049a7bc in run_event_loop(Connection*, short) ()

#20 0x000000000042f607 in event_handler(int, short, void*) ()

#21 0x00007f31a0401107 in event_process_active_single_queue.isra.26 () from /opt/couchbase/bin/../lib/libevent_core.so.2.1.8

#22 0x00007f31a040167f in event_base_loop () from /opt/couchbase/bin/../lib/libevent_core.so.2.1.8

#23 0x000000000046ff59 in worker_libevent(void*) ()

#24 0x00007f31a1113f27 in platform_thread_wrap(void*) () from /opt/couchbase/bin/../lib/libplatform_so.so.0.1.0

#25 0x00007f319eb42e25 in start_thread () from /lib64/libpthread.so.0

#26 0x00007f319e86cbad in clone () from /lib64/libc.so.6