Crash tearing down tap client

Perhaps a double-shutdown?

Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x00000000000000d8
Crashed Thread: 0 Dispatch queue: com.apple.main-thread

Thread 0 Crashed: Dispatch queue: com.apple.main-thread
0 libSystem.B.dylib 0x00007fff80119b37 pthread_mutex_lock + 31
1 ep.0.so 0x000000010020301d Mutex::acquire() + 33 (mutex.hh:46)
2 ep.0.so 0x0000000100203128 LockHolder::lock() + 24 (locks.hh:31)
3 ep.0.so 0x0000000100215cf0 LockHolder::LockHolder(Mutex&) + 44 (locks.hh:20)
4 ep.0.so 0x000000010021a17a TapClientConnection::stop() + 30 (tapclient.hh:111)
5 ep.0.so 0x000000010021a23e EventuallyPersistentEngine::stopReplication() + 80 (ep_engine.h:978)
6 ep.0.so 0x000000010021a38b EventuallyPersistentEngine::destroy() + 21 (ep_engine.h:406)
7 ep.0.so 0x000000010021025e EvpDestroy + 29 (ep_engine.cc:48)
8 engine_testapp 0x00000001000016aa destroy_engine + 26 (engine_testapp.c:103)
9 engine_testapp 0x0000000100001f0e main + 1902 (engine_testapp.c:139)
10 engine_testapp 0x0000000100001684 start + 52