(lldb) f 1
frame #1: 0x0000000108f94b60 mongo`mongo::mozjs::MozJSImplScope::registerOperation(this=0x00007f9adc800400, txn=0x0000000000000000) + 16 at implscope.cpp:139
   136 	
   137 	void MozJSImplScope::registerOperation(OperationContext* txn) {
   138 	    invariant(_opId == 0);
-> 139 	    _opId = txn->getOpID();
   140 	
   141 	    _engine->registerOperation(txn, this);
   142 	}
(lldb) p txn
(mongo::OperationContext *) $0 = 0x0000000000000000

* thread #25: tid = 0x0018, 0x0000000108f94b60 mongo`mongo::mozjs::MozJSImplScope::registerOperation(mongo::OperationContext*) [inlined] mongo::OperationContext::getOpID(this=0x0000000000000000) const at operation_context.h:156, stop reason = signal SIGSTOP
  * frame #0: 0x0000000108f94b60 mongo`mongo::mozjs::MozJSImplScope::registerOperation(mongo::OperationContext*) [inlined] mongo::OperationContext::getOpID(this=0x0000000000000000) const at operation_context.h:156
    frame #1: 0x0000000108f94b60 mongo`mongo::mozjs::MozJSImplScope::registerOperation(this=0x00007f9adc800400, txn=0x0000000000000000) + 16 at implscope.cpp:139
    frame #2: 0x0000000108fbbbf9 mongo`mongo::mozjs::MozJSProxyScope::implThread(void*) [inlined] std::__1::function<void ()>::operator()() const + 393 at functional:1756
    frame #3: 0x0000000108fbbbe1 mongo`mongo::mozjs::MozJSProxyScope::implThread(arg=0x00007f9adea22f70) + 369 at proxyscope.cpp:351
    frame #4: 0x0000000108f8692c mongo`nspr::Thread::ThreadRoutine(arg=0x00007f9adea21fd0) + 28 at PosixNSPR.cpp:56
    frame #5: 0x0000000108f86f61 mongo`void* std::__1::__thread_proxy<std::__1::tuple<void* (*)(void*), nspr::Thread*> >(void*) [inlined] decltype(__f=<unavailable>)(void*)>(fp)(std::__1::forward<nspr::Thread*>(fp0))) std::__1::__invoke<void* (*)(void*), nspr::Thread*>(void* (*&&)(void*), nspr::Thread*&&) + 97 at __functional_base:413
    frame #6: 0x0000000108f86f58 mongo`void* std::__1::__thread_proxy<std::__1::tuple<void* (*)(void*), nspr::Thread*> >(void*) [inlined] void std::__1::__thread_execute<void* (*)(void*), nspr::Thread*, 1ul>(std::__1::tuple<void* (*)(void*), nspr::Thread*>&, std::__1::__tuple_indices<1ul>) at thread:332
    frame #7: 0x0000000108f86f58 mongo`void* std::__1::__thread_proxy<std::__1::tuple<void* (*)(void*), nspr::Thread*> >(__vp=0x00007f9adea21ee0) + 88 at thread:342
    frame #8: 0x00007fff8373f05a libsystem_pthread.dylib`_pthread_body + 131
    frame #9: 0x00007fff8373efd7 libsystem_pthread.dylib`_pthread_start + 176
    frame #10: 0x00007fff8373c3ed libsystem_pthread.dylib`thread_start + 13