Secondary couldn't signal OplogWaiters to advance the lastCommittedOpTime in Chained replication mode

A replSet has three members, aa/bb/cc. In this situation , aa->bb->cc , aa is primary and the sync source of bb , bb is the sync source of cc . aa advances the lastCommittedOpTime , and it signal OplogWaiters , so bb can advance his lastCommittedOpTime , but sometimes cc couldn't advance its lastCommittedOpTime.
 
The key problem here is the _localOplogCollection always is NULL for secondary.

@oplog.cpp 

void signalOplogWaiters() { 

if (_localOplogCollection) { 

_localOplogCollection->notifyCappedWaitersIfNeeded(); 

} 

}

the only way to set _localOplogCollection is in function getLocalOplogCollection，and called here.

void logOp(OperationContext* txn, 

const char* opstr, 

const char* ns, 

const BSONObj& obj, 

const BSONObj* o2, 

bool fromMigrate) 

void logOps(OperationContext* txn, 

const char* opstr, 

const NamespaceString& nss, 

std::vector<BSONObj>::const_iterator begin, 

std::vector<BSONObj>::const_iterator end, 

bool fromMigrate)

but for secondary ，it returned early because of oplog Disabled. 

So because _localOplogCollection = NULL , when secondary update _lastCommittedOpTime , it cann't signal OplogWaiters.