Problem

The cloneCollection command calls the copyCollection method on the Cloner class

https://github.com/mongodb/mongo/blob/master/src/mongo/db/commands/clone_collection.cpp#L143

Cloner cloner;
...
return cloner.copyCollection(txn, collection, query, errmsg, true, false, copyIndexes);

The Cloner class does the following

https://github.com/mongodb/mongo/blob/master/src/mongo/db/cloner.cpp#L113

            // XXX: can probably take dblock instead
            Lock::GlobalWrite lk(txn->lockState());

Solution

cloneCollection (since the source is a different server) probably only needs to take out a DB_X on the local (i.e. destination) server. However, all usages of should be checked in case the lock type has to be determined by the caller function (in this case cloneCollection)