It looks like as of 4.2 the renameCollection command supports doing a rename - even with dropTarget: true - with only a MODE_IX lock, not MODE_X as is currently taken. At the very least, that lock acquisition could use a comment indicating why MODE_X is required.

I think if we change the DB lock to MODE_X, we'll need to acquire both collection locks in MODE_X (and in the right order) to inspect and verify the index catalog and collection options, but that should be fine and is an improvement.