Lucene appears to be inconsistent on this behaviour; the FSDirectory documents a FileNotfoundException but actually returns 0 while the RAMDirectory implements the FileNotfoundException but is totally synchronized,
I assume the FSDirectory is more reliable in this case and we got a reported error about this, my rationale is that the race condition which FSDirectory can handle is never an issue in the synchronized blocks of RAMDirectory.

Inspecting the code of Lucene, there appears to be no harm done in returning 0 for unexistend metadata.