I'm not too familer with NFS, but have you checked from another machine that the file is locked? It could be that your local machine considers it locked, but the filesystem as a whole doesn't.

Also, I belive there are a few OSes that force locking for everyone (by making the file readable only by a random uid that the process runs under, IIRC), but Linux isn't one of them.

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Reinvent a rounder wheel.

Note: All code is untested, unless otherwise stated