Issue spawned from this thread:
When IndexFileDeleter lists the directory, looking for segments_X
files to load, if it hits a FNFE on opening such a file it should
catch this and treat it as if the file does not exist.
On NFS (and possibly other file systems), a directory listing is not
guaranteed to be "current"/coherent. Specifically, if machine #1 has
just removed file "segments_n" and shortly thereafer machine #2 does a
dir listing, it's possible (likely?) that the dir listing will still
show that segments_n exists.
I think the fix is simple: catch the FNFE and just handle it as if the
segments_n does not in fact exist.