Inverse of keyToFileName: recover the cache key and hash tag a file
name encodes, or null when the name is not one this encoding can produce (a
foreign or truncated file). Round-trips through keyToFileName so only a
canonical encoding is accepted. Used by the orphan reconcile to re-index chunk
files the persisted index never recorded.
Inverse of keyToFileName: recover the cache key and hash tag a file name encodes, or
nullwhen the name is not one this encoding can produce (a foreign or truncated file). Round-trips throughkeyToFileNameso only a canonical encoding is accepted. Used by the orphan reconcile to re-index chunk files the persisted index never recorded.