// rangeReader calls ioutil.ReadAll on the given byte range [start, n). It uses
// optimizations for different kinds of VFSs.
func rangeReader(fs rwvfs.FileSystem, name string, f io.ReadSeeker, start, n int64) (io.Reader, error) {
if fs, ok := fs.(rwvfs.FetcherOpener); ok {
// Clone f so we can parallelize it.
var err error
f, err = fs.OpenFetcher(name)
if err != nil {
return nil, err
}
if err := f.(rwvfs.Fetcher).Fetch(start, start+n); err != nil {
return nil, err
}
}
if _, err := f.Seek(start, 0); err != nil {
return nil, err
}
return f, nil
}