tar support

[moskomule]

Hi, the document says

PFIO supports various container file formats to aggregate many small files into single large file with metadata mapping, e.g. HDF5, ZIP and Tar (and more in future)

but does PFIO currently support tar files at the moment or plan to add its support in the future?

When I quickly try it as follows, pfio did not recognize a tar file:

with pfio.v2.from_url('file:///absolute_path/to/tar_file.tar') as f:
    # raises ValueError: /absolute_path/to/tar_file.tar must be a directory
    ...

[kuenishi]

I have been hesitating to support tar so far, because the format of tar needs seeking every head of the file entry to create directory tree map inside the tarball. The expected implementation of tar container will be very inefficient at the very beginning of opening the tarball as a PFIO container. Supporting tar format aiming for small tarballs for utility won't be bad, but it's in lower priority for us. Absolutely, contribution will be welcomed.
See also: https://superuser.com/a/538798