ValueError: buffer size must be strictly positive when opening an empty file in S3 with "rb" mode

[belltailjp]

In case we try to open an empty (zero-bytes) file on S3 in "rb" mode using pfio v2 API with the default buffering option, we get this error.

>>> import pfio; pfio.__version__
'2.2.0'
>>> with pfio.v2.open_url('s3://<bucket>/empty_file.dat', 'rb') as f:
...     pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.8/contextlib.py", line 113, in __enter__
    return next(self.gen)
  File "/usr/local/lib/python3.8/site-packages/pfio/v2/fs.py", line 310, in open_url
    with fs.open(filename, mode) as fp:
  File "/usr/local/lib/python3.8/site-packages/pfio/v2/s3.py", line 407, in open
    obj = io.BufferedReader(obj, buffer_size=bs)
ValueError: buffer size must be strictly positive

This is because
https://github.com/pfnet/pfio/blob/2.2.0/pfio/v2/s3.py#L401-L407

    bs = min(obj.content_length, DEFAULT_MAX_BUFFER_SIZE)

this will make bs to be zero, which is then passed to BufferedReader as the buffer size causing the error.

In case the content length of the object to be read is zero, we of course don't need buffering at all, so I think we can simply skip wrapping obj by BufferedReader.