rasterio chunks argument causes loading from s3 to fail

[rabernat]

Code Sample, a copy-pastable example if possible

# This works
url = 's3://landsat-pds/L8/139/045/LC81390452014295LGN00/LC81390452014295LGN00_B1.TIF'
ds = xr.open_rasterio(url)
# this doesn't
ds = xr.open_rasterio(url, chunks=512)

The error is

---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-17-8b55d7e920b8> in <module>()
      6 # https://aws.amazon.com/public-datasets/landsat/
      7 # 512x512 chunking
----> 8 ds = xr.open_rasterio(url, chunks=512)
      9 ds

~/miniconda3/envs/geo_scipy/lib/python3.6/site-packages/xarray-0.10.0-py3.6.egg/xarray/backends/rasterio_.py in open_rasterio(filename, chunks, cache, lock)
    172         from dask.base import tokenize
    173         # augment the token with the file modification time
--> 174         mtime = os.path.getmtime(filename)
    175         token = tokenize(filename, mtime, chunks)
    176         name_prefix = 'open_rasterio-%s' % token

~/miniconda3/envs/geo_scipy/lib/python3.6/genericpath.py in getmtime(filename)
     53 def getmtime(filename):
     54     """Return the last modification time of a file, reported by os.stat()."""
---> 55     return os.stat(filename).st_mtime
     56 
     57 

FileNotFoundError: [Errno 2] No such file or directory: 's3://landsat-pds/L8/139/045/LC81390452014295LGN00/LC81390452014295LGN00_B1.TIF'

Problem description

It is pretty clear that the current xarray code expects to receive a filename. (The name of the argument is filename.) But rasterio's open function accepts a much wider range of dataset identifiers. The tokenizing function should be updated to allow for this. Seems like it should be a pretty easy fix.

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.2.final.0 python-bits: 64 OS: Darwin OS-release: 16.7.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8

xarray: 0.10.0
pandas: 0.20.3
numpy: 1.13.1
scipy: 0.19.1
netCDF4: 1.3.1
h5netcdf: 0.4.1
Nio: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.16.0
matplotlib: 2.1.0
cartopy: 0.15.1
seaborn: 0.8.1
setuptools: 36.3.0
pip: 9.0.1
conda: None
pytest: 3.2.1
IPython: 6.1.0
sphinx: 1.6.5

[rabernat]

Note that, in order for this s3 example to work, you need to have boto installed and aws credentials enabled:
https://aws.amazon.com/developers/getting-started/python/