id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
427768540,MDU6SXNzdWU0Mjc3Njg1NDA=,2862,cannot properly .close() a dataset opened with `chunks` argument?,24368151,open,0,,,2,2019-04-01T15:23:06Z,2019-04-08T12:29:48Z,,NONE,,,,"I want to do operations on a copy of a dataset and then overwrite the NetCDF it was read from:
```python
f = xr.open_dataset('dataset.nc')
n = f.copy(deep=True)
f.close()
n.to_netcdf(path='dataset.nc')
```
#### Problem description
The above works, however if I use the `chunks` argument while opening the dataset, a `KeyError: 'tcw'` is thrown. The NetCDF on disk is also corrupted. It happens both with `deep=True` or `deep=False` .

#### Expected Output
Although not an expert of Dask, it kind of makes sense that close doesn't really close when evaluated lazily if I still need things from it afterwards, so maybe that is the correct behaviour and we should just handle the exception ? or should I `compute()` after closing? really not sure. 

#### Output of ``xr.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None

xarray: 0.11.0
pandas: 0.24.1
numpy: 1.15.4
scipy: None
netCDF4: 1.4.2
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: 1.0.3.4
PseudonetCDF: None
rasterio: 1.0.13
iris: None
bottleneck: None
cyordereddict: None
dask: 1.1.4
distributed: 1.26.0
matplotlib: None
cartopy: None
seaborn: None
setuptools: 40.7.3
pip: 19.0.1
conda: None
pytest: 4.2.1
IPython: 7.2.0
sphinx: None
</details>
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