html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/3781#issuecomment-702348129,https://api.github.com/repos/pydata/xarray/issues/3781,702348129,MDEyOklzc3VlQ29tbWVudDcwMjM0ODEyOQ==,7933853,2020-10-01T19:24:48Z,2020-10-01T20:00:27Z,NONE,"I think I ran into a similar problem when combining dask-chunked DataSets (originating from `open_mfdataset`) with Python's native `multiprocessing` package. I get no error message, and the headers of the files are created, but then the script hangs indefinitely. The use case is: combining and resampling of variables into ~1000 different NetCDF files, which I want to distribute over different processes using `multiprocessing`. 

**MCVE Code Sample**
```python
import xarray as xr
from multiprocessing import Pool
import os

if (False): 
    """"""
    Load data without using dask
    """"""
    ds = xr.open_dataset(""http://www.esrl.noaa.gov/psd/thredds/dodsC/Datasets/ncep.reanalysis/surface/air.sig995.1960.nc"")
else:
    """"""
    Load data using dask
    """"""
    ds = xr.open_dataset(""http://www.esrl.noaa.gov/psd/thredds/dodsC/Datasets/ncep.reanalysis/surface/air.sig995.1960.nc"", chunks={})

print(ds.nbytes / 1e6, 'MB')

print('chunks', ds.air.chunks) # chunks is empty without dask

outdir = '/glade/scratch/lvank' # change this to some temporary directory on your system

def do_work(n):
    print(n)
    ds.to_netcdf(os.path.join(outdir, f'{n}.nc'))

tasks = range(10)

with Pool(processes=2) as pool:
    pool.map(do_work, tasks)
    
print('done')
```

**Expected Output**
The NetCDF copies in `outdir` named `0.nc` to `9.nc` should be created for both cases (with and without Dask). 

**Problem Description**
In the case with Dask, when the if-statement evaluates to `False`, the files are not created and the program hangs. 

**Output of xr.show_versions()**
<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.8.5 (default, Sep  4 2020, 07:30:14) 
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 3.10.0-1127.13.1.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: 1.10.4
libnetcdf: 4.7.3

xarray: 0.16.1
pandas: 1.1.1
numpy: 1.19.1
scipy: 1.5.2
netCDF4: 1.5.3
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: 1.2.1
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.27.0
distributed: 2.28.0
matplotlib: 3.3.1
cartopy: None
seaborn: None
numbagg: None
pint: None
setuptools: 49.6.0.post20200925
pip: 20.2.2
conda: None
pytest: None
IPython: 7.18.1
sphinx: None
```
</details>","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,567678992