Hangs while saving netcdf file opened using xr.open_mfdataset with lock=None

I am testing out code that uses xarray to process netcdf files, in particular to join multiple netcdf files into one along shared dimensions. This was working well, except sometimes when saving the netcdf file the process would hang.

I was able to whittle it down to this simple example: https://github.com/jessicaaustin/xarray_netcdf_hanging_issue

This is the code snippet at the core of the example:

 # If you set lock=False then this runs fine every time.
 # Setting lock=None causes it to intermittently hang on mfd.to_netcdf
 with xr.open_mfdataset(['dataset.nc'], combine='by_coords', lock=None) as mfd:
     p = os.path.join('tmp', 'xarray_{}.nc'.format(uuid.uuid4().hex))
     print(f"Writing data to {p}")
     mfd.to_netcdf(p)
     print("complete")

If you run this once, it's typically fine. But run it over and over again in a loop, and it'll eventually hang on mfd.to_netcdf. However if I set lock=False then it runs fine every time.

I've seen this with the following combos:

• xarray=0.14.1
• dask=2.9.1
• netcdf4=1.5.3

and

• xarray=0.15.1
• dask=2.14.0
• netcdf4=1.5.3

And I've tried it with different netcdf files and different computers.

Versions

Start by running the linked minimal example repeatedly with xr.open_mfdataset(..., lock=None) and mfd.to_netcdf, then compare it with lock=False using the reported xarray, dask, and netCDF4 versions. Trace the open_mfdataset and to_netcdf entry points to isolate the intermittent lock behavior. Done means repeated saves no longer hang and regression coverage preserves the working lock=False case.