KeyError: 'lon_bnds' raised occasionally when opening (mostly NetCDF) datasets

[forman]

Describe the bug

We sometimes see KeyError: 'lon_bnds' raised when opening datasets, mostly NetCDF. Cause of the problem is that an (internal) xcube function get_dataset_bounds() expects bounds coordinates, e.g. lon_bnds, to always be in xarray.Dataset.coords. However, xarray often puts such coordinates in xarray.Dataset.data_vars. The name of bounds coordinates are usually given by the value of a coordinate attribute named bounds - according to CF conventions.

Expected behavior

1. check if given bounds coordinate is contained in dataset
2. get bounds coordinate from xarray.Dataset.variables, not xarray.Dataset.coords

Traceback

...
File ~/projects/xcube/xcube/core/store/descriptor.py:432, in _determine_bbox(data)
    429 def _determine_bbox(data: xr.Dataset) \
    430         -> Optional[Tuple[float, float, float, float]]:
    431     try:
--> 432         return get_dataset_bounds(data)
    433     except ValueError:
    434         if 'geospatial_lon_min' in data.attrs and \
    435                 'geospatial_lat_min' in data.attrs and \
    436                 'geospatial_lon_max' in data.attrs and \
    437                 'geospatial_lat_max' in data.attrs:

File ~/projects/xcube/xcube/core/geom.py:726, in get_dataset_bounds(dataset, xy_var_names)
    724 x_bnds_name = get_dataset_bounds_var_name(dataset, x_name)
    725 if x_bnds_name:
--> 726     x_bnds_var = dataset.coords[x_bnds_name]
    727     x1 = x_bnds_var[0, 0]
    728     x2 = x_bnds_var[0, 1]

File ~/mamba/envs/xcube/Lib/site-packages/xarray/core/coordinates.py:581, in DatasetCoordinates.__getitem__(self, key)
    579 def __getitem__(self, key: Hashable) -> DataArray:
    580     if key in self._data.data_vars:
--> 581         raise KeyError(key)
    582     return self._data[key]

KeyError: 'lon_bnds'