Conflicting _FillValue and missing_value on write

What happened?

see also #7191

If missing_value and _FillValue is an attribute of a DataArray it can't be written out to file if these two contradict:

ValueError: Variable 'test' has conflicting _FillValue (nan) and missing_value (1.0). Cannot encode data.

This happens, if missing_value is an attribute of a specific netCDF Dataset of an existing file. On read the missing_value will be masked with np.nan on the data and it will be preserved within encoding. On write, _FillValue will be added as attribute by xarray (if not available, at least for floating point types), too. So far so good.

The error first manifests if you read back this file and try to write it again. There is no warning on the second read, that the two _FillValue and missing_value are differing. Only on the second write.

What did you expect to happen?

The file should be written on the second roundtrip.

There are at least two solutions to this:

1. Mask missing_value on read and purge missing_value completely in favor of _FillValue.
2. Do not handle missing_value at all, but let the user take action.

Minimal Complete Verifiable Example

import numpy as np
import netCDF4 as nc
import xarray as xr

with nc.Dataset("test-no-fillval-01.nc", mode="w") as ds:
    x = ds.createDimension("x", 4)
    test = ds.createVariable("test", "f4", ("x",), fill_value=None)
    test.missing_value = 1.
    test.valid_min = 2.
    test.valid_max = 10.
    test[:] = np.array([0.0, np.nan, 1.0, 8.0], dtype="f4")
with nc.Dataset("test-no-fillval-01.nc") as ds:
    print(ds["test"])
    print(ds["test"][:])


with xr.open_dataset("test-no-fillval-01.nc").load() as roundtrip:
    print(roundtrip)
    print(roundtrip["test"].attrs)
    print(roundtrip["test"].encoding)
    roundtrip.to_netcdf("test-no-fillval-02.nc")

with xr.open_dataset("test-no-fillval-02.nc").load() as roundtrip:
    print(roundtrip)
    print(roundtrip["test"].attrs)
    print(roundtrip["test"].encoding)
    roundtrip.to_netcdf("test-no-fillval-03.nc")

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

<class 'netCDF4._netCDF4.Variable'>
float32 test(x)
    missing_value: 1.0
    valid_min: 2.0
    valid_max: 10.0
unlimited dimensions: 
current shape = (4,)
filling on, default _FillValue of 9.969209968386869e+36 used

<xarray.Dataset>
Dimensions:  (x: 4)
Dimensions without coordinates: x
Data variables:
    test     (x) float32 0.0 nan nan 8.0
{'valid_min': 2.0, 'valid_max': 10.0}
{'zlib': False, 'szip': False, 'zstd': False, 'bzip2': False, 'blosc': False, 'shuffle': False, 'complevel': 0, 'fletcher32': False, 'contiguous': True, 'chunksizes': None, 'source': 'test-no-fillval-01.nc', 'original_shape': (4,), 'dtype': dtype('float32'), 'missing_value': 1.0}

<xarray.Dataset>
Dimensions:  (x: 4)
Dimensions without coordinates: x
Data variables:
    test     (x) float32 0.0 nan nan 8.0
{'valid_min': 2.0, 'valid_max': 10.0}
{'zlib': False, 'szip': False, 'zstd': False, 'bzip2': False, 'blosc': False, 'shuffle': False, 'complevel': 0, 'fletcher32': False, 'contiguous': True, 'chunksizes': None, 'source': 'test-no-fillval-02.nc', 'original_shape': (4,), 'dtype': dtype('float32'), 'missing_value': 1.0, '_FillValue': nan}

File /home/kai/miniconda/envs/xarray_311/lib/python3.11/site-packages/xarray/coding/variables.py:167, in CFMaskCoder.encode(self, variable, name)
    160 mv = encoding.get("missing_value")
    162 if (
    163     fv is not None
    164     and mv is not None
    165     and not duck_array_ops.allclose_or_equiv(fv, mv)
    166 ):
--> 167     raise ValueError(
    168         f"Variable {name!r} has conflicting _FillValue ({fv}) and missing_value ({mv}). Cannot encode data."
    169     )
    171 if fv is not None:
    172     # Ensure _FillValue is cast to same dtype as data's
    173     encoding["_FillValue"] = dtype.type(fv)

ValueError: Variable 'test' has conflicting _FillValue (nan) and missing_value (1.0). Cannot encode data.

Anything else we need to know?

The adding of _FillValue on write happens here:

https://github.com/pydata/xarray/blob/d4db16699f30ad1dc3e6861601247abf4ac96567/xarray/conventions.py#L300

https://github.com/pydata/xarray/blob/d4db16699f30ad1dc3e6861601247abf4ac96567/xarray/conventions.py#L144-L152

Environment

Start with the _FillValue handling in xarray/conventions.py at the linked lines, then inspect CFMaskCoder.encode in xarray/coding/variables.py where the conflict is raised. Reproduce the issue with the provided netCDF4/xarray MVCE and add focused coverage for the second roundtrip. Done means the demonstrated dataset can be written successfully through the second roundtrip, with the missing-value behavior made explicit.