FIX: do not cast _FillValue/missing_value in CFMaskCoder if _Unsigned is provided (#8852)

* FIX: do not cast _FillValue/missing_value in CFMaskCoder if _Unsigned is provided

* add whats-new.rst entry

* add comment on _Unsigned

Author: kmuehlbauer

doc/whats-new.rst

@@ -57,6 +57,9 @@ Bug fixes
   `CFMaskCoder`/`CFScaleOffsetCoder` (:issue:`2304`, :issue:`5597`,
   :issue:`7691`, :pull:`8713`, see also discussion in :pull:`7654`).
   By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
+- do not cast `_FillValue`/`missing_value` in `CFMaskCoder` if `_Unsigned` is provided
+  (:issue:`8844`, :pull:`8852`).
+  By `Kai Mühlbauer <https://github.com/kmuehlbauer>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/coding/variables.py

@@ -309,6 +309,9 @@ def encode(self, variable: Variable, name: T_Name = None):
         dtype = np.dtype(encoding.get("dtype", data.dtype))
         fv = encoding.get("_FillValue")
         mv = encoding.get("missing_value")
+        # to properly handle _FillValue/missing_value below [a], [b]
+        # we need to check if unsigned data is written as signed data
+        unsigned = encoding.get("_Unsigned") is not None
 
         fv_exists = fv is not None
         mv_exists = mv is not None
@@ -323,13 +326,19 @@ def encode(self, variable: Variable, name: T_Name = None):
 
         if fv_exists:
             # Ensure _FillValue is cast to same dtype as data's
-            encoding["_FillValue"] = dtype.type(fv)
+            # [a] need to skip this if _Unsigned is available
+            if not unsigned:
+                encoding["_FillValue"] = dtype.type(fv)
             fill_value = pop_to(encoding, attrs, "_FillValue", name=name)
 
         if mv_exists:
             # try to use _FillValue, if it exists to align both values
             # or use missing_value and ensure it's cast to same dtype as data's
-            encoding["missing_value"] = attrs.get("_FillValue", dtype.type(mv))
+            # [b] need to provide mv verbatim if _Unsigned is available
+            encoding["missing_value"] = attrs.get(
+                "_FillValue",
+                (dtype.type(mv) if not unsigned else mv),
+            )
             fill_value = pop_to(encoding, attrs, "missing_value", name=name)
 
         # apply fillna
@@ -522,7 +531,6 @@ def encode(self, variable: Variable, name: T_Name = None) -> Variable:
     def decode(self, variable: Variable, name: T_Name = None) -> Variable:
         if "_Unsigned" in variable.attrs:
             dims, data, attrs, encoding = unpack_for_decoding(variable)
-
             unsigned = pop_to(attrs, encoding, "_Unsigned")
 
             if data.dtype.kind == "i":