fix(OneOf): fail coercion when the single OneOf field is invalidly provided by a default

Also provides the previously omitted undefined key in the execution
test for accepting a good variable with an undefined key.

Replicates graphql/graphql-js@5042e18

Author: Cito

src/graphql/utilities/coerce_input_value.py

@@ -72,11 +72,13 @@ def coerce_input_value(input_value: Any, type_: GraphQLInputType) -> Any:
 
         coerced_dict: dict[str, Any] = {}
         fields = type_.fields
-        if any(
-            field_value is not Undefined and field_name not in fields
-            for field_name, field_value in input_value.items()
-        ):
-            return Undefined  # Invalid: intentionally return no value.
+        defined_field_count = 0
+        for field_name, field_value in input_value.items():
+            if field_value is Undefined:
+                continue
+            defined_field_count += 1
+            if field_name not in fields:
+                return Undefined  # Invalid: intentionally return no value.
         for field_name, field in fields.items():
             field_value = input_value.get(field_name, Undefined)
             if field_value is Undefined:
@@ -94,8 +96,8 @@ def coerce_input_value(input_value: Any, type_: GraphQLInputType) -> Any:
 
         if type_.is_one_of:
             keys = list(coerced_dict)
-            if len(keys) != 1:
-                # Invalid: not exactly one key, intentionally return no value.
+            if defined_field_count != 1 or len(keys) != 1:
+                # Invalid: intentionally return no value.
                 return Undefined
             if coerced_dict[keys[0]] is None:
                 # Invalid: value not non-null, intentionally return no value.

tests/execution/test_oneof.py

@@ -4,6 +4,7 @@
 
 from graphql.execution import ExecutionResult, execute
 from graphql.language import parse
+from graphql.pyutils import Undefined
 from graphql.utilities import build_schema
 
 if TYPE_CHECKING:
@@ -97,7 +98,9 @@ def accepts_a_good_variable_with_an_undefined_key():
                   }
                 }
                 """
-            result = execute_query(query, root_value, {"input": {"a": "abc"}})
+            result = execute_query(
+                query, root_value, {"input": {"a": "abc", "b": Undefined}}
+            )
 
             assert result == ({"test": {"a": "abc", "b": None}}, None)
 

tests/utilities/test_coerce_input_value.py

@@ -194,6 +194,16 @@ def describe_for_graphql_input_object_that_is_one_of():
             is_one_of=True,
         )
 
+        TestInvalidOneOfInputObjectWithDefault = GraphQLInputObjectType(
+            "TestInvalidOneOfInputObjectWithDefault",
+            {
+                "foo": GraphQLInputField(
+                    GraphQLInt, default=GraphQLDefaultInput(value=123)
+                ),
+            },
+            is_one_of=True,
+        )
+
         def returns_for_valid_input():
             _test({"foo": 123}, TestInputObject, {"foo": 123})
 
@@ -203,6 +213,16 @@ def invalid_if_more_than_one_field_is_specified():
         def invalid_if_the_one_field_is_null():
             _test({"bar": None}, TestInputObject, Undefined)
 
+        def invalid_if_an_omitted_field_would_be_filled_by_a_default():
+            _test({}, TestInvalidOneOfInputObjectWithDefault, Undefined)
+
+        def invalid_if_an_undefined_field_would_be_filled_by_a_default():
+            _test(
+                {"foo": Undefined},
+                TestInvalidOneOfInputObjectWithDefault,
+                Undefined,
+            )
+
         def invalid_for_an_invalid_field():
             _test({"foo": nan}, TestInputObject, Undefined)