Transform optional enums to match pre kube 2.0.0 format

kube-core/src/schema.rs

@@ -5,9 +5,12 @@
 // Used in docs
 #[allow(unused_imports)] use schemars::generate::SchemaSettings;
 
-use schemars::{transform::Transform, JsonSchema};
+use schemars::{
+    transform::{transform_subschemas, Transform},
+    JsonSchema,
+};
 use serde::{Deserialize, Serialize};
-use serde_json::Value;
+use serde_json::{json, Value};
 use std::collections::{btree_map::Entry, BTreeMap, BTreeSet};
 
 /// schemars [`Visitor`] that rewrites a [`Schema`] to conform to Kubernetes' "structural schema" rules
@@ -27,6 +30,42 @@ use std::collections::{btree_map::Entry, BTreeMap, BTreeSet};
 #[derive(Debug, Clone)]
 pub struct StructuralSchemaRewriter;
 
+/// Recursively restructures JSON Schema objects so that the Option<Enum> object
+/// is returned per k8s CRD schema expectations.
+///
+/// In kube 2.x the schema output behavior for `Option<Enum>` types changed.
+///
+/// Previously given an enum like:
+///
+/// ```rust
+/// enum LogLevel {
+///     Debug,
+///     Info,
+///     Error,
+/// }
+/// ```
+///
+/// The following would be generated for Optional<LogLevel>:
+///
+/// ```json
+/// { "enum": ["Debug", "Info", "Error"], "type": "string", "nullable": true }
+/// ```
+///
+/// Now, schemars generates `anyOf` for `Option<LogLevel>` like:
+///
+/// ```json
+/// {
+///   "anyOf": [
+///     { "enum": ["Debug", "Info", "Error"], "type": "string" },
+///     { "enum": [null], "nullable": true }
+///   ]
+/// }
+/// ```
+///
+/// This transform implementation prevents this specific case from happening.
+#[derive(Debug, Clone, Default)]
+pub struct OptionalEnum;
+
 /// A JSON Schema.
 #[allow(clippy::large_enum_variant)]
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema)]
@@ -305,6 +344,41 @@ impl Transform for StructuralSchemaRewriter {
     }
 }
 
+impl Transform for OptionalEnum {
+    fn transform(&mut self, schema: &mut schemars::Schema) {
+        transform_subschemas(self, schema);
+
+        let Some(obj) = schema.as_object_mut().filter(|o| o.len() == 1) else {
+            return;
+        };
+
+        let arr = obj
+            .get("anyOf")
+            .iter()
+            .flat_map(|any_of| any_of.as_array())
+            .last()
+            .cloned()
+            .unwrap_or_default();
+
+        let [first, second] = arr.as_slice() else {
+            return;
+        };
+        let (Some(first), Some(second)) = (first.as_object(), second.as_object()) else {
+            return;
+        };
+
+        if first.contains_key("enum")
+            && !first.contains_key("nullable")
+            && second.get("enum") == Some(&json!([null]))
+            && second.get("nullable") == Some(&json!(true))
+        {
+            obj.remove("anyOf");
+            obj.append(&mut first.clone());
+            obj.insert("nullable".to_string(), Value::Bool(true));
+        }
+    }
+}
+
 /// Bring all plain enum values up to the root schema,
 /// since Kubernetes doesn't allow subschemas to define enum options.
 ///

kube-derive/src/custom_resource.rs

@@ -677,6 +677,7 @@ pub(crate) fn derive(input: proc_macro2::TokenStream) -> proc_macro2::TokenStrea
                 })
                 .with_transform(#schemars::transform::AddNullable::default())
                 .with_transform(#kube_core::schema::StructuralSchemaRewriter)
+                .with_transform(#kube_core::schema::OptionalEnum)
                 .into_generator();
             let schema = generate.into_root_schema_for::<Self>();
         }

kube-derive/tests/crd_schema_test.rs

@@ -9,6 +9,7 @@ use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 use std::collections::{HashMap, HashSet};
 
+
 // See `crd_derive_schema` example for how the schema generated from this struct affects defaulting and validation.
 #[derive(CustomResource, Serialize, Deserialize, Debug, PartialEq, Clone, KubeSchema)]
 #[kube(
@@ -77,6 +78,8 @@ struct FooSpec {
 
     #[x_kube(merge_strategy = ListMerge::Set)]
     x_kubernetes_set: Vec<String>,
+
+    optional_enum: Option<Gender>,
 }
 
 #[derive(Clone, Debug, PartialEq, Deserialize, Serialize, JsonSchema)]
@@ -187,6 +190,7 @@ fn test_serialized_matches_expected() {
             my_list: vec!["".into()],
             set: HashSet::from(["foo".to_owned()]),
             x_kubernetes_set: vec![],
+            optional_enum: Some(Gender::Other),
         }))
         .unwrap(),
         serde_json::json!({
@@ -222,6 +226,7 @@ fn test_serialized_matches_expected() {
                 "myList": [""],
                 "set": ["foo"],
                 "xKubernetesSet": [],
+                "optionalEnum": "Other",
             }
         })
     )
@@ -410,6 +415,15 @@ fn test_crd_schema_matches_expected() {
                                                 },
                                                 "x-kubernetes-list-type": "set",
                                             },
+                                            "optionalEnum": {
+                                                "nullable": true,
+                                                "type": "string",
+                                                "enum": [
+                                                    "Female",
+                                                    "Male",
+                                                    "Other"
+                                                ],
+                                            }
                                         },
                                         "required": [
                                             "complexEnum",