validate K8s resource references against the RFC 1123 hostname format Kubernetes uses

Author: ThoSap

operator/build.gradle.kts

@@ -20,6 +20,7 @@ dependencies {
      * Fabric8 Kubernetes Client
      */
     implementation("io.fabric8:generator-annotations")
+    implementation("io.fabric8:crd-generator-api-v2")
 
     /**
      * jOOQ

operator/src/main/java/it/aboutbits/postgresql/core/ClusterReference.java

@@ -1,5 +1,6 @@
 package it.aboutbits.postgresql.core;
 
+import io.fabric8.crdv2.generator.v1.SchemaCustomizer;
 import io.fabric8.generator.annotation.Required;
 import io.fabric8.generator.annotation.ValidationRule;
 import lombok.Getter;
@@ -10,6 +11,7 @@
 @NullMarked
 @Getter
 @Setter
+@SchemaCustomizer(HostnameRFC1123Customizer.class)
 public class ClusterReference {
     @Required
     @ValidationRule(

operator/src/main/java/it/aboutbits/postgresql/core/HostnameRFC1123Customizer.java

@@ -0,0 +1,81 @@
+package it.aboutbits.postgresql.core;
+
+import io.fabric8.crdv2.generator.v1.SchemaCustomizer;
+import io.fabric8.kubernetes.api.model.apiextensions.v1.JSONSchemaProps;
+import io.fabric8.kubernetes.client.utils.KubernetesSerialization;
+import org.jspecify.annotations.NullMarked;
+
+import java.util.Arrays;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+/// A [SchemaCustomizer.Customizer] that sets the `format` of string properties
+/// to `"hostname"` (RFC 1123) in the generated CRD JSON Schema.
+///
+/// This customizer is intended to be used with the
+/// [@SchemaCustomizer][SchemaCustomizer] annotation on a class whose properties
+/// should be validated as RFC 1123 hostnames by the Kubernetes API server.
+///
+/// ### Behavior
+///
+/// - If `input` is **blank** (the default), the `"hostname"` format
+///   is applied to **all** string properties of the annotated class.
+/// - If `input` contains a **comma-separated list** of field names,
+///   the format is applied **only** to the specified properties.
+///
+/// ### Usage examples
+///
+/// **Apply to all string properties:**
+///
+/// ```java
+/// @SchemaCustomizer(HostnameRFC1123Customizer.class)
+/// public class SecretRef {
+///     private String name = ""; // gets format: "hostname"
+///     private String namespace; // gets format: "hostname"
+/// }
+/// ```
+///
+/// **Apply to specific properties only:**
+///
+/// ```java
+/// @SchemaCustomizer(value = HostnameRFC1123Customizer.class, input = "host")
+/// public class ClusterConnectionSpec {
+///     private String host = "";        // gets format: "hostname"
+///     private String anotherHost = ""; // gets format: "hostname"
+///     private String database = "";    // unchanged
+/// }
+/// ```
+///
+/// @see SchemaCustomizer
+/// @see SchemaCustomizer.Customizer
+@NullMarked
+public class HostnameRFC1123Customizer implements SchemaCustomizer.Customizer {
+    @Override
+    public JSONSchemaProps apply(
+            JSONSchemaProps jsonSchemaProps,
+            String input,
+            KubernetesSerialization kubernetesSerialization
+    ) {
+        var properties = jsonSchemaProps.getProperties();
+        if (properties == null) {
+            return jsonSchemaProps;
+        }
+
+        var targetFields = input.isBlank()
+                ? Set.<String>of()
+                : Arrays.stream(input.split(","))
+                        .map(String::trim)
+                        .collect(Collectors.toSet());
+
+        for (var entry : properties.entrySet()) {
+            var prop = entry.getValue();
+            if ("string".equals(prop.getType())) {
+                if (targetFields.isEmpty() || targetFields.contains(entry.getKey())) {
+                    prop.setFormat("hostname");
+                }
+            }
+        }
+
+        return jsonSchemaProps;
+    }
+}

operator/src/main/java/it/aboutbits/postgresql/core/SecretRef.java

@@ -1,5 +1,6 @@
 package it.aboutbits.postgresql.core;
 
+import io.fabric8.crdv2.generator.v1.SchemaCustomizer;
 import io.fabric8.generator.annotation.Required;
 import io.fabric8.generator.annotation.ValidationRule;
 import lombok.Getter;
@@ -10,6 +11,7 @@
 @NullMarked
 @Getter
 @Setter
+@SchemaCustomizer(HostnameRFC1123Customizer.class)
 public class SecretRef {
     @Required
     @ValidationRule(

operator/src/main/java/it/aboutbits/postgresql/crd/clusterconnection/ClusterConnectionSpec.java

@@ -1,10 +1,12 @@
 package it.aboutbits.postgresql.crd.clusterconnection;
 
+import io.fabric8.crdv2.generator.v1.SchemaCustomizer;
 import io.fabric8.generator.annotation.Max;
 import io.fabric8.generator.annotation.Min;
 import io.fabric8.generator.annotation.Required;
 import io.fabric8.generator.annotation.ValidationRule;
 import it.aboutbits.postgresql.core.SecretRef;
+import it.aboutbits.postgresql.core.HostnameRFC1123Customizer;
 import lombok.Getter;
 import lombok.Setter;
 import org.jspecify.annotations.NullMarked;
@@ -15,6 +17,7 @@
 @NullMarked
 @Getter
 @Setter
+@SchemaCustomizer(value = HostnameRFC1123Customizer.class, input = "host")
 public class ClusterConnectionSpec {
     @Required
     @ValidationRule(