fix(avro): order "null" first in union when default is null

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AvroSchemaCodegen.java

@@ -16,6 +16,7 @@
 
 package org.openapitools.codegen.languages;
 
+import com.fasterxml.jackson.databind.JsonNode;
 import io.swagger.v3.oas.models.media.Schema;
 import lombok.Getter;
 import lombok.Setter;
@@ -170,6 +171,17 @@ public String toDefaultValue(Schema p) {
             return null;
         }
 
+        // The Swagger Parser represents an explicit `default: null` (common with
+        // `nullable: true`, e.g. via allOf composition) as a Jackson NullNode rather than a
+        // Java null. Treating it as a concrete default produces an invalid Avro union such as
+        // `["Foo", "null"]` with `"default": null`, because Avro requires a union's default
+        // value to match its FIRST branch. Treat an explicit null default as "no default" so
+        // the field falls through to the nullable-union form `["null", "Foo"]` with
+        // `"default": null`, which is valid.
+        if (p.getDefault() instanceof JsonNode && ((JsonNode) p.getDefault()).isNull()) {
+            return null;
+        }
+
         if (ModelUtils.isDateSchema(p) || ModelUtils.isDateTimeSchema(p) || ModelUtils.isStringSchema(p)) {
             return "\"" + p.getDefault().toString() + "\"";
         }

modules/openapi-generator/src/test/resources/3_0/issue6268.yaml

@@ -19,12 +19,26 @@ paths:
           description: successful operation
 components:
   schemas:
+    AvroDefaultNullReference:
+      type: object
+      properties:
+        id:
+          type: string
     SampleModelToTestAvroDefaultValues:
       type: object
       required:
         - tagsRequired
         - tagsRequiredWithDefault
       properties:
+        nullableScalarWithNullDefault:
+          type: string
+          nullable: true
+          default: null
+        nullableModelWithNullDefault:
+          nullable: true
+          default: null
+          allOf:
+            - $ref: '#/components/schemas/AvroDefaultNullReference'
         name:
           type: string
           default: 'defaultName'

samples/openapi3/schema/petstore/avro-schema-issue6268/.openapi-generator/FILES

@@ -1 +1,2 @@
+AvroDefaultNullReference.avsc
 SampleModelToTestAvroDefaultValues.avsc

samples/openapi3/schema/petstore/avro-schema-issue6268/AvroDefaultNullReference.avsc

@@ -0,0 +1,15 @@
+{
+  "namespace": "model",
+  "type": "record",
+  "doc": "",
+  "name": "AvroDefaultNullReference",
+  "fields": [
+    {
+      "name": "id",
+      "type": ["null", "string"],
+      "doc": "",
+      "default": null
+    }
+  ]
+
+}

samples/openapi3/schema/petstore/avro-schema-issue6268/SampleModelToTestAvroDefaultValues.avsc

@@ -4,6 +4,18 @@
   "doc": "",
   "name": "SampleModelToTestAvroDefaultValues",
   "fields": [
+    {
+      "name": "nullableScalarWithNullDefault",
+      "type": ["null", "string"],
+      "doc": "",
+      "default": null
+    },
+    {
+      "name": "nullableModelWithNullDefault",
+      "type": ["null", "model.AvroDefaultNullReference"],
+      "doc": "",
+      "default": null
+    },
     {
       "name": "name",
       "type": ["string", "null"],