fix(converter): prevent x-nullable loss for shared $ref responses

The convert(vendorExtensions) method mutated the original map when
filtering out x-nullable, x-example, and x-examples extensions.

When multiple endpoints referenced the same response via $ref, the
first conversion removed x-nullable from the shared source object,
causing subsequent conversions to miss it and produce nullable=null.

Create a new map instead of mutating the original to preserve vendor
extensions for all endpoints sharing the same response definition.

Author: esaezgil

modules/swagger-parser-v2-converter/src/main/java/io/swagger/v3/parser/converter/SwaggerConverter.java

@@ -655,10 +655,14 @@ public Operation convert(io.swagger.models.Operation v2Operation) {
 
     private Map<String, Object> convert(Map<String, Object> vendorExtensions) {
         if (vendorExtensions != null && vendorExtensions.size() > 0) {
-            vendorExtensions.entrySet().removeIf(extension -> (
-                    extension.getKey().equals("x-example")) ||
-                    extension.getKey().equals("x-examples") ||
-                    extension.getKey().equals("x-nullable"));
+            Map<String, Object> result = new LinkedHashMap<>();
+            for (Map.Entry<String, Object> entry : vendorExtensions.entrySet()) {
+                String key = entry.getKey();
+                if (!key.equals("x-example") && !key.equals("x-examples") && !key.equals("x-nullable")) {
+                    result.put(key, entry.getValue());
+                }
+            }
+            return result;
         }
 
         return vendorExtensions;

modules/swagger-parser/src/test/java/io/swagger/parser/OpenAPIParserTest.java

@@ -686,6 +686,32 @@ public void testIssue1552() throws Exception {
         assertNotNull(schema.getProperties().get("foo"));
     }
 
+    @Test(description = "Issue 2269: preserve x-nullable in shared responses for swagger 2.0 specs")
+    public void testSwagger2SharedResponseNullable() {
+        ParseOptions options = new ParseOptions();
+        options.setResolve(true);
+        options.setResolveFully(true);
+
+        SwaggerParseResult result = new OpenAPIParser().readLocation("issue2269.yaml", null, options);
+
+        assertNotNull(result);
+        assertNotNull(result.getOpenAPI());
+        OpenAPI openAPI = result.getOpenAPI();
+
+        Schema endpoint1Field = (Schema) openAPI.getPaths().get("/endpoint1").getGet()
+                .getResponses().get("200").getContent().values().iterator().next()
+                .getSchema().getProperties().get("optional_field");
+        assertNotNull(endpoint1Field, "Endpoint 1 should have optional_field");
+        assertEquals(endpoint1Field.getNullable(), Boolean.TRUE, "Endpoint 1 optional_field should be nullable");
+
+        Schema endpoint2Field = (Schema) openAPI.getPaths().get("/endpoint2").getGet()
+                .getResponses().get("200").getContent().values().iterator().next()
+                .getSchema().getProperties().get("optional_field");
+        assertNotNull(endpoint2Field, "Endpoint 2 should have optional_field");
+        assertEquals(endpoint2Field.getNullable(), Boolean.TRUE, "Endpoint 2 optional_field should be nullable");
+
+    }
+
     @org.testng.annotations.Test(description = "convert response schema")
     public void testIssue1552AdditionalProps() throws Exception {
         ParseOptions options = new ParseOptions();

modules/swagger-parser/src/test/resources/issue2269.yaml

@@ -0,0 +1,38 @@
+swagger: '2.0'
+info:
+  version: 1.0.0
+  title: Swagger 2.0 Shared Response Test
+  description: Test case for verifying x-nullable is preserved when multiple endpoints share the same response definition
+
+responses:
+  SharedResponse:
+    description: A shared response with nullable field
+    schema:
+      type: object
+      properties:
+        data:
+          type: string
+          description: Required field
+        optional_field:
+          type: integer
+          description: Optional nullable field
+          x-nullable: true
+      required:
+        - data
+
+paths:
+  /endpoint1:
+    get:
+      operationId: endpoint1
+      summary: First endpoint using shared response
+      responses:
+        '200':
+          $ref: '#/responses/SharedResponse'
+
+  /endpoint2:
+    get:
+      operationId: endpoint2
+      summary: Second endpoint using shared response
+      responses:
+        '200':
+          $ref: '#/responses/SharedResponse'