fix: fixes handling of empty objects during marshaller sync that was impacting docs upgrading with nullable only schemas

marshaller/syncer.go

@@ -249,6 +249,16 @@ func syncChanges(ctx context.Context, source any, target any, valueNode *yaml.No
 		}
 	}
 
+	// Ensure we have a valid YAML node even for empty structs
+	if valueNode == nil {
+		// Create an empty mapping node for empty structs
+		valueNode = &yaml.Node{
+			Kind:  yaml.MappingNode,
+			Tag:   "!!map",
+			Style: yaml.FlowStyle,
+		}
+	}
+
 	// Populate the RootNode of the target with the result
 	if coreModel, ok := t.Addr().Interface().(CoreModeler); ok {
 		coreModel.SetRootNode(valueNode)

openapi/testdata/upgrade/expected_minimal_nullable_upgraded.json

@@ -0,0 +1,27 @@
+{
+  "openapi": "3.1.1",
+  "info": {
+    "title": "Test API",
+    "version": "1.0.0"
+  },
+  "paths": {},
+  "components": {
+    "schemas": {
+      "TestSchema": {
+        "type": "object",
+        "properties": {
+          "parent": {
+            "oneOf": [
+              {},
+              {
+                "type": [
+                  "null"
+                ]
+              }
+            ]
+          }
+        }
+      }
+    }
+  }
+}

openapi/testdata/upgrade/minimal_nullable.json

@@ -0,0 +1,20 @@
+{
+  "openapi": "3.0.0",
+  "info": {
+    "title": "Test API",
+    "version": "1.0.0"
+  },
+  "paths": {},
+  "components": {
+    "schemas": {
+      "TestSchema": {
+        "type": "object",
+        "properties": {
+          "parent": {
+            "nullable": true
+          }
+        }
+      }
+    }
+  }
+}

openapi/upgrade.go

@@ -135,7 +135,7 @@ func upgradeNullableSchema(schema *oas3.Schema) {
 		nullSchema := createNullSchema()
 		clone := *schema
 		newSchema := oas3.Schema{}
-		newSchema.OneOf = []*oas3.JSONSchema[oas3.Referenceable]{nullSchema, oas3.NewJSONSchemaFromSchema[oas3.Referenceable](&clone)}
+		newSchema.OneOf = []*oas3.JSONSchema[oas3.Referenceable]{oas3.NewJSONSchemaFromSchema[oas3.Referenceable](&clone), nullSchema}
 		*schema = newSchema
 	}
 }

openapi/upgrade_test.go

@@ -51,6 +51,13 @@ func TestUpgrade_Success(t *testing.T) {
 			options:      []openapi.Option[openapi.UpgradeOptions]{openapi.WithUpgradeSamePatchVersion()},
 			description:  "3.1.0 should upgrade with WithUpgradeSamePatchVersion option",
 		},
+		{
+			name:         "upgrade_nullable_schema",
+			inputFile:    "testdata/upgrade/minimal_nullable.json",
+			expectedFile: "testdata/upgrade/expected_minimal_nullable_upgraded.json",
+			options:      nil,
+			description:  "nullable schema should upgrade to oneOf without panic",
+		},
 	}
 
 	for _, tt := range tests {