fix(openapi-yaml): preserve format:byte examples (#23623)

Jackson's default YAMLMapper serializes byte[] values with a !!binary
tag and re-base64-encodes the payload. swagger-parser stores
`format: byte` examples as the raw bytes of the input base64 string,
so the default behavior produced double-encoded YAML, e.g.:

  example: !!binary |-
    YUdWc2JHOEs=

Register a ByteArraySerializer in the OpenAPIModule that writes
byte[] as its UTF-8 string representation, restoring the original
base64 literal in the regenerated YAML (e.g. `example: aGVsbG8K`).

Adds a regression test and fixture for issue #19929.

Author: pastuxso

modules/openapi-generator/src/main/java/org/openapitools/codegen/serializer/ByteArraySerializer.java

@@ -0,0 +1,31 @@
+package org.openapitools.codegen.serializer;
+
+import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.databind.SerializerProvider;
+import com.fasterxml.jackson.databind.ser.std.StdSerializer;
+
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+
+/**
+ * Serializes {@code byte[]} as a UTF-8 string, preserving the original base64 representation
+ * of an OpenAPI {@code format: byte} example. swagger-parser stores {@code format: byte}
+ * examples as the raw bytes of the input base64 string (not the decoded content), so writing
+ * those bytes back as a string round-trips to the user's original value. This prevents
+ * Jackson's default YAML behavior of emitting a {@code !!binary} block with a re-encoded payload.
+ *
+ * <p>Load-bearing assumption: this only round-trips correctly because swagger-parser stores the
+ * raw input bytes. If swagger-parser ever decodes {@code format: byte} examples internally, this
+ * serializer must switch to {@code Base64.getEncoder().encodeToString(value)}.
+ */
+public class ByteArraySerializer extends StdSerializer<byte[]> {
+
+    public ByteArraySerializer() {
+        super(byte[].class);
+    }
+
+    @Override
+    public void serialize(byte[] value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
+        gen.writeString(new String(value, StandardCharsets.UTF_8));
+    }
+}

modules/openapi-generator/src/main/java/org/openapitools/codegen/serializer/SerializerUtils.java

@@ -66,6 +66,7 @@ public static String toJsonString(OpenAPI openAPI) {
     private static SimpleModule createModule() {
         SimpleModule module = new SimpleModule("OpenAPIModule");
         module.addSerializer(OpenAPI.class, new OpenAPISerializer());
+        module.addSerializer(byte[].class, new ByteArraySerializer());
         return module;
     }
 }

modules/openapi-generator/src/test/java/org/openapitools/codegen/yaml/YamlGeneratorTest.java

@@ -18,6 +18,8 @@
 package org.openapitools.codegen.yaml;
 
 import io.swagger.v3.oas.models.OpenAPI;
+import io.swagger.v3.oas.models.media.Schema;
+import io.swagger.v3.oas.models.parameters.Parameter;
 import org.openapitools.codegen.ClientOptInput;
 import org.openapitools.codegen.DefaultGenerator;
 import org.openapitools.codegen.TestUtils;
@@ -27,6 +29,7 @@
 import org.testng.annotations.Test;
 
 import java.io.File;
+import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.HashMap;
@@ -149,4 +152,37 @@ public void testIssue18622() throws Exception {
         Assert.assertEquals(actual.getComponents().getSchemas().get("myresponse").getExample(),
                 expected.getComponents().getSchemas().get("myresponse").getExample());
     }
+
+    @Test
+    public void testIssue19929() throws Exception {
+        Map<String, Object> properties = new HashMap<>();
+        properties.put(OpenAPIYamlGenerator.OUTPUT_NAME, "issue_19929.yaml");
+
+        File output = Files.createTempDirectory("issue_19929").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("openapi-yaml")
+                .setAdditionalProperties(properties)
+                .setInputSpec("src/test/resources/3_0/issue_19929.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        final ClientOptInput clientOptInput = configurator.toClientOptInput();
+        DefaultGenerator generator = new DefaultGenerator();
+        generator.opts(clientOptInput).generate();
+
+        Path generated = Path.of(output.getAbsolutePath(), "issue_19929.yaml");
+        String generatedYaml = new String(Files.readAllBytes(generated), StandardCharsets.UTF_8);
+
+        Assert.assertFalse(generatedYaml.contains("!!binary"),
+                "openapi-yaml output must not emit !!binary for format: byte examples. Output was:\n" + generatedYaml);
+        Assert.assertTrue(generatedYaml.contains("aGVsbG8K"),
+                "openapi-yaml output should preserve the original base64 example. Output was:\n" + generatedYaml);
+
+        OpenAPI roundTripped = TestUtils.parseSpec(generated.toString());
+        Parameter coordinates = roundTripped.getPaths().get("/operationsTest/").getGet().getParameters().get(0);
+        Schema<?> dataSchema = (Schema<?>) coordinates.getContent().get("application/json").getSchema().getProperties().get("data");
+        byte[] exampleBytes = (byte[]) dataSchema.getExample();
+        Assert.assertEquals(new String(exampleBytes, StandardCharsets.UTF_8), "aGVsbG8K");
+    }
 }

modules/openapi-generator/src/test/resources/3_0/issue_19929.yaml

@@ -0,0 +1,24 @@
+openapi: 3.0.3
+info:
+  title: TestRestService2.v2
+  description: Hello
+  version: 2.2.2
+paths:
+  /operationsTest/:
+    get:
+      responses:
+        '200':
+          description: Empty response.
+      parameters:
+        - in: query
+          name: coordinates
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  data:
+                    type: string
+                    format: byte
+                    description: 'Base64 encoded JSON with description of audited operation.'
+                    example: 'aGVsbG8K'