Fix parsing/serialization of packed proto3 custom options

We have an obscure bug where packed-by-default repeated custom options defined
in proto3 files are not correctly treated as packed when dynamic extensions are
used. This CL introduces a simple fix and adds a test verifying the correct
behavior.

The bug was not a major problem in most cases, since parsers will accept both
the packed and unpacked encodings. It is useful to fix this to ensure that
descriptors are serialized in a consistent way, though.

PiperOrigin-RevId: 513249881

src/google/protobuf/extension_set_heavy.cc

@@ -285,7 +285,7 @@ bool DescriptorPoolExtensionFinder::Find(int number, ExtensionInfo* output) {
   } else {
     output->type = extension->type();
     output->is_repeated = extension->is_repeated();
-    output->is_packed = extension->options().packed();
+    output->is_packed = extension->is_packed();
     output->descriptor = extension;
     if (extension->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
       output->message_info.prototype =

src/google/protobuf/extension_set_unittest.cc

@@ -1359,6 +1359,54 @@ TEST(ExtensionSetTest, DynamicExtensions) {
   }
 }
 
+TEST(ExtensionSetTest, Proto3PackedDynamicExtensions) {
+  // Regression test for b/271121265. This test case verifies that
+  // packed-by-default repeated custom options in proto3 are correctly
+  // serialized in packed form when dynamic extensions are used.
+
+  // Create a custom option in proto3 and load this into an overlay
+  // DescriptorPool with a DynamicMessageFactory.
+  google::protobuf::FileDescriptorProto file_descriptor_proto;
+  file_descriptor_proto.set_syntax("proto3");
+  file_descriptor_proto.set_name(
+      "third_party/protobuf/unittest_proto3_packed_extension.proto");
+  file_descriptor_proto.set_package("proto3_unittest");
+  file_descriptor_proto.add_dependency(
+      DescriptorProto::descriptor()->file()->name());
+  FieldDescriptorProto* extension = file_descriptor_proto.add_extension();
+  extension->set_name("repeated_int32_option");
+  extension->set_extendee(MessageOptions().GetTypeName());
+  extension->set_number(50009);
+  extension->set_label(FieldDescriptorProto::LABEL_REPEATED);
+  extension->set_type(FieldDescriptorProto::TYPE_INT32);
+  extension->set_json_name("repeatedInt32Option");
+  google::protobuf::DescriptorPool pool(DescriptorPool::generated_pool());
+  ASSERT_NE(pool.BuildFile(file_descriptor_proto), nullptr);
+  DynamicMessageFactory factory;
+  factory.SetDelegateToGeneratedFactory(true);
+
+  // Create a serialized MessageOptions proto equivalent to:
+  // [proto3_unittest.repeated_int32_option]: 1
+  UnknownFieldSet unknown_fields;
+  unknown_fields.AddVarint(50009, 1);
+  std::string serialized_options;
+  ASSERT_TRUE(unknown_fields.SerializeToString(&serialized_options));
+
+  // Parse the MessageOptions using our custom extension registry.
+  io::ArrayInputStream input_stream(serialized_options.data(),
+                                    serialized_options.size());
+  io::CodedInputStream coded_stream(&input_stream);
+  coded_stream.SetExtensionRegistry(&pool, &factory);
+  MessageOptions message_options;
+  ASSERT_TRUE(message_options.ParseFromCodedStream(&coded_stream));
+
+  // Finally, serialize the proto again and verify that the repeated option has
+  // been correctly serialized in packed form.
+  std::string reserialized_options;
+  ASSERT_TRUE(message_options.SerializeToString(&reserialized_options));
+  EXPECT_EQ(reserialized_options, "\xca\xb5\x18\x01\x01");
+}
+
 TEST(ExtensionSetTest, BoolExtension) {
   unittest::TestAllExtensions msg;
   uint8_t wire_bytes[2] = {13 * 8, 42 /* out of bounds payload for bool */};