hpb: Introduce add_alias for repeated

Similar to set_alias for singular submessages, we augment hpb with the ability to add already-allocated messages via the generated function add_alias for repeated messages.

PiperOrigin-RevId: 684136800

hpb_generator/gen_repeated_fields.cc

@@ -43,6 +43,7 @@ void WriteRepeatedFieldUsingAccessors(const protobuf::FieldDescriptor* field,
       output(
           R"cc(
             using $0Access::add_$1;
+            using $0Access::add_alias_$1;
             using $0Access::mutable_$1;
           )cc",
           class_name, resolved_field_name);
@@ -89,6 +90,12 @@ void WriteRepeatedFieldsInMessageHeader(const protobuf::Descriptor* desc,
           const ::hpb::RepeatedField<const $4>::CProxy $2() const;
           ::hpb::Ptr<::hpb::RepeatedField<$4>> mutable_$2();
           absl::StatusOr<$0> add_$2();
+          /**
+           * Re-points submsg of repeated field to given target.
+           *
+           * REQUIRES: both messages must be in the same arena.
+           */
+          bool add_alias_$2($0 target);
           $0 mutable_$2(size_t index) const;
         )cc",
         MessagePtrConstType(field, /* const */ false),   // $0
@@ -149,13 +156,26 @@ void WriteRepeatedMessageAccessor(const protobuf::Descriptor* message,
           if (!new_msg) {
             return ::hpb::MessageAllocationError();
           }
-          return hpb::interop::upb::MakeHandle<$4>((upb_Message*)new_msg, $5);
+          return hpb::interop::upb::MakeHandle<$4>((upb_Message *)new_msg, $5);
+        }
+
+        bool $0::add_alias_$2($1 target) {
+          ABSL_CHECK_EQ(arena_, hpb::interop::upb::GetArena(target));
+          size_t size = 0;
+          $3_$2(msg_, &size);
+          auto elements = $3_resize_$2(msg_, size + 1, arena_);
+          if (!elements) {
+            return false;
+          }
+          elements[size] = ($9 *)hpb::interop::upb::GetMessage(target);
+          return true;
         }
       )cc",
       class_name, MessagePtrConstType(field, /* const */ false),
       resolved_field_name, MessageName(message),
       MessageBaseType(field, /* maybe_const */ false), arena_expression,
-      upbc_name);
+      upbc_name, ClassName(message), field->index(),
+      upb::generator::CApiMessageType(field->message_type()->full_name()));
   output(
       R"cc(
         $1 $0::mutable_$2(size_t index) const {

hpb_generator/tests/set_alias.proto

@@ -17,3 +17,7 @@ message Parent {
   optional int32 x = 1;
   optional Child child = 2;
 }
+
+message ParentWithRepeated {
+  repeated Child children = 1;
+}

hpb_generator/tests/test_generated.cc

@@ -41,6 +41,7 @@ using ::hpb_unittest::protos::container_ext;
 using ::hpb_unittest::protos::ContainerExtension;
 using ::hpb_unittest::protos::other_ext;
 using ::hpb_unittest::protos::Parent;
+using ::hpb_unittest::protos::ParentWithRepeated;
 using ::hpb_unittest::protos::RED;
 using ::hpb_unittest::protos::TestEnum;
 using ::hpb_unittest::protos::TestModel;
@@ -1273,4 +1274,29 @@ TEST(CppGeneratedCode, SetAliasFailsForDifferentArena) {
   EXPECT_DEATH(parent.set_alias_child(child), "hpb::interop::upb::GetArena");
 }
 
+TEST(CppGeneratedCode, SetAliasRepeated) {
+  hpb::Arena arena;
+  auto child = hpb::CreateMessage<Child>(arena);
+  child.set_peeps(1611);
+  auto parent1 = hpb::CreateMessage<ParentWithRepeated>(arena);
+  auto parent2 = hpb::CreateMessage<ParentWithRepeated>(arena);
+  parent1.add_alias_children(child);
+  parent2.add_alias_children(child);
+
+  ASSERT_EQ(parent1.children(0)->peeps(), parent2.children(0)->peeps());
+  ASSERT_EQ(hpb::interop::upb::GetMessage(parent1.children(0)),
+            hpb::interop::upb::GetMessage(parent2.children(0)));
+  auto childPtr = hpb::Ptr<Child>(child);
+  ASSERT_EQ(hpb::interop::upb::GetMessage(childPtr),
+            hpb::interop::upb::GetMessage(parent1.children(0)));
+}
+
+TEST(CppGeneratedCode, SetAliasRepeatedFailsForDifferentArena) {
+  hpb::Arena arena;
+  auto child = hpb::CreateMessage<Child>(arena);
+  hpb::Arena different_arena;
+  auto parent = hpb::CreateMessage<ParentWithRepeated>(different_arena);
+  EXPECT_DEATH(parent.add_alias_children(child), "hpb::interop::upb::GetArena");
+}
+
 }  // namespace