Merge pull request #2799 from atn34/atn34/fix-void

Encode Void as a valid flatbuffers message

flow/flat_buffers.cpp

@@ -488,6 +488,10 @@ TEST_CASE("/flow/FlatBuffers/Standalone") {
 // Meant to be run with valgrind or asan, to catch heap buffer overflows
 TEST_CASE("/flow/FlatBuffers/Void") {
 	Standalone<StringRef> msg = ObjectWriter::toValue(Void(), Unversioned());
+	// Manually verified to be a valid flatbuffers message. This is technically brittle since there are other valid
+	// encodings of this message, but our implementation is unlikely to change.
+	ASSERT(msg == LiteralStringRef("\x14\x00\x00\x00J\xad\x1e\x00\x00\x00\x04\x00\x04\x00\x06\x00\x08\x00\x04\x00\x06"
+	                               "\x00\x00\x00\x04\x00\x00\x00\x12\x00\x00\x00"));
 	auto buffer = std::make_unique<uint8_t[]>(msg.size()); // Make a heap allocation of precisely the right size, so
 	                                                       // that asan or valgrind will catch any overflows
 	memcpy(buffer.get(), msg.begin(), msg.size());

flow/serialize.h

@@ -82,20 +82,21 @@ inline typename Archive::READER& operator >> (Archive& ar, Item& item ) {
 	return ar;
 }
 
-template <class Archive>
-void serializer(Archive& ar) {}
-
 template <class Archive, class Item, class... Items>
 typename Archive::WRITER& serializer(Archive& ar, const Item& item, const Items&... items) {
 	save(ar, item);
-	serializer(ar, items...);
+	if constexpr (sizeof...(Items) > 0) {
+		serializer(ar, items...);
+	}
 	return ar;
 }
 
 template <class Archive, class Item, class... Items>
 typename Archive::READER& serializer(Archive& ar, Item& item, Items&... items) {
 	load(ar, item);
-	serializer(ar, items...);
+	if constexpr (sizeof...(Items) > 0) {
+		serializer(ar, items...);
+	}
 	return ar;
 }