src: handle empty external string resources

src/node_union_bytes.h

@@ -66,6 +66,10 @@ class UnionBytes {
   UnionBytes& operator=(UnionBytes&&) = default;
 
   bool is_one_byte() const { return one_byte_resource_ != nullptr; }
+  size_t length() const {
+    return is_one_byte() ? one_byte_resource_->length()
+                         : two_byte_resource_->length();
+  }
 
   v8::Local<v8::String> ToStringChecked(v8::Isolate* isolate) const;
 

src/util.cc

@@ -636,6 +636,12 @@ void SetConstructorFunction(Isolate* isolate,
 }
 
 Local<String> UnionBytes::ToStringChecked(Isolate* isolate) const {
+  if (length() == 0) [[unlikely]] {
+    // V8 requires non-null data pointers for empty external strings,
+    // but we don't guarantee that. Solve this by not creating an
+    // external string at all in that case.
+    return String::Empty(isolate);
+  }
   if (is_one_byte()) {
     return String::NewExternalOneByte(isolate, one_byte_resource_)
         .ToLocalChecked();

test/cctest/test_util.cc

@@ -3,6 +3,7 @@
 #include "gtest/gtest.h"
 #include "node_options-inl.h"
 #include "node_test_fixture.h"
+#include "node_union_bytes.h"
 #include "simdutf.h"
 #include "util-inl.h"
 #include "v8-function-callback.h"
@@ -312,6 +313,19 @@ TEST_F(UtilTest, DumpJavaScriptStackWithNoIsolate) {
   node::DumpJavaScriptBacktrace(stderr);
 }
 
+TEST_F(UtilTest, EmptyUnionBytesToString) {
+  const v8::HandleScope handle_scope(isolate_);
+  node::StaticExternalOneByteResource one_byte_resource(nullptr, 0, nullptr);
+  node::StaticExternalTwoByteResource two_byte_resource(nullptr, 0, nullptr);
+
+  EXPECT_EQ(
+      0,
+      node::UnionBytes(&one_byte_resource).ToStringChecked(isolate_)->Length());
+  EXPECT_EQ(
+      0,
+      node::UnionBytes(&two_byte_resource).ToStringChecked(isolate_)->Length());
+}
+
 TEST_F(UtilTest, DetermineSpecificErrorType) {
   const v8::HandleScope handle_scope(isolate_);
   Argv argv;