src: copy-construct data when cloning threadsafe COW

Initialize the cloned data directly while holding the source read lock,
so payloads do not need to be default-constructible or copy-assignable.

Signed-off-by: Archkon <180910180+Archkon@users.noreply.github.com>

Author: Archkon

src/node_threadsafe_cow-inl.h

@@ -42,10 +42,11 @@ T* ThreadsafeCopyOnWrite<T>::Write::operator->() {
 }
 
 template <typename T>
-ThreadsafeCopyOnWrite<T>::Impl::Impl(const Impl& other) {
-  RwLock::ScopedReadLock lock(other.mutex);
-  data = other.data;
-}
+ThreadsafeCopyOnWrite<T>::Impl::Impl(const Impl& other)
+    : data([&other]() {
+        RwLock::ScopedReadLock lock(other.mutex);
+        return other.data;
+      }()) {}
 
 }  // namespace node
 

test/cctest/test_per_process.cc

@@ -6,6 +6,7 @@
 
 #include <string>
 
+using node::ThreadsafeCopyOnWrite;
 using node::builtins::BuiltinLoader;
 using node::builtins::BuiltinSourceMap;
 
@@ -18,6 +19,22 @@ class PerProcessTest : public ::testing::Test {
 
 namespace {
 
+struct CopyConstructOnly {
+  explicit CopyConstructOnly(int value) : value(value) {}
+  CopyConstructOnly() = delete;
+  CopyConstructOnly(const CopyConstructOnly&) = default;
+  CopyConstructOnly& operator=(const CopyConstructOnly&) = delete;
+
+  int value;
+};
+
+TEST(ThreadsafeCopyOnWriteTest, CloneCopyConstructsData) {
+  const ThreadsafeCopyOnWrite<CopyConstructOnly> original(42);
+  auto copy = original;
+
+  EXPECT_EQ(copy.write()->value, 42);
+}
+
 TEST_F(PerProcessTest, EmbeddedSources) {
   const auto& sources = PerProcessTest::get_sources_for_test();
   ASSERT_TRUE(std::any_of(sources.cbegin(), sources.cend(), [](auto p) {