JNI local-reference accumulation in the string array helpers

[K-ANOY]

Describe the bug, including details regarding any error messages, version, and platform.

There is a JNI local-reference issue in the dataset helpers that convert Java String[] arrays into C++ containers. They get each element with GetObjectArrayElement() and convert it, but never delete the resulting local reference.

Files:

• dataset/src/main/cpp/jni_util.cc
• dataset/src/main/cpp/jni_wrapper.cc

Functions:

• ToStringVector (jni_util.cc)
• ToStringMap, LoadNamedTables (jni_wrapper.cc)

Relevant code in ToStringVector():

std::vector<std::string> ToStringVector(JNIEnv* env, jobjectArray& str_array) {
  int length = env->GetArrayLength(str_array);
  std::vector<std::string> vector;
  for (int i = 0; i < length; i++) {
    auto string = reinterpret_cast<jstring>(env->GetObjectArrayElement(str_array, i));
    vector.push_back(JStringToCString(env, string));
  }
  return vector;
}

JStringToCString() does correctly release the native UTF chars it acquires:

const char* chars = env->GetStringUTFChars(string, nullptr);
std::string ret(chars);
env->ReleaseStringUTFChars(string, chars);
return ret;

but that release only matches GetStringUTFChars(). It does not delete the jstring local reference that GetObjectArrayElement() returned, so one reference remains live per element until the native method returns.

ToStringMap() has the same pattern with two references per iteration:

for (int i = 0; i < length; i += 2) {
  auto key = reinterpret_cast<jstring>(env->GetObjectArrayElement(str_array, i));
  auto value = reinterpret_cast<jstring>(env->GetObjectArrayElement(str_array, i + 1));
  map[JStringToCString(env, key)] = JStringToCString(env, value);
}

LoadNamedTables() also takes two per iteration, and has three JniThrow() exits inside the loop body — the odd-length check and the two std::stol failure handlers — so on those paths the references acquired in the current iteration are abandoned mid-loop.

These are local references, so they are reclaimed when the native method returns, and the inputs are option maps, partition columns, and named-table lists — typically tens of entries. There is no observable leak or reachable failure here; the reference count is simply higher than necessary for the duration of the call.

Suggested fix:

auto string = reinterpret_cast<jstring>(env->GetObjectArrayElement(str_array, i));
vector.push_back(JStringToCString(env, string));
env->DeleteLocalRef(string);

For the key/value loops, delete both key and value, including on the JniThrow() paths in LoadNamedTables().

[lidavidm]

Is this really a problem then?