Comparator thread-local reusable buffers do not delete their native wrapper #14975

File: java/rocksjni/comparatorjnicallback.cc

Functions: ComparatorJniCallback::ComparatorJniCallback, ComparatorJniCallback::GetBuffer

When comparator buffer reuse is configured with ReusedSynchronisationType::THREAD_LOCAL, GetBuffer allocates a ThreadLocalBuf for each thread-local reused buffer:

jobject jtl_buf = env->NewGlobalRef(ByteBufferJni::construct(
    env, m_options->direct_buffer, m_options->max_reused_buffer_size,
    m_jbytebuffer_clazz));
if (jtl_buf == nullptr) {
  // exception thrown: OutOfMemoryError
  return nullptr;
}
tlb = new ThreadLocalBuf(m_jvm, m_options->direct_buffer, jtl_buf);
tl_buf->Reset(tlb);

The associated ThreadLocalPtr unref handler releases the direct-buffer backing
memory and deletes the JNI global reference, but it never deletes the
ThreadLocalBuf object itself:

UnrefHandler unref = [](void* ptr) {
  ThreadLocalBuf* tlb = reinterpret_cast<ThreadLocalBuf*>(ptr);
  jboolean attached_thread = JNI_FALSE;
  JNIEnv* _env = JniUtil::getJniEnv(tlb->jvm, &attached_thread);
  if (_env != nullptr) {
    if (tlb->direct_buffer) {
      void* buf = _env->GetDirectBufferAddress(tlb->jbuf);
      delete[] static_cast<char*>(buf);
    }
    _env->DeleteGlobalRef(tlb->jbuf);
    JniUtil::releaseJniEnv(tlb->jvm, attached_thread);
  }
};

ThreadLocalPtr does call the handler for stored values on thread exit and when
the ThreadLocalPtr is destroyed, so the missing delete tlb leaks the native
wrapper allocated for every thread-local comparator buffer.

Suggested fix

Delete the wrapper after releasing its JNI/direct-buffer resources:

 UnrefHandler unref = [](void* ptr) {
   ThreadLocalBuf* tlb = reinterpret_cast<ThreadLocalBuf*>(ptr);
   jboolean attached_thread = JNI_FALSE;
   JNIEnv* _env = JniUtil::getJniEnv(tlb->jvm, &attached_thread);
   if (_env != nullptr) {
     if (tlb->direct_buffer) {
       void* buf = _env->GetDirectBufferAddress(tlb->jbuf);
       delete[] static_cast<char*>(buf);
     }
     _env->DeleteGlobalRef(tlb->jbuf);
     JniUtil::releaseJniEnv(tlb->jvm, attached_thread);
   }
+  delete tlb;
 };

If getJniEnv can fail during shutdown, the code may also need a policy for
the still-held JNI global reference. The immediate native leak is the missing
delete of ThreadLocalBuf.