pub fn upgrade(&self) -> Option<RocksDB<CF>> {
        Some(RocksDB {
            inner: self.inner.upgrade()?,
            sync_writes: self.sync_writes,
            stalled_writes_retries: self.stalled_writes_retries,
            _cf: PhantomData,
        })
    }
Tries to upgrade to a strong reference to RocksDB. If the RocksDB instance has been dropped, returns `None`.
core/lib/storage/src/db.rs