presto-main/src/main/java/io/prestosql/operator/ReferenceCount.java/ReferenceCount/release
class ReferenceCount:    public synchronized void release()
    {
        checkState(!freeFuture.isDone(), "Reference has already been freed");
        count--;
        if (count == 0) {
            freeFuture.set(null);
        }
    }