Function check_bundle_base_tx 

fn check_bundle_base_tx(
    tx: &Transaction<'_>,
    id: &str,
    base: &str,
) -> Result<(), StoreError>

The base check behind bundle_upsert_with_version_based / bundle_upsert_system_if_changed_based, run inside the caller’s own transaction so nothing can land between the check and the write.