Question: How to find a transaction’s data manager, if any? about transaction

[d-maurer]

The typical approach is to let the data manager remember whether it has already joined the transaction or not.

Of course, this requires a different handling of the data manager. In your code above, you (ab)use the transaction itself to manage the data manager. Typically, the data manager exists independent of the transaction (it may e.g. be managed in a thread local "variable"). The application interacts primarily with the data manager (transactions are relevant only at very few points) and the data manager decides when to join the transaction (and knows if it has already done this). If the data manager has joined the transaction, it will get informed when the transaction is aborted/committed (which implicitly creates a new transaction). It can then reset its joined state).