+ * Note that if the TDB_ALLOW_NESTING flag is set, a tdb_transaction_start()
+ * within a transaction will succeed, but it's not a real transaction:
+ * (1) An inner transaction which is committed is not actually committed until
+ * the outer transaction is; if the outer transaction is cancelled, the
+ * inner ones are discarded.
+ * (2) tdb_transaction_cancel() marks the outer transaction as having an error,
+ * so the final tdb_transaction_commit() will fail.
+ * (3) the outer transaction will see the results of the inner transaction.
+ *