I wrote:

...

Can't use a DBI from an aborted child txn, unlike aborted main txn:

(...) Well, closing the DBI in mdb_txn_abort() caused problems. A fix in the opposite direction is easy, if we want DBIs to work that way: mdb_env_open() can open the DBI in any parent txns too.

To get more consistent behavior, if that wasn't clear: Any parent txn will receive the DBI independently of commit/abort, just like the MDB_env. Future children of a preexisting txn still differ, which is probably for the best. Changing that would take locking.