Support GTID_Anonymous to replicate MariaDB ⮕ MySQL with GTID_MODE = ON

MDEV-8685 added an acknowledgement that MySQL's anonymous GTIDs are a thing, enough to ignore them. I understand that MySQL 5.7.6+ adds these when a GTID is not present. This work is described in MySQL WL#7592.

MySQL 8.0.23+ offers the ability to turn these anonymous GTID records into local or specified-base GTID with ASSIGN_GTIDS_TO_ANONYMOUS_TRANSACTIONS - the LOCAL option essentially upgrades the binlog to be compatible with GTID-only downstream replicas, with the caveats noted.

This feature does not appear to work when there is no such record, as in replication from a MariaDB 10.11 server running in GTID_MODE = OFF, where a timestamp is inserted instead. Instead, an error is thrown trying to execute the relay log, as the replica's GTID_MODE = ON. So it'd be nice if MariaDB could operate in a compatibility mode that added these records to the binlog instead of timestamps (and potentially BEGIN) so that it was possible to feed a GTID-enabled MySQL replica in this manner.

Why? I'd like to use a cloud provider's MySQL hosting for a partial regional read-only replica, keeping my core services MariaDB, rather than switch to MySQL - which isn't supported on Debian, although I got the Ubuntu 22.04 LTS jammy-updates package working anyway to try to work around this issue.

Starting replication to the provider requires GTID_MODE = ON (not ON_PERMISSIVE), and uses AUTO_POSITION = 1, which causes replication to fail if I switch it back, as there are now anonymous transactions in the mix (e.g. "MySQL Errno - 13114: Got fatal error 1236 from master when reading data from binary log: 'Cannot replicate anonymous transaction when AUTO_POSITION = 1, at file ./binlog.000027, position 197...'").

I'm given to understand that it may be possible to go MariaDB -> MySQL 5.7 -> MySQL 8.0, but this seems ill-advised, especially as MySQL 5.7 is no longer generally supported. (It was tolerable to add a MySQL 8 replica, as I can use a full read-only replica anyway.)

I requested the provider add MariaDB (or consider relaxing their replication requirements), but given their focus is understandably on well-funded startups, this seems unlikely to happen. Getting ASSIGN_GTIDS_TO_ANONYMOUS_TRANSACTIONS enabled might be easier, but would still require the MySQL intermediary unless MariaDB also implements them.