As the description of the main task
MDEV-452 already mentions, MySQL implementation has the variable explicit_defaults_for_timestamp which modifies the behavior of auto-updated temporal columns. Since the planned release approaches and we don't have the variable yet, we either need to add it now, or decide to release without it, and add it in further versions. Either is okay as long as it's not completely forgotten.
The definition of MySQL's implementation can be found here:
What we should do in MariaDB:
- Add this new variable and have it work as MySQL when enabled.
- Have it disabled as default, to not cause incompatibilities with old applications
- Always keep the variable around (not make it deprecated) as we don't want to have it default.
- Add a MYSQL mode where, if enabled, this variable is set.
TODO: check how replication of this variable works in MySQL.