Server part fix

https://github.com/kamil-holubicki/percona-xtradb-cluster/commit/610f42a30c1ba22037d4f5aa2169fce0271f4673

Waiting for release-8.0.31 -> 8.0 merge to create PR.

The initial mixed-version-node-is-writable problem seems related to initialization. PXC 8.0 initializes wsrep_protocol_version = wsrep_max_protocol_version. When a node joins a cluster, there seems to be some assumption that the view and wsrep_protocol_version will be initialized via SST. This doesn't work when upgrading from 5.7 to 8.0, so wsrep_protocol_version remains at 4 even in a mixed-mode cluster effectively disabling the pxc-strict-mode check.

The wsrep_protocol_version does get updated later in a rolling upgrade once a new cluster view is formed. E.g. when the second node exits the cluster and a new view is formed, then wsrep_protocol_version downgrades back to 3 and pxc-strict-mode starts being enforced.

However, even if a node manages to enforce pxc_strict_mode initially, when the node state is Donor/Desynced (or any not-synced state) this check is disabled again.

This seems to be a defect in the block_write_while_in_rolling_upgrade implementation which disables the check if the node isn't strictly in the "synced" state:

Could relate to

Can not reproduced if you restart mysql 8.0 after its initial SST and upgrade.

In case you do not reproduce, simply set up a 3 nodes cluster with 2 5.7 and 1 8.0 node.

It can be repeated by removing the 8.0 datadir to force SST.