`read_barrier(session2)` was supposed to ensure `node2` has caught up on schema
before a CL=ALL write. But `patient_cql_connection(node2)` creates a
cluster-aware driver session `(TokenAwarePolicy(DCAwareRoundRobinPolicy()))`
that can route the barrier CQL statement to any node — not necessarily `node2`.
If the barrier runs on `node1` or `node3` (which already have the new schema),
it's a no-op, and `node2` remains stale, thus the observed `WriteFailure`.
The fix is to switch to `patient_exclusive_cql_connection(node2)`,
which uses `WhiteListRoundRobinPolicy([node2_ip])` to pin all CQL to `node2`.
This is already the established pattern used by other tests in the same file.
Fixes: SCYLLADB-1139
No need to backport yet, appeared only on master.
Closesscylladb/scylladb#29151
32225797cd