In effb9fb3cb migration request handler
(called when a node requests schema pull) was extended with a
`system.scylla_local` mutation:
```
cm.emplace_back(co_await self._sys_ks.local().get_group0_schema_version());
```
This mutation is empty if the GROUP0_SCHEMA_VERSIONING feature is
disabled.
Nevertheless, it turned out to cause problems during upgrades.
The following scenario shows the problem:
We upgrade from 5.2 to enterprise version with the aforementioned patch.
In 5.2, `system.scylla_local` does not use schema commitlog.
After the first node upgrades to the enterprise version, it immediately
on boot creates a new enterprise-only table
(`system_replicated_keys.encrypted_keys`) -- the specific table is not
important, only the fact that a schema change is performed.
This happens before the restarting node notices other nodes being UP, so
the schema change is not immediately pushed to the other nodes.
Instead, soon after boot, the other non-upgraded nodes pull the schema
from the upgraded node.
The upgraded node attaches a `system.scylla_local` mutation to the
vector of returned mutations.
The non-upgraded nodes try to apply this vector of mutations. Because
some of these mutations are for tables that already use schema
commitlog, while the `system.scylla_local` table does not use schema
commitlog, this triggers the following error (even though the mutation
is empty):
```
Cannot apply atomically across commitlog domains: system.scylla_local, system_schema.keyspaces
```
Fortunately, the fix is simple -- instead of attaching an empty
mutation, do not attach a mutation at all if the handler of migration
request notices that group0_schema_version is not present.
Note that group0_schema_version is only present if the
GROUP0_SCHEMA_VERSIONING feature is enabled, which happens only after
the whole upgrade finishes.
Refs: scylladb/scylladb#16414
Not using "Fixes" because the issue will only be fixed once this PR is
merged to `master` and the commit is cherry-picked onto next-enterprise.
Closesscylladb/scylladb#16416
6a4106edf3