ALTER tablets-enabled KEYSPACES (KS) may fail due to
`group0_concurrent_modification`, in which case it's repeated by a `for`
loop surrounding the code. But because raft's `add_entry` consumes the
raft's guard (by `std::move`'ing the guard object), retries of ALTER KS
will use a moved-from guard object, which is UB, potentially a crash.
The fix is to remove the before mentioned `for` loop altogether and rethrow the exception, as the `rf_change` event
will be repeated by the topology state machine if it receives the
concurrent modification exception, because the event will remain present
in the global requests queue, hence it's going to be executed as the
very next event.
Note: refactor is implemented in the follow-up commit.
Fixes: scylladb/scylladb#21102
Should be backported to every 6.x branch, as it may lead to a crash.
Closesscylladb/scylladb#21121
* github.com:scylladb/scylladb:
test: add UT to test retrying ALTER tablets KEYSPACE
cql/tablets: fix indentation in `rf_change` event handler
cql/tablets: fix retrying ALTER tablets KEYSPACE
f5c60e538d