The comparator used to sort per-IP client rows was not a strict-weak-ordering (it could return true in both directions for some pairs), which makes `std::ranges::sort` behavior undefined. A concrete pair that breaks it (and is realistic in system.clients):
a = (port=9042, client_type="cql")
b = (port=10000, client_type="alternator")
With the current comparator:
cmp(a,b) = (9042 < 10000) || ("cql" < "alternator") = true || false = true
cmp(b,a) = (10000 < 9042) || ("alternator" < "cql") = false || true = true
So both directions are true, meaning there is no valid ordering that sort can achieve.
The fix is to sort lexicographically by (port, client_type) to match the table's clustering key and ensure deterministic ordering.
Closesscylladb/scylladb#28844
a31cb18324