The guard should stop refreshing the ERM when the number of tablets changes. Tablet splits or merges invalidate the `tablet_id` field (`_tablet`), which means the guard can no longer correctly protect ongoing operations from tablet migrations.
The problem is specific to LWT, since `tablet_metadata_guard` is used mostly for heavy topology operations, which exclude with split and merge. The guard was used for LWT as an optimization -- we don't need to block topology operations or migrations of unrelated tablets. In the future, we could use the guard for regular reads/writes as well (via the `token_metadata_guard` wrapper).
Fixes [scylladb/scylladb#26437](https://github.com/scylladb/scylladb/issues/26437)
backports: need to backport to 2025.4 since the bug is relevant to LWT over tablets.
Closesscylladb/scylladb#26619
* github.com:scylladb/scylladb:
test_tablets_lwt: add test_tablets_merge_waits_for_lwt
test.py: add universalasync_typed_wrap
tablet_metadata_guard: fix split/merge handling
tablet_metadata_guard: add debug logs
paxos_state: shards_for_writes: improve the error message
storage_service: barrier_and_drain – change log level to info
topology_coordinator: fix log message
564cebd0e6