System indices can be snapshotted and are therefore potential candidates
to be mounted as searchable snapshot indices. As of today nothing
prevents a snapshot to be mounted under an index name starting with .
and this can lead to conflicting situations because searchable snapshot
indices are read-only and Elasticsearch expects some system indices
to be writable; because searchable snapshot indices will soon use an
internal system index (#60522) to speed up recoveries and we should
prevent the system index to be itself a searchable snapshot index
(leading to some deadlock situation for recovery).
This commit introduces a changes to prevent snapshots to be mounted
as a system index.
787dfda4c1