Today we empty the searchable snapshots cache when cleanly closing a
shard, but leak cache files in some cases involving an unclean shutdown.
Such leaks are not permanent, they are cleaned up on shard relocation or
deletion, but they still might last for arbitrarily long until that
happens. This commit introduces a cleanup process that runs during node
startup to catch such leaks sooner.
Also, today we permit searchable snapshots to be held on custom data
paths, and store the corresponding cache files within the custom
location. Supporting this feature would make the cleanup process
significantly more complicated since it would require each node to parse
the index metadata for the shards it held before shutdown. Yet, this
feature is undocumented and offers minimal benefits to searchable
snapshots. Therefore with this commit we forbid custom data paths for
searchable snapshot shards.
6ffdb19a2a