Module Superbol_cache.Prune

run ~store ~policy applies policy to the cache held in store, and is the number of entries it reclaimed.

An age bound goes first, then a size bound reclaims the least recently used of whatever is left until the cache fits. Values are shared between entries, so one is only freed once no entry names it any more; whatever is left unnamed is swept at the end. Should an entry fail to decode, values are left alone for that round --- sweeping without having read every entry would reclaim ones that are still in use --- and so is the size bound, which could not be told what evicting an entry would free.

Subtrees belonging to another on-disk format are reclaimed only once everything in them has gone stale, so that an instance built against a neighbouring format sharing the root keeps working.

Pruning takes the cache's lock, and is a no-op --- reported as Ok 0 --- when another instance is already at it.