cache

type Cache interface {
	io.Closer

	// Keys returns the list of all cached digests in LRU order.
	Keys() isolated.HexDigests

	// Touch updates the LRU position of an item to ensure it is kept in the
	// cache.
	//
	// Returns true if item is in cache.
	Touch(digest isolated.HexDigest) bool

	// Evict removes item from cache if it's there.
	Evict(digest isolated.HexDigest)

	// Add reads data from src and stores it in cache.
	Add(digest isolated.HexDigest, src io.Reader) error

	// Read returns contents of the cached item.
	Read(digest isolated.HexDigest) (io.ReadCloser, error)

	// Hardlink ensures file at |dest| has the same content as cached |digest|.
	//
	// Note that the behavior when dest already exists is undefined. It will work
	// on all POSIX and may or may not fail on Windows depending on the
	// implementation used. Do not rely on this behavior.
	Hardlink(digest isolated.HexDigest, dest string, perm os.FileMode) error
}

type Policies struct {
	// MaxSize trims if the cache gets larger than this value. If 0, the cache is
	// effectively a leak.
	MaxSize units.Size
	// MaxItems is the maximum number of items to keep in the cache. If 0, do not
	// enforce a limit.
	MaxItems int
	// MinFreeSpace trims if disk free space becomes lower than this value. If 0,
	// it unconditionally fills the disk. Only makes sense when using disk based
	// cache.
	//
	// BUG: Implement Policies.MinFreeSpace.
	MinFreeSpace units.Size
}