Clarify CacheValidity option and add NoCache/MaxCache consts

Signed-off-by: Cody Soyland <codysoyland@github.com>

pkg/tuf/options.go

@@ -16,6 +16,7 @@ package tuf
 
 import (
  "embed"
+ "math"
  "os"
  "path/filepath"
 
@@ -25,11 +26,20 @@ import (
 //go:embed repository
 var embeddedRepo embed.FS
 
-const DefaultMirror = "https://tuf-repo-cdn.sigstore.dev"
+const (
+ DefaultMirror = "https://tuf-repo-cdn.sigstore.dev"
+
+ // The following caching values can be used for the CacheValidity option
+ NoCache = 0
+ MaxCache = math.MaxInt
+)
 
 // Options represent the various options for a Sigstore TUF Client
 type Options struct {
- // CacheValidity period in days (default 0)
+ // CacheValidity period in days (default 0). Note that the client will
+ // always refresh the cache if the metadata is expired, so this is not an
+ // optimal control for air-gapped environments. Use const MaxCache to only
+ // update the cache when the metadata is expired.
  CacheValidity int
  // ForceCache controls if the cache should be used without update
  // as long as the metadata is valid