Add cache purge

Signed-off-by: jolheiser <john.olheiser@gmail.com>

bench.txt

@@ -1,9 +0,0 @@
-go test -benchmem -bench=.
-goos: linux
-goarch: amd64
-pkg: go.jolheiser.com/overlay
-cpu: Intel(R) Core(TM) i7-4700MQ CPU @ 2.40GHz
-BenchmarkCache-8     	134959974	         9.003 ns/op	       0 B/op	       0 allocs/op
-BenchmarkNoCache-8   	  897212	      1369 ns/op	     280 B/op	       4 allocs/op
-PASS
-ok  	go.jolheiser.com/overlay	3.360s

overlay.go

@@ -18,12 +18,17 @@ type FS struct {
 	cache   map[string]bool
 }
 
+// PurgeCache purges the cache
+func (f *FS) PurgeCache() {
+	f.cache = make(map[string]bool)
+}
+
 func (f *FS) apn(name string) string {
 	return path.Join(f.root, name)
 }
 
 func (f *FS) exists(name string) bool {
-	if has, ok := f.cache[name]; ok && f.doCache {
+	if has, ok := f.cache[name]; ok {
 		return has
 	}
 	_, err := os.Stat(f.apn(name))
@@ -65,7 +70,6 @@ func New(root string, fs fs.FS, opts ...Option) (*FS, error) {
 	x := &FS{
 		fs:    fs,
 		root:  root,
-		doCache: true,
 		cache: make(map[string]bool),
 	}
 
@@ -94,13 +98,3 @@ func WithSub(sub string) Option {
 		return
 	}
 }
-
-// WithCaching sets a caching mode for an FS
-// Caching avoids subsequent os.Stat to determine if a file exists on disk
-// See bench.txt for differences in usage
-func WithCaching(doCache bool) Option {
-	return func(x *FS) error {
-		x.doCache = doCache
-		return nil
-	}
-}