Overlay file system
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package overlay
import (
"io/fs"
"os"
"path"
)
// Interface guard
// fs.ReadFileFS also fulfills fs.FS
var _ fs.ReadFileFS = (*FS)(nil)
// FS is an overlay File System
type FS struct {
fs fs.FS
root string
doCache bool
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 {
return has
}
_, err := os.Stat(f.apn(name))
if err != nil {
f.cache[name] = false
return false
}
f.cache[name] = true
return true
}
// Open opens an fs.File, preferring disk
func (f *FS) Open(name string) (fs.File, error) {
if f.exists(name) {
return os.Open(f.apn(name))
}
return f.fs.Open(name)
}
// ReadFile reads a file, preferring disk
func (f *FS) ReadFile(name string) ([]byte, error) {
if f.exists(name) {
return os.ReadFile(f.apn(name))
}
return fs.ReadFile(f.fs, name)
}
// ReadDir reads []fs.DirEntry
// This method will prefer EMBEDDED, because that is the "real" FS for overlay
func (f *FS) ReadDir(name string) ([]fs.DirEntry, error) {
return fs.ReadDir(f.fs, name)
}
// Option is a functional option for an FS
type Option func(*FS) error
// New returns a new FS
func New(root string, fs fs.FS, opts ...Option) (*FS, error) {
x := &FS{
fs: fs,
root: root,
cache: make(map[string]bool),
}
for _, opt := range opts {
if err := opt(x); err != nil {
return x, err
}
}
return x, nil
}
// MustNew returns New and panics on error
func MustNew(root string, fs fs.FS, opts ...Option) *FS {
f, err := New(root, fs, opts...)
if err != nil {
panic(err)
}
return f
}
// WithSub sets a fs.Sub for an FS
func WithSub(sub string) Option {
return func(x *FS) (err error) {
x.fs, err = fs.Sub(x.fs, sub)
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
}
}