--- /dev/null
+package afero
+
+import (
+ "os"
+ "syscall"
+ "time"
+)
+
+// If the cache duration is 0, cache time will be unlimited, i.e. once
+// a file is in the layer, the base will never be read again for this file.
+//
+// For cache times greater than 0, the modification time of a file is
+// checked. Note that a lot of file system implementations only allow a
+// resolution of a second for timestamps... or as the godoc for os.Chtimes()
+// states: "The underlying filesystem may truncate or round the values to a
+// less precise time unit."
+//
+// This caching union will forward all write calls also to the base file
+// system first. To prevent writing to the base Fs, wrap it in a read-only
+// filter - Note: this will also make the overlay read-only, for writing files
+// in the overlay, use the overlay Fs directly, not via the union Fs.
+type CacheOnReadFs struct {
+ base Fs
+ layer Fs
+ cacheTime time.Duration
+}
+
+func NewCacheOnReadFs(base Fs, layer Fs, cacheTime time.Duration) Fs {
+ return &CacheOnReadFs{base: base, layer: layer, cacheTime: cacheTime}
+}
+
+type cacheState int
+
+const (
+ // not present in the overlay, unknown if it exists in the base:
+ cacheMiss cacheState = iota
+ // present in the overlay and in base, base file is newer:
+ cacheStale
+ // present in the overlay - with cache time == 0 it may exist in the base,
+ // with cacheTime > 0 it exists in the base and is same age or newer in the
+ // overlay
+ cacheHit
+ // happens if someone writes directly to the overlay without
+ // going through this union
+ cacheLocal
+)
+
+func (u *CacheOnReadFs) cacheStatus(name string) (state cacheState, fi os.FileInfo, err error) {
+ var lfi, bfi os.FileInfo
+ lfi, err = u.layer.Stat(name)
+ if err == nil {
+ if u.cacheTime == 0 {
+ return cacheHit, lfi, nil
+ }
+ if lfi.ModTime().Add(u.cacheTime).Before(time.Now()) {
+ bfi, err = u.base.Stat(name)
+ if err != nil {
+ return cacheLocal, lfi, nil
+ }
+ if bfi.ModTime().After(lfi.ModTime()) {
+ return cacheStale, bfi, nil
+ }
+ }
+ return cacheHit, lfi, nil
+ }
+
+ if err == syscall.ENOENT || os.IsNotExist(err) {
+ return cacheMiss, nil, nil
+ }
+
+ return cacheMiss, nil, err
+}
+
+func (u *CacheOnReadFs) copyToLayer(name string) error {
+ return copyToLayer(u.base, u.layer, name)
+}
+
+func (u *CacheOnReadFs) Chtimes(name string, atime, mtime time.Time) error {
+ st, _, err := u.cacheStatus(name)
+ if err != nil {
+ return err
+ }
+ switch st {
+ case cacheLocal:
+ case cacheHit:
+ err = u.base.Chtimes(name, atime, mtime)
+ case cacheStale, cacheMiss:
+ if err := u.copyToLayer(name); err != nil {
+ return err
+ }
+ err = u.base.Chtimes(name, atime, mtime)
+ }
+ if err != nil {
+ return err
+ }
+ return u.layer.Chtimes(name, atime, mtime)
+}
+
+func (u *CacheOnReadFs) Chmod(name string, mode os.FileMode) error {
+ st, _, err := u.cacheStatus(name)
+ if err != nil {
+ return err
+ }
+ switch st {
+ case cacheLocal:
+ case cacheHit:
+ err = u.base.Chmod(name, mode)
+ case cacheStale, cacheMiss:
+ if err := u.copyToLayer(name); err != nil {
+ return err
+ }
+ err = u.base.Chmod(name, mode)
+ }
+ if err != nil {
+ return err
+ }
+ return u.layer.Chmod(name, mode)
+}
+
+func (u *CacheOnReadFs) Stat(name string) (os.FileInfo, error) {
+ st, fi, err := u.cacheStatus(name)
+ if err != nil {
+ return nil, err
+ }
+ switch st {
+ case cacheMiss:
+ return u.base.Stat(name)
+ default: // cacheStale has base, cacheHit and cacheLocal the layer os.FileInfo
+ return fi, nil
+ }
+}
+
+func (u *CacheOnReadFs) Rename(oldname, newname string) error {
+ st, _, err := u.cacheStatus(oldname)
+ if err != nil {
+ return err
+ }
+ switch st {
+ case cacheLocal:
+ case cacheHit:
+ err = u.base.Rename(oldname, newname)
+ case cacheStale, cacheMiss:
+ if err := u.copyToLayer(oldname); err != nil {
+ return err
+ }
+ err = u.base.Rename(oldname, newname)
+ }
+ if err != nil {
+ return err
+ }
+ return u.layer.Rename(oldname, newname)
+}
+
+func (u *CacheOnReadFs) Remove(name string) error {
+ st, _, err := u.cacheStatus(name)
+ if err != nil {
+ return err
+ }
+ switch st {
+ case cacheLocal:
+ case cacheHit, cacheStale, cacheMiss:
+ err = u.base.Remove(name)
+ }
+ if err != nil {
+ return err
+ }
+ return u.layer.Remove(name)
+}
+
+func (u *CacheOnReadFs) RemoveAll(name string) error {
+ st, _, err := u.cacheStatus(name)
+ if err != nil {
+ return err
+ }
+ switch st {
+ case cacheLocal:
+ case cacheHit, cacheStale, cacheMiss:
+ err = u.base.RemoveAll(name)
+ }
+ if err != nil {
+ return err
+ }
+ return u.layer.RemoveAll(name)
+}
+
+func (u *CacheOnReadFs) OpenFile(name string, flag int, perm os.FileMode) (File, error) {
+ st, _, err := u.cacheStatus(name)
+ if err != nil {
+ return nil, err
+ }
+ switch st {
+ case cacheLocal, cacheHit:
+ default:
+ if err := u.copyToLayer(name); err != nil {
+ return nil, err
+ }
+ }
+ if flag&(os.O_WRONLY|syscall.O_RDWR|os.O_APPEND|os.O_CREATE|os.O_TRUNC) != 0 {
+ bfi, err := u.base.OpenFile(name, flag, perm)
+ if err != nil {
+ return nil, err
+ }
+ lfi, err := u.layer.OpenFile(name, flag, perm)
+ if err != nil {
+ bfi.Close() // oops, what if O_TRUNC was set and file opening in the layer failed...?
+ return nil, err
+ }
+ return &UnionFile{base: bfi, layer: lfi}, nil
+ }
+ return u.layer.OpenFile(name, flag, perm)
+}
+
+func (u *CacheOnReadFs) Open(name string) (File, error) {
+ st, fi, err := u.cacheStatus(name)
+ if err != nil {
+ return nil, err
+ }
+
+ switch st {
+ case cacheLocal:
+ return u.layer.Open(name)
+
+ case cacheMiss:
+ bfi, err := u.base.Stat(name)
+ if err != nil {
+ return nil, err
+ }
+ if bfi.IsDir() {
+ return u.base.Open(name)
+ }
+ if err := u.copyToLayer(name); err != nil {
+ return nil, err
+ }
+ return u.layer.Open(name)
+
+ case cacheStale:
+ if !fi.IsDir() {
+ if err := u.copyToLayer(name); err != nil {
+ return nil, err
+ }
+ return u.layer.Open(name)
+ }
+ case cacheHit:
+ if !fi.IsDir() {
+ return u.layer.Open(name)
+ }
+ }
+ // the dirs from cacheHit, cacheStale fall down here:
+ bfile, _ := u.base.Open(name)
+ lfile, err := u.layer.Open(name)
+ if err != nil && bfile == nil {
+ return nil, err
+ }
+ return &UnionFile{base: bfile, layer: lfile}, nil
+}
+
+func (u *CacheOnReadFs) Mkdir(name string, perm os.FileMode) error {
+ err := u.base.Mkdir(name, perm)
+ if err != nil {
+ return err
+ }
+ return u.layer.MkdirAll(name, perm) // yes, MkdirAll... we cannot assume it exists in the cache
+}
+
+func (u *CacheOnReadFs) Name() string {
+ return "CacheOnReadFs"
+}
+
+func (u *CacheOnReadFs) MkdirAll(name string, perm os.FileMode) error {
+ err := u.base.MkdirAll(name, perm)
+ if err != nil {
+ return err
+ }
+ return u.layer.MkdirAll(name, perm)
+}
+
+func (u *CacheOnReadFs) Create(name string) (File, error) {
+ bfh, err := u.base.Create(name)
+ if err != nil {
+ return nil, err
+ }
+ lfh, err := u.layer.Create(name)
+ if err != nil {
+ // oops, see comment about OS_TRUNC above, should we remove? then we have to
+ // remember if the file did not exist before
+ bfh.Close()
+ return nil, err
+ }
+ return &UnionFile{base: bfh, layer: lfh}, nil
+}
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