Add cache

This commits adds rudimentary support for a cache directory, enabled by
default. The cache directory is created if it does not exist. The cache
is used if there's anything in it, newly created snapshot and index
files are written to the cache automatically.

internal/cache/cache.go

@@ -0,0 +1,122 @@
+package cache
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strconv"
+
+	"github.com/pkg/errors"
+	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/fs"
+	"github.com/restic/restic/internal/restic"
+)
+
+// Cache manages a local cache.
+type Cache struct {
+	Path string
+}
+
+const dirMode = 0700
+const fileMode = 0600
+
+func readVersion(dir string) (v uint, err error) {
+	buf, err := ioutil.ReadFile(filepath.Join(dir, "version"))
+	if os.IsNotExist(err) {
+		return 0, nil
+	}
+
+	if err != nil {
+		return 0, errors.Wrap(err, "ReadFile")
+	}
+
+	ver, err := strconv.ParseUint(string(buf), 10, 32)
+	if err != nil {
+		return 0, errors.Wrap(err, "ParseUint")
+	}
+
+	return uint(ver), nil
+}
+
+const cacheVersion = 1
+
+// ensure Cache implements restic.Cache
+var _ restic.Cache = &Cache{}
+
+var cacheLayoutPaths = map[restic.FileType]string{
+	restic.DataFile:     "data",
+	restic.SnapshotFile: "snapshots",
+	restic.IndexFile:    "index",
+}
+
+// New returns a new cache for the repo ID at dir. If dir is the empty string,
+// the default cache location (according to the XDG standard) is used.
+func New(id string, dir string) (c *Cache, err error) {
+	if dir == "" {
+		dir, err = getXDGCacheDir()
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	cachedir := filepath.Join(dir, id)
+	debug.Log("using cache dir %v", cachedir)
+
+	v, err := readVersion(cachedir)
+	if err != nil {
+		return nil, err
+	}
+
+	if v > cacheVersion {
+		return nil, errors.New("cache version is newer")
+	}
+
+	// create the repo cache dir if it does not exist yet
+	if err = fs.MkdirAll(cachedir, dirMode); err != nil {
+		return nil, err
+	}
+
+	if v < cacheVersion {
+		err = ioutil.WriteFile(filepath.Join(cachedir, "version"), []byte(fmt.Sprintf("%d", cacheVersion)), 0644)
+		if err != nil {
+			return nil, errors.Wrap(err, "WriteFile")
+		}
+	}
+
+	for _, p := range cacheLayoutPaths {
+		if err = fs.MkdirAll(filepath.Join(cachedir, p), dirMode); err != nil {
+			return nil, err
+		}
+	}
+
+	c = &Cache{
+		Path: cachedir,
+	}
+
+	return c, nil
+}
+
+// errNoSuchFile is returned when a file is not cached.
+type errNoSuchFile struct {
+	Type string
+	Name string
+}
+
+func (e errNoSuchFile) Error() string {
+	return fmt.Sprintf("file %v (%v) is not cached", e.Name, e.Type)
+}
+
+// IsNotExist returns true if the error was caused by a non-existing file.
+func (c *Cache) IsNotExist(err error) bool {
+	_, ok := errors.Cause(err).(errNoSuchFile)
+	return ok
+}
+
+// Wrap returns a backend with a cache.
+func (c *Cache) Wrap(be restic.Backend) restic.Backend {
+	return &Backend{
+		Backend: be,
+		Cache:   c,
+	}
+}