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/dir.go

@@ -0,0 +1,49 @@
+package cache
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/pkg/errors"
+	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/fs"
+)
+
+// getXDGCacheDir returns the cache directory according to XDG basedir spec, see
+// http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
+func getXDGCacheDir() (string, error) {
+	xdgcache := os.Getenv("XDG_CACHE_HOME")
+	home := os.Getenv("HOME")
+
+	if xdgcache == "" && home == "" {
+		return "", errors.New("unable to locate cache directory (XDG_CACHE_HOME and HOME unset)")
+	}
+
+	cachedir := ""
+	if xdgcache != "" {
+		cachedir = filepath.Join(xdgcache, "restic")
+	} else if home != "" {
+		cachedir = filepath.Join(home, ".cache", "restic")
+	}
+
+	fi, err := fs.Stat(cachedir)
+	if os.IsNotExist(errors.Cause(err)) {
+		err = fs.MkdirAll(cachedir, 0700)
+		if err != nil {
+			return "", errors.Wrap(err, "MkdirAll")
+		}
+
+		fi, err = fs.Stat(cachedir)
+		debug.Log("create cache dir %v", cachedir)
+	}
+
+	if err != nil {
+		return "", errors.Wrap(err, "Stat")
+	}
+
+	if !fi.IsDir() {
+		return "", errors.Errorf("cache dir %v is not a directory", cachedir)
+	}
+
+	return cachedir, nil
+}