cache: Restructure New to remove redundant operations

New and its helpers used to create the cache directories several times
over. They now only do so once. The added test ensures that the cache is
produced in a consistent state when parts are deleted.

internal/cache/cache_test.go

@@ -0,0 +1,46 @@
+package cache
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/restic/restic/internal/restic"
+	rtest "github.com/restic/restic/internal/test"
+)
+
+func TestNew(t *testing.T) {
+	parent := rtest.TempDir(t)
+	basedir := filepath.Join(parent, "cache")
+	id := restic.NewRandomID().String()
+	tagFile := filepath.Join(basedir, "CACHEDIR.TAG")
+	versionFile := filepath.Join(basedir, id, "version")
+
+	const (
+		stepCreate = iota
+		stepComplete
+		stepRmTag
+		stepRmVersion
+		stepEnd
+	)
+
+	for step := stepCreate; step < stepEnd; step++ {
+		switch step {
+		case stepRmTag:
+			rtest.OK(t, os.Remove(tagFile))
+		case stepRmVersion:
+			rtest.OK(t, os.Remove(versionFile))
+		}
+
+		c, err := New(id, basedir)
+		rtest.OK(t, err)
+		rtest.Equals(t, basedir, c.Base)
+		rtest.Equals(t, step == stepCreate, c.Created)
+
+		for _, name := range []string{tagFile, versionFile} {
+			info, err := os.Lstat(name)
+			rtest.OK(t, err)
+			rtest.Assert(t, info.Mode().IsRegular(), "")
+		}
+	}
+}