cache: move to backend package

internal/backend/cache/dir_test.go

@@ -0,0 +1,31 @@
+package cache
+
+import (
+	"os"
+	"testing"
+
+	rtest "github.com/restic/restic/internal/test"
+)
+
+// DefaultDir should honor RESTIC_CACHE_DIR on all platforms.
+func TestCacheDirEnv(t *testing.T) {
+	cachedir := os.Getenv("RESTIC_CACHE_DIR")
+
+	if cachedir == "" {
+		cachedir = "/doesnt/exist"
+		err := os.Setenv("RESTIC_CACHE_DIR", cachedir)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer func() {
+			err := os.Unsetenv("RESTIC_CACHE_DIR")
+			if err != nil {
+				t.Fatal(err)
+			}
+		}()
+	}
+
+	dir, err := DefaultDir()
+	rtest.Equals(t, cachedir, dir)
+	rtest.OK(t, err)
+}