dirs: rewrite and test StripRootDir

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

dirs/dirs.go

@@ -20,8 +20,10 @@
 package dirs
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
+	"strings"
 )
 
 // the various file paths
@@ -75,10 +77,17 @@ func init() {
 
 // StripRootDir strips the custom global root directory from the specified argument.
 func StripRootDir(dir string) string {
-	if GlobalRootDir == "/" {
-		return dir
+	if !filepath.IsAbs(dir) {
+		panic(fmt.Sprintf("supplied path is not absolute %q", dir))
 	}
-	return dir[len(GlobalRootDir):]
+	if !strings.HasPrefix(dir, GlobalRootDir) {
+		panic(fmt.Sprintf("supplied path is not related to global root %q", dir))
+	}
+	result, err := filepath.Rel(GlobalRootDir, dir)
+	if err != nil {
+		panic(err)
+	}
+	return "/" + result
 }
 
 // SetRootDir allows settings a new global root directory, this is useful

dirs/dirs_test.go

@@ -0,0 +1,49 @@
+// -*- Mode: Go; indent-tabs-mode: t -*-
+
+/*
+ * Copyright (C) 2016 Canonical Ltd
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+package dirs_test
+
+import (
+	"testing"
+
+	. "gopkg.in/check.v1"
+
+	"github.com/snapcore/snapd/dirs"
+)
+
+// Hook up check.v1 into the "go test" runner
+func Test(t *testing.T) { TestingT(t) }
+
+var _ = Suite(&DirsTestSuite{})
+
+type DirsTestSuite struct{}
+
+func (s *DirsTestSuite) TestStripRootDir(c *C) {
+	// strip does nothing if the default (empty) root directory is used
+	c.Check(dirs.StripRootDir("/foo/bar"), Equals, "/foo/bar")
+	// strip only works on absolute paths
+	c.Check(func() { dirs.StripRootDir("relative") }, Panics, `supplied path is not absolute "relative"`)
+	// with an alternate root
+	dirs.SetRootDir("/alt/")
+	defer dirs.SetRootDir("")
+	// strip behaves as expected, returning absolute paths without the prefix
+	c.Check(dirs.StripRootDir("/alt/foo/bar"), Equals, "/foo/bar")
+	// strip only works on paths that begin with the global root directory
+	c.Check(func() { dirs.StripRootDir("/other/foo/bar") }, Panics, `supplied path is not related to global root "/other/foo/bar"`)
+}