Fix BasePathFs symlink

1. Support related path
2. Make ReadlinkIfPossible return path under basepath

Fixes #282

basepath.go

@@ -185,9 +185,12 @@ func (b *BasePathFs) LstatIfPossible(name string) (os.FileInfo, bool, error) {
 }
 
 func (b *BasePathFs) SymlinkIfPossible(oldname, newname string) error {
-	oldname, err := b.RealPath(oldname)
+	var err error
-	if err != nil {
+	if filepath.IsAbs(oldname) {
-		return &os.LinkError{Op: "symlink", Old: oldname, New: newname, Err: err}
+		oldname, err = b.RealPath(oldname)
+		if err != nil {
+			return &os.LinkError{Op: "symlink", Old: oldname, New: newname, Err: err}
+		}
 	}
 	newname, err = b.RealPath(newname)
 	if err != nil {
@@ -205,7 +208,14 @@ func (b *BasePathFs) ReadlinkIfPossible(name string) (string, error) {
 		return "", &os.PathError{Op: "readlink", Path: name, Err: err}
 	}
 	if reader, ok := b.source.(LinkReader); ok {
-		return reader.ReadlinkIfPossible(name)
+		linkpath, err := reader.ReadlinkIfPossible(name)
+		if err != nil {
+			return "", err
+		}
+		if filepath.IsAbs(linkpath) {
+			linkpath = strings.TrimPrefix(filepath.Clean(linkpath), filepath.Clean(b.path))
+		}
+		return linkpath, nil
 	}
 	return "", &os.PathError{Op: "readlink", Path: name, Err: ErrNoReadlink}
 }

basepath_test.go

@@ -188,3 +188,44 @@ func TestBasePathTempFile(t *testing.T) {
 		t.Fatalf("TempFile realpath leaked: expected %s, got %s", expected, actual)
 	}
 }
+
+func TestBasePathSymlink(t *testing.T) {
+	type SymlinkTest struct {
+		Testcase, Link, Target string
+	}
+	symlinkTests := []SymlinkTest{
+		{Testcase: "Absolute path", Link: "/foo/baz/abs_link", Target: "/foo/file"},
+		{Testcase: "Relative path", Link: "/foo/baz/ref_link", Target: "../file"},
+	}
+
+	fs := NewOsFs()
+	baseDir, err := TempDir(fs, "", "base")
+	if err != nil {
+		t.Fatal("error creating tempDir", err)
+	}
+	defer fs.RemoveAll(baseDir)
+
+	var bp Fs = NewBasePathFs(fs, baseDir).(*BasePathFs)
+	bp.MkdirAll("/foo/baz", 0777)
+	bp.Create("/foo/file")
+	linker, _ := bp.(Linker)
+	lreader, _ := bp.(LinkReader)
+
+	for _, test := range symlinkTests {
+		err = linker.SymlinkIfPossible(test.Target, test.Link)
+		if err != nil {
+			t.Errorf("%s: error creating symlink %s", test.Testcase, err.Error())
+		}
+		linkpath, err := lreader.ReadlinkIfPossible(test.Link)
+		if err != nil {
+			t.Errorf("%s: error read symlink %s", test.Testcase, err.Error())
+		}
+		if linkpath != test.Target {
+			t.Errorf("%s: link not match %s != %s", test.Testcase, linkpath, test.Target)
+		}
+		_, err = bp.Stat(test.Link)
+		if err != nil {
+			t.Errorf("%s: error stat symlink %s", test.Testcase, err.Error())
+		}
+	}
+}