Fix JoinPathSegments to disallow dot segments (#221)

JoinPathSegments does not fail for dot segments but should.

This PR also exports ValidatePathSegment for convenience.

Signed-off-by: Andrew Harding <andrew@spirl.com>

v2/spiffeid/path.go

@@ -23,7 +23,7 @@ func FormatPath(format string, args ...interface{}) (string, error) {
 func JoinPathSegments(segments ...string) (string, error) {
 	var builder strings.Builder
 	for _, segment := range segments {
-		if err := validatePathSegment(segment); err != nil {
+		if err := ValidatePathSegment(segment); err != nil {
 			return "", err
 		}
 		builder.WriteByte('/')
@@ -71,9 +71,15 @@ func ValidatePath(path string) error {
 	return nil
 }
 
-func validatePathSegment(segment string) error {
-	if segment == "" {
+// ValidatePathSegment validates that a string is a conformant segment for
+// inclusion in the path for a SPIFFE ID.
+// See https://github.com/spiffe/spiffe/blob/main/standards/SPIFFE-ID.md#22-path
+func ValidatePathSegment(segment string) error {
+	switch segment {
+	case "":
 		return errEmptySegment
+	case ".", "..":
+		return errDotSegment
 	}
 	for i := 0; i < len(segment); i++ {
 		if !isValidPathSegmentChar(segment[i]) {

v2/spiffeid/path_test.go

@@ -0,0 +1,58 @@
+package spiffeid
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestJoinPathSegments(t *testing.T) {
+	assertBad := func(t *testing.T, expectErr error, segments ...string) {
+		_, err := JoinPathSegments(segments...)
+		assert.ErrorIs(t, err, expectErr)
+	}
+	assertOK := func(t *testing.T, expectPath string, segments ...string) {
+		path, err := JoinPathSegments(segments...)
+		if assert.NoError(t, err) {
+			assert.Equal(t, expectPath, path)
+		}
+	}
+
+	t.Run("empty", func(t *testing.T) {
+		assertBad(t, errEmptySegment, "")
+	})
+	t.Run("single dot", func(t *testing.T) {
+		assertBad(t, errDotSegment, ".")
+	})
+	t.Run("double dot", func(t *testing.T) {
+		assertBad(t, errDotSegment, "..")
+	})
+	t.Run("invalid char", func(t *testing.T) {
+		assertBad(t, errBadPathSegmentChar, "/")
+	})
+	t.Run("valid segment", func(t *testing.T) {
+		assertOK(t, "/a", "a")
+	})
+	t.Run("valid segments", func(t *testing.T) {
+		assertOK(t, "/a/b", "a", "b")
+	})
+}
+
+func TestValidatePathSegment(t *testing.T) {
+	t.Run("empty", func(t *testing.T) {
+		require.ErrorIs(t, ValidatePathSegment(""), errEmptySegment)
+	})
+	t.Run("single dot", func(t *testing.T) {
+		require.ErrorIs(t, ValidatePathSegment("."), errDotSegment)
+	})
+	t.Run("double dot", func(t *testing.T) {
+		require.ErrorIs(t, ValidatePathSegment(".."), errDotSegment)
+	})
+	t.Run("invalid char", func(t *testing.T) {
+		require.ErrorIs(t, ValidatePathSegment("/"), errBadPathSegmentChar)
+	})
+	t.Run("valid segment", func(t *testing.T) {
+		require.NoError(t, ValidatePathSegment("a"))
+	})
+}