fix invalid path (#598)

path.go

@@ -83,6 +83,9 @@ end:
 }
 
 func parsePathDot(b *PathBuilder, buf []rune, cursor int) (*PathBuilder, []rune, int, error) {
+	if b.root == nil || b.node == nil {
+		return nil, nil, 0, fmt.Errorf("required '$' character at first: %w", ErrInvalidPathString)
+	}
 	length := len(buf)
 	if cursor+1 < length && buf[cursor+1] == '.' {
 		b, buf, c, err := parsePathRecursive(b, buf, cursor)
@@ -119,6 +122,10 @@ end:
 }
 
 func parseQuotedKey(b *PathBuilder, buf []rune, cursor int) (*PathBuilder, []rune, int, error) {
+	if b.root == nil || b.node == nil {
+		return nil, nil, 0, fmt.Errorf("required '$' character at first: %w", ErrInvalidPathString)
+	}
+
 	cursor++ // skip single quote
 	start := cursor
 	length := len(buf)
@@ -156,6 +163,10 @@ end:
 }
 
 func parsePathIndex(b *PathBuilder, buf []rune, cursor int) (*PathBuilder, []rune, int, error) {
+	if b.root == nil || b.node == nil {
+		return nil, nil, 0, fmt.Errorf("required '$' character at first: %w", ErrInvalidPathString)
+	}
+
 	length := len(buf)
 	cursor++ // skip '[' character
 	if length <= cursor {

path_test.go

@@ -603,6 +603,33 @@ building:
 	}
 }
 
+func TestInvalidPath(t *testing.T) {
+	tests := []struct {
+		name string
+		path string
+	}{
+		{
+			name: "missing root with dot",
+			path: ".foo",
+		},
+		{
+			name: "missing root with index",
+			path: "foo[0]",
+		},
+		{
+			name: "missing root with recursive",
+			path: "..foo",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if _, err := yaml.PathString(test.path); err == nil {
+				t.Fatal("expected error")
+			}
+		})
+	}
+}
+
 func ExamplePath_AnnotateSource() {
 	yml := `
 a: 1