From 6f80c5700f899d360742c6dbd5ee8c944d5297cb Mon Sep 17 00:00:00 2001
From: hbc <me@hbc.rocks>
Date: Thu, 30 Jan 2025 16:56:28 -0800
Subject: [PATCH] fix: empty document should not panic on String() (#630)

fixes #629
---
 ast/ast.go            |  4 +++-
 parser/parser_test.go | 43 ++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/ast/ast.go b/ast/ast.go
index 7f342275..d9607fa9 100644
--- a/ast/ast.go
+++ b/ast/ast.go
@@ -576,7 +576,9 @@ func (d *DocumentNode) String() string {
 	if d.Start != nil {
 		doc = append(doc, d.Start.Value)
 	}
-	doc = append(doc, d.Body.String())
+	if d.Body != nil {
+		doc = append(doc, d.Body.String())
+	}
 	if d.End != nil {
 		doc = append(doc, d.End.Value)
 	}
diff --git a/parser/parser_test.go b/parser/parser_test.go
index 8a809253..487bfe10 100644
--- a/parser/parser_test.go
+++ b/parser/parser_test.go
@@ -16,6 +16,7 @@ import (
 
 func TestParser(t *testing.T) {
 	sources := []string{
+		"",
 		"null\n",
 		"0_",
 		"{}\n",
@@ -144,12 +145,52 @@ foo: zzz
 `,
 	}
 	for _, src := range sources {
-		if _, err := parser.Parse(lexer.Tokenize(src), 0); err != nil {
+		f, err := parser.Parse(lexer.Tokenize(src), 0)
+		if err != nil {
 			t.Fatalf("parse error: source [%s]: %+v", src, err)
 		}
+		_ = f.String() // ensure no panic
 	}
 }
 
+func TestParseEmptyDocument(t *testing.T) {
+	t.Run("empty document", func(t *testing.T) {
+		f, err := parser.ParseBytes([]byte(""), parser.ParseComments)
+		if err != nil {
+			t.Fatal(err)
+		}
+		got := f.String()
+		expected := "\n"
+		if got != expected {
+			t.Fatalf("failed to parse comment:\nexpected:\n%q\ngot:\n%q", expected, got)
+		}
+	})
+
+	t.Run("empty document with comment (parse comment = off)", func(t *testing.T) {
+		f, err := parser.ParseBytes([]byte("# comment"), 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		got := f.String()
+		expected := "\n"
+		if got != expected {
+			t.Fatalf("failed to parse comment:\nexpected:\n%q\ngot:\n%q", expected, got)
+		}
+	})
+
+	t.Run("empty document with comment (parse comment = on)", func(t *testing.T) {
+		f, err := parser.ParseBytes([]byte("# comment"), parser.ParseComments)
+		if err != nil {
+			t.Fatal(err)
+		}
+		got := f.String()
+		expected := "# comment\n"
+		if got != expected {
+			t.Fatalf("failed to parse comment:\nexpected:\n%q\ngot:\n%q", expected, got)
+		}
+	})
+}
+
 func TestParseComplicatedDocument(t *testing.T) {
 	tests := []struct {
 		source string