support to disallow duplicate map key at parsing

decode.go

@@ -38,7 +38,7 @@ type Decoder struct {
 	isResolvedReference  bool
 	validator            StructValidator
 	disallowUnknownField bool
-	disallowDuplicateKey bool
+	allowDuplicateMapKey bool
 	useOrderedMap        bool
 	useJSONUnmarshaler   bool
 	parsedFile           *ast.File
@@ -60,7 +60,7 @@ func NewDecoder(r io.Reader, opts ...DecodeOption) *Decoder {
 		isRecursiveDir:       false,
 		isResolvedReference:  false,
 		disallowUnknownField: false,
-		disallowDuplicateKey: false,
+		allowDuplicateMapKey: false,
 		useOrderedMap:        false,
 	}
 }
@@ -1570,7 +1570,7 @@ func (d *Decoder) validateDuplicateKey(keyMap map[string]struct{}, key interface
 	if !ok {
 		return nil
 	}
-	if d.disallowDuplicateKey {
+	if !d.allowDuplicateMapKey {
 		if _, exists := keyMap[k]; exists {
 			return errDuplicateKey(fmt.Sprintf(`duplicate key "%s"`, k), keyNode.GetToken())
 		}
@@ -1806,7 +1806,11 @@ func (d *Decoder) parse(bytes []byte) (*ast.File, error) {
 	if d.toCommentMap != nil {
 		parseMode = parser.ParseComments
 	}
-	f, err := parser.ParseBytes(bytes, parseMode)
+	var opts []parser.Option
+	if d.allowDuplicateMapKey {
+		opts = append(opts, parser.AllowDuplicateMapKey())
+	}
+	f, err := parser.ParseBytes(bytes, parseMode, opts...)
 	if err != nil {
 		return nil, err
 	}

decode_test.go

@@ -956,10 +956,6 @@ func TestDecoder(t *testing.T) {
 			"a: &a [1, 2]\nb: *a\n",
 			struct{ B []int }{[]int{1, 2}},
 		},
-		{
-			"&0: *0\n*0:\n*0:",
-			map[string]any{"null": nil},
-		},
 		{
 			"key1: &anchor\n  subkey: *anchor\nkey2: *anchor\n",
 			map[string]any{
@@ -1502,7 +1498,7 @@ items:
 		Items []*Item
 	}
 	buf := bytes.NewBufferString(yml)
-	dec := yaml.NewDecoder(buf)
+	dec := yaml.NewDecoder(buf, yaml.AllowDuplicateMapKey())
 	var v T
 	if err := dec.Decode(&v); err != nil {
 		t.Fatalf("%+v", err)
@@ -1834,39 +1830,23 @@ children:
 	})
 }
 
-func TestDecoder_DisallowDuplicateKey(t *testing.T) {
+func TestDecoder_AllowDuplicateMapKey(t *testing.T) {
 	yml := `
 a: b
 a: c
-`
-	expected := `
-[3:1] duplicate key "a"
-   2 | a: b
->  3 | a: c
-       ^
 `
 	t.Run("map", func(t *testing.T) {
 		var v map[string]string
-		err := yaml.NewDecoder(strings.NewReader(yml), yaml.DisallowDuplicateKey()).Decode(&v)
-		if err == nil {
-			t.Fatal("decoding should fail")
-		}
-		actual := "\n" + err.Error()
-		if expected != actual {
-			t.Fatalf("expected:[%s] actual:[%s]", expected, actual)
+		if err := yaml.NewDecoder(strings.NewReader(yml), yaml.AllowDuplicateMapKey()).Decode(&v); err != nil {
+			t.Fatal(err)
 		}
 	})
 	t.Run("struct", func(t *testing.T) {
 		var v struct {
 			A string
 		}
-		err := yaml.NewDecoder(strings.NewReader(yml), yaml.DisallowDuplicateKey()).Decode(&v)
-		if err == nil {
-			t.Fatal("decoding should fail")
-		}
-		actual := "\n" + err.Error()
-		if expected != actual {
-			t.Fatalf("expected:[%s] actual:[%s]", expected, actual)
+		if err := yaml.NewDecoder(strings.NewReader(yml), yaml.AllowDuplicateMapKey()).Decode(&v); err != nil {
+			t.Fatal(err)
 		}
 	})
 }
@@ -3099,3 +3079,23 @@ nested:
 		t.Fatal("decoder doesn't preserve struct defaults")
 	}
 }
+
+func TestDecodeError(t *testing.T) {
+	tests := []struct {
+		name   string
+		source string
+	}{
+		{
+			name:   "duplicated map key name with anchor-alias",
+			source: "&0: *0\n*0:\n*0:",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			var v any
+			if err := yaml.Unmarshal([]byte(test.source), &v); err == nil {
+				t.Fatal("cannot catch decode error")
+			}
+		})
+	}
+}

option.go

@@ -54,7 +54,6 @@ func Validator(v StructValidator) DecodeOption {
 func Strict() DecodeOption {
 	return func(d *Decoder) error {
 		d.disallowUnknownField = true
-		d.disallowDuplicateKey = true
 		return nil
 	}
 }
@@ -69,10 +68,10 @@ func DisallowUnknownField() DecodeOption {
 	}
 }
 
-// DisallowDuplicateKey causes an error when mapping keys that are duplicates
-func DisallowDuplicateKey() DecodeOption {
+// AllowDuplicateMapKey ignore syntax error when mapping keys that are duplicates.
+func AllowDuplicateMapKey() DecodeOption {
 	return func(d *Decoder) error {
-		d.disallowDuplicateKey = true
+		d.allowDuplicateMapKey = true
 		return nil
 	}
 }

parser/option.go

@@ -0,0 +1,12 @@
+package parser
+
+// Option represents parser's option.
+type Option func(p *parser)
+
+// AllowDuplicateMapKey allow the use of keys with the same name in the same map,
+// but by default, this is not permitted.
+func AllowDuplicateMapKey() Option {
+	return func(p *parser) {
+		p.allowDuplicateMapKey = true
+	}
+}

parser/parser.go

@@ -12,12 +12,14 @@ import (
 )
 
 type parser struct {
-	idx    int
-	size   int
-	tokens token.Tokens
+	idx                  int
+	size                 int
+	tokens               token.Tokens
+	pathMap              map[string]ast.Node
+	allowDuplicateMapKey bool
 }
 
-func newParser(tokens token.Tokens, mode Mode) *parser {
+func newParser(tokens token.Tokens, mode Mode, opts []Option) *parser {
 	filteredTokens := []*token.Token{}
 	if mode&ParseComments != 0 {
 		filteredTokens = tokens
@@ -31,11 +33,16 @@ func newParser(tokens token.Tokens, mode Mode) *parser {
 			filteredTokens = append(filteredTokens, tk)
 		}
 	}
-	return &parser{
-		idx:    0,
-		size:   len(filteredTokens),
-		tokens: token.Tokens(filteredTokens),
+	p := &parser{
+		idx:     0,
+		size:    len(filteredTokens),
+		tokens:  token.Tokens(filteredTokens),
+		pathMap: make(map[string]ast.Node),
 	}
+	for _, opt := range opts {
+		opt(p)
+	}
+	return p
 }
 
 func (p *parser) next() bool {
@@ -281,7 +288,16 @@ func (p *parser) existsNewLineCharacter(src string) bool {
 	return false
 }
 
-func (p *parser) validateMapKey(tk *token.Token) error {
+func (p *parser) validateMapKey(tk *token.Token, keyPath string) error {
+	if !p.allowDuplicateMapKey {
+		if n, exists := p.pathMap[keyPath]; exists {
+			pos := n.GetToken().Position
+			return errors.ErrSyntax(
+				fmt.Sprintf("mapping key %q already defined at [%d:%d]", tk.Value, pos.Line, pos.Column),
+				tk,
+			)
+		}
+	}
 	if tk.Type != token.StringType {
 		return nil
 	}
@@ -323,7 +339,7 @@ func (p *parser) createMapValueNode(ctx *context, key ast.MapKeyNode, colonToken
 	if tk.Type == token.CommentType {
 		comment = p.parseCommentOnly(ctx)
 		if comment != nil {
-			comment.SetPath(ctx.withChild(key.GetToken().Value).path)
+			comment.SetPath(ctx.withChild(p.mapKeyText(key)).path)
 		}
 		tk = p.currentToken()
 	}
@@ -397,16 +413,28 @@ func (p *parser) validateMapValue(ctx *context, key, value ast.Node) error {
 	return nil
 }
 
+func (p *parser) mapKeyText(n ast.Node) string {
+	switch nn := n.(type) {
+	case *ast.MappingKeyNode:
+		return p.mapKeyText(nn.Value)
+	case *ast.TagNode:
+		return p.mapKeyText(nn.Value)
+	}
+	return n.GetToken().Value
+}
+
 func (p *parser) parseMappingValue(ctx *context) (ast.Node, error) {
 	key, err := p.parseMapKey(ctx)
 	if err != nil {
 		return nil, err
 	}
-	keyText := key.GetToken().Value
-	key.SetPath(ctx.withChild(keyText).path)
-	if err := p.validateMapKey(key.GetToken()); err != nil {
+	keyText := p.mapKeyText(key)
+	keyPath := ctx.withChild(keyText).path
+	key.SetPath(keyPath)
+	if err := p.validateMapKey(key.GetToken(), keyPath); err != nil {
 		return nil, err
 	}
+	p.pathMap[keyPath] = key
 	p.progress(1) // progress to mapping value token
 	if ctx.isFlow {
 		// if "{key}" or "{key," style, returns MappingValueNode.
@@ -557,7 +585,7 @@ func (p *parser) parseFlowMapNullValue(ctx *context, key ast.MapKeyNode) (*ast.M
 		return nil, err
 	}
 	node := ast.MappingValue(tk, key, value)
-	node.SetPath(ctx.withChild(key.GetToken().Value).path)
+	node.SetPath(ctx.withChild(p.mapKeyText(key)).path)
 	return node, nil
 }
 
@@ -868,7 +896,7 @@ func (p *parser) parseMappingKey(ctx *context) (*ast.MappingKeyNode, error) {
 	node := ast.MappingKey(keyTk)
 	node.SetPath(ctx.path)
 	p.progress(1) // skip mapping key token
-	value, err := p.parseToken(ctx.withChild(keyTk.Value), p.currentToken())
+	value, err := p.parseToken(ctx, p.currentToken())
 	if err != nil {
 		return nil, err
 	}
@@ -962,34 +990,34 @@ const (
 )
 
 // ParseBytes parse from byte slice, and returns ast.File
-func ParseBytes(bytes []byte, mode Mode) (*ast.File, error) {
+func ParseBytes(bytes []byte, mode Mode, opts ...Option) (*ast.File, error) {
 	tokens := lexer.Tokenize(string(bytes))
-	f, err := Parse(tokens, mode)
+	f, err := Parse(tokens, mode, opts...)
 	if err != nil {
 		return nil, err
 	}
 	return f, nil
 }
 
 // Parse parse from token instances, and returns ast.File
-func Parse(tokens token.Tokens, mode Mode) (*ast.File, error) {
+func Parse(tokens token.Tokens, mode Mode, opts ...Option) (*ast.File, error) {
 	if tk := tokens.InvalidToken(); tk != nil {
 		return nil, errors.ErrSyntax("found invalid token", tk)
 	}
-	f, err := newParser(tokens, mode).parse(newContext())
+	f, err := newParser(tokens, mode, opts).parse(newContext())
 	if err != nil {
 		return nil, err
 	}
 	return f, nil
 }
 
 // Parse parse from filename, and returns ast.File
-func ParseFile(filename string, mode Mode) (*ast.File, error) {
+func ParseFile(filename string, mode Mode, opts ...Option) (*ast.File, error) {
 	file, err := os.ReadFile(filename)
 	if err != nil {
 		return nil, err
 	}
-	f, err := ParseBytes(file, mode)
+	f, err := ParseBytes(file, mode, opts...)
 	if err != nil {
 		return nil, err
 	}

parser/parser_test.go

@@ -1284,6 +1284,42 @@ a:
 [1:4] found an invalid key for this map
 >  1 | [a]: v
           ^
+`,
+		},
+		{
+			`
+foo:
+  bar:
+    foo: 2
+  baz:
+    foo: 3
+foo: 2
+`,
+			`
+[7:1] mapping key "foo" already defined at [2:1]
+   4 |     foo: 2
+   5 |   baz:
+   6 |     foo: 3
+>  7 | foo: 2
+       ^
+`,
+		},
+		{
+			`
+foo:
+  bar:
+    foo: 2
+  baz:
+    foo: 3
+    foo: 4
+`,
+			`
+[7:5] mapping key "foo" already defined at [6:5]
+   4 |     foo: 2
+   5 |   baz:
+   6 |     foo: 3
+>  7 |     foo: 4
+           ^
 `,
 		},
 	}