do not allow forward references

decode.go

@@ -22,10 +22,12 @@ type node struct {
 	kind         int
 	line, column int
 	tag          string
-	value        string
-	implicit     bool
-	children     []*node
-	anchors      map[string]*node
+	// For an alias node, alias holds the resolved alias.
+	alias    *node
+	value    string
+	implicit bool
+	children []*node
+	anchors  map[string]*node
 }
 
 // ----------------------------------------------------------------------------
@@ -146,6 +148,10 @@ func (p *parser) document() *node {
 func (p *parser) alias() *node {
 	n := p.node(aliasNode)
 	n.value = string(p.event.anchor)
+	n.alias = p.doc.anchors[n.value]
+	if n.alias == nil {
+		failf("unknown anchor '%s' referenced", n.value)
+	}
 	p.skip()
 	return n
 }
@@ -187,7 +193,7 @@ func (p *parser) mapping() *node {
 
 type decoder struct {
 	doc     *node
-	aliases map[string]bool
+	aliases map[*node]bool
 	mapType reflect.Type
 	terrors []string
 	strict  bool
@@ -202,7 +208,7 @@ var (
 
 func newDecoder(strict bool) *decoder {
 	d := &decoder{mapType: defaultMapType, strict: strict}
-	d.aliases = make(map[string]bool)
+	d.aliases = make(map[*node]bool)
 	return d
 }
 
@@ -308,16 +314,13 @@ func (d *decoder) document(n *node, out reflect.Value) (good bool) {
 }
 
 func (d *decoder) alias(n *node, out reflect.Value) (good bool) {
-	an, ok := d.doc.anchors[n.value]
-	if !ok {
-		failf("unknown anchor '%s' referenced", n.value)
-	}
-	if d.aliases[n.value] {
+	if d.aliases[n] {
+		// TODO this could actually be allowed in some circumstances.
 		failf("anchor '%s' value contains itself", n.value)
 	}
-	d.aliases[n.value] = true
-	good = d.unmarshal(an, out)
-	delete(d.aliases, n.value)
+	d.aliases[n] = true
+	good = d.unmarshal(n.alias, out)
+	delete(d.aliases, n)
 	return good
 }
 

decode_test.go

@@ -2,13 +2,14 @@ package yaml_test
 
 import (
 	"errors"
-	. "gopkg.in/check.v1"
-	"gopkg.in/yaml.v2"
 	"math"
 	"net"
 	"reflect"
 	"strings"
 	"time"
+
+	. "gopkg.in/check.v1"
+	"gopkg.in/yaml.v2"
 )
 
 var unmarshalIntTest = 123
@@ -425,13 +426,6 @@ var unmarshalTests = []struct {
 	}, {
 		"a: &a [1, 2]\nb: *a",
 		&struct{ B []int }{[]int{1, 2}},
-	}, {
-		"b: *a\na: &a {c: 1}",
-		&struct {
-			A, B struct {
-				C int
-			}
-		}{struct{ C int }{1}, struct{ C int }{1}},
 	},
 
 	// Bug #1133337
@@ -611,6 +605,16 @@ var unmarshalTests = []struct {
 		"a: 123456E1\n",
 		M{"a": "123456E1"},
 	},
+	// yaml-test-suite 3GZX: Spec Example 7.1. Alias Nodes
+	{
+		"First occurrence: &anchor Foo\nSecond occurrence: *anchor\nOverride anchor: &anchor Bar\nReuse anchor: *anchor\n",
+		map[interface{}]interface{}{
+			"Reuse anchor":      "Bar",
+			"First occurrence":  "Foo",
+			"Second occurrence": "Foo",
+			"Override anchor":   "Bar",
+		},
+	},
 }
 
 type M map[interface{}]interface{}
@@ -670,6 +674,7 @@ var unmarshalErrorTests = []struct {
 	{"a: !!binary ==", "yaml: !!binary value contains invalid base64 data"},
 	{"{[.]}", `yaml: invalid map key: \[\]interface \{\}\{"\."\}`},
 	{"{{.}}", `yaml: invalid map key: map\[interface\ \{\}\]interface \{\}\{".":interface \{\}\(nil\)\}`},
+	{"b: *a\na: &a {c: 1}", `yaml: unknown anchor 'a' referenced`},
 	{"%TAG !%79! tag:yaml.org,2002:\n---\nv: !%79!int '1'", "yaml: did not find expected whitespace"},
 }