do not read ahead unnecessarily

There's no need to read beyond the end of a document end
marker, but the code was doing that by relying on each
parser method to read the next token, even though it
didn't need it.

We change the parser methods to read tokens only when needed.

decode.go

@@ -66,11 +66,7 @@ func (p *parser) init() {
 	if p.doneInit {
 		return
 	}
-	p.skip()
-	if p.event.typ != yaml_STREAM_START_EVENT {
-		panic("expected stream start event, got " + strconv.Itoa(int(p.event.typ)))
-	}
-	p.skip()
+	p.expect(yaml_STREAM_START_EVENT)
 	p.doneInit = true
 }
 
@@ -81,16 +77,35 @@ func (p *parser) destroy() {
 	yaml_parser_delete(&p.parser)
 }
 
-func (p *parser) skip() {
-	if p.event.typ != yaml_NO_EVENT {
-		if p.event.typ == yaml_STREAM_END_EVENT {
-			failf("attempted to go past the end of stream; corrupted value?")
+// expect consumes an event from the event stream and
+// checks that it's of the expected type.
+func (p *parser) expect(e yaml_event_type_t) {
+	if p.event.typ == yaml_NO_EVENT {
+		if !yaml_parser_parse(&p.parser, &p.event) {
+			p.fail()
 		}
-		yaml_event_delete(&p.event)
+	}
+	if p.event.typ == yaml_STREAM_END_EVENT {
+		failf("attempted to go past the end of stream; corrupted value?")
+	}
+	if p.event.typ != e {
+		p.parser.problem = fmt.Sprintf("expected %s event but got %s", e, p.event.typ)
+		p.fail()
+	}
+	yaml_event_delete(&p.event)
+	p.event.typ = yaml_NO_EVENT
+}
+
+// peek peeks at the next event in the event stream,
+// puts the results into p.event and returns the event type.
+func (p *parser) peek() yaml_event_type_t {
+	if p.event.typ != yaml_NO_EVENT {
+		return p.event.typ
 	}
 	if !yaml_parser_parse(&p.parser, &p.event) {
 		p.fail()
 	}
+	return p.event.typ
 }
 
 func (p *parser) fail() {
@@ -121,7 +136,7 @@ func (p *parser) anchor(n *node, anchor []byte) {
 
 func (p *parser) parse() *node {
 	p.init()
-	switch p.event.typ {
+	switch p.peek() {
 	case yaml_SCALAR_EVENT:
 		return p.scalar()
 	case yaml_ALIAS_EVENT:
@@ -136,7 +151,7 @@ func (p *parser) parse() *node {
 		// Happens when attempting to decode an empty buffer.
 		return nil
 	default:
-		panic("attempted to parse unknown event: " + strconv.Itoa(int(p.event.typ)))
+		panic("attempted to parse unknown event: " + p.event.typ.String())
 	}
 }
 
@@ -152,12 +167,9 @@ func (p *parser) document() *node {
 	n := p.node(documentNode)
 	n.anchors = make(map[string]*node)
 	p.doc = n
-	p.skip()
+	p.expect(yaml_DOCUMENT_START_EVENT)
 	n.children = append(n.children, p.parse())
-	if p.event.typ != yaml_DOCUMENT_END_EVENT {
-		panic("expected end of document event but got " + strconv.Itoa(int(p.event.typ)))
-	}
-	p.skip()
+	p.expect(yaml_DOCUMENT_END_EVENT)
 	return n
 }
 
@@ -168,7 +180,7 @@ func (p *parser) alias() *node {
 	if n.alias == nil {
 		failf("unknown anchor '%s' referenced", n.value)
 	}
-	p.skip()
+	p.expect(yaml_ALIAS_EVENT)
 	return n
 }
 
@@ -178,29 +190,29 @@ func (p *parser) scalar() *node {
 	n.tag = string(p.event.tag)
 	n.implicit = p.event.implicit
 	p.anchor(n, p.event.anchor)
-	p.skip()
+	p.expect(yaml_SCALAR_EVENT)
 	return n
 }
 
 func (p *parser) sequence() *node {
 	n := p.node(sequenceNode)
 	p.anchor(n, p.event.anchor)
-	p.skip()
-	for p.event.typ != yaml_SEQUENCE_END_EVENT {
+	p.expect(yaml_SEQUENCE_START_EVENT)
+	for p.peek() != yaml_SEQUENCE_END_EVENT {
 		n.children = append(n.children, p.parse())
 	}
-	p.skip()
+	p.expect(yaml_SEQUENCE_END_EVENT)
 	return n
 }
 
 func (p *parser) mapping() *node {
 	n := p.node(mappingNode)
 	p.anchor(n, p.event.anchor)
-	p.skip()
-	for p.event.typ != yaml_MAPPING_END_EVENT {
+	p.expect(yaml_MAPPING_START_EVENT)
+	for p.peek() != yaml_MAPPING_END_EVENT {
 		n.children = append(n.children, p.parse(), p.parse())
 	}
-	p.skip()
+	p.expect(yaml_MAPPING_END_EVENT)
 	return n
 }
 

decode_test.go

@@ -617,6 +617,11 @@ var unmarshalTests = []struct {
 			"Override anchor":   "Bar",
 		},
 	},
+	// Single document with garbage following it.
+	{
+		"---\nhello\n...\n}not yaml",
+		"hello",
+	},
 }
 
 type M map[interface{}]interface{}

yamlh.go

@@ -1,6 +1,7 @@
 package yaml
 
 import (
+	"fmt"
 	"io"
 )
 
@@ -239,6 +240,27 @@ const (
 	yaml_MAPPING_END_EVENT    // A MAPPING-END event.
 )
 
+var eventStrings = []string{
+	yaml_NO_EVENT:             "none",
+	yaml_STREAM_START_EVENT:   "stream start",
+	yaml_STREAM_END_EVENT:     "stream end",
+	yaml_DOCUMENT_START_EVENT: "document start",
+	yaml_DOCUMENT_END_EVENT:   "document end",
+	yaml_ALIAS_EVENT:          "alias",
+	yaml_SCALAR_EVENT:         "scalar",
+	yaml_SEQUENCE_START_EVENT: "sequence start",
+	yaml_SEQUENCE_END_EVENT:   "sequence end",
+	yaml_MAPPING_START_EVENT:  "mapping start",
+	yaml_MAPPING_END_EVENT:    "mapping end",
+}
+
+func (e yaml_event_type_t) String() string {
+	if e < 0 || int(e) >= len(eventStrings) {
+		return fmt.Sprintf("unknown event %d", e)
+	}
+	return eventStrings[e]
+}
+
 // The event structure.
 type yaml_event_t struct {