json: keep track of error offset in SyntaxError

R=rsc
CC=golang-dev
https://golang.org/cl/4430043

src/pkg/json/decode_test.go

@@ -71,7 +71,7 @@ var unmarshalTests = []unmarshalTest{
 	{`{"X":"a", "y":"b", "Z":"c"}`, new(badTag), badTag{"a", "b", "c"}, nil},
 
 	// syntax errors
-	{`{"X": "foo", "Y"}`, nil, nil, SyntaxError("invalid character '}' after object key")},
+	{`{"X": "foo", "Y"}`, nil, nil, &SyntaxError{"invalid character '}' after object key", 17}},
 
 	// composite tests
 	{allValueIndent, new(All), allValue, nil},
@@ -125,12 +125,12 @@ func TestMarshalBadUTF8(t *testing.T) {
 }
 
 func TestUnmarshal(t *testing.T) {
-	var scan scanner
 	for i, tt := range unmarshalTests {
+		var scan scanner
 		in := []byte(tt.in)
 		if err := checkValid(in, &scan); err != nil {
 			if !reflect.DeepEqual(err, tt.err) {
-				t.Errorf("#%d: checkValid: %v", i, err)
+				t.Errorf("#%d: checkValid: %#v", i, err)
 				continue
 			}
 		}

src/pkg/json/scanner.go

@@ -23,6 +23,7 @@ import (
 func checkValid(data []byte, scan *scanner) os.Error {
 	scan.reset()
 	for _, c := range data {
+		scan.bytes++
 		if scan.step(scan, int(c)) == scanError {
 			return scan.err
 		}
@@ -56,10 +57,12 @@ func nextValue(data []byte, scan *scanner) (value, rest []byte, err os.Error) {
 }
 
 // A SyntaxError is a description of a JSON syntax error.
-type SyntaxError string
-
-func (e SyntaxError) String() string { return string(e) }
+type SyntaxError struct {
+	msg    string // description of error
+	Offset int64  // error occurred after reading Offset bytes
+}
 
+func (e *SyntaxError) String() string { return e.msg }
 
 // A scanner is a JSON scanning state machine.
 // Callers call scan.reset() and then pass bytes in one at a time
@@ -89,6 +92,9 @@ type scanner struct {
 	// 1-byte redo (see undo method)
 	redoCode  int
 	redoState func(*scanner, int) int
+
+	// total bytes consumed, updated by decoder.Decode
+	bytes int64
 }
 
 // These values are returned by the state transition functions
@@ -148,7 +154,7 @@ func (s *scanner) eof() int {
 		return scanEnd
 	}
 	if s.err == nil {
-		s.err = SyntaxError("unexpected end of JSON input")
+		s.err = &SyntaxError{"unexpected end of JSON input", s.bytes}
 	}
 	return scanError
 }
@@ -581,7 +587,7 @@ func stateError(s *scanner, c int) int {
 // error records an error and switches to the error state.
 func (s *scanner) error(c int, context string) int {
 	s.step = stateError
-	s.err = SyntaxError("invalid character " + quoteChar(c) + " " + context)
+	s.err = &SyntaxError{"invalid character " + quoteChar(c) + " " + context, s.bytes}
 	return scanError
 }
 

src/pkg/json/stream.go

@@ -23,8 +23,8 @@ func NewDecoder(r io.Reader) *Decoder {
 	return &Decoder{r: r}
 }
 
-// Decode reads the next JSON-encoded value from the
-// connection and stores it in the value pointed to by v.
+// Decode reads the next JSON-encoded value from its
+// input and stores it in the value pointed to by v.
 //
 // See the documentation for Unmarshal for details about
 // the conversion of JSON into a Go value.
@@ -62,6 +62,7 @@ Input:
 	for {
 		// Look in the buffer for a new value.
 		for i, c := range dec.buf[scanp:] {
+			dec.scan.bytes++
 			v := dec.scan.step(&dec.scan, int(c))
 			if v == scanEnd {
 				scanp += i