encoding/json: Include the offset of a SyntaxError

When a SyntaxError occurs, report the current offset within the stream.
The code already accounted for the offset within the current buffer
being scanned. By including how much data was already scanned, the
current offset can be computed.

Fixes #22478

Change-Id: I91ecd4cad0b85a5c1556bc597f3ee914e769af01
Reviewed-on: https://go-review.googlesource.com/74251Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/json/stream.go

@@ -16,6 +16,7 @@ type Decoder struct {
 	buf     []byte
 	d       decodeState
 	scanp   int   // start of unread data in buf
+	scanned int64 // amount of data already scanned
 	scan    scanner
 	err     error
 
@@ -55,7 +56,7 @@ func (dec *Decoder) Decode(v interface{}) error {
 	}
 
 	if !dec.tokenValueAllowed() {
-		return &SyntaxError{msg: "not at beginning of value"}
+		return &SyntaxError{msg: "not at beginning of value", Offset: dec.offset()}
 	}
 
 	// Read whole value into buffer.
@@ -140,6 +141,7 @@ func (dec *Decoder) refill() error {
 	// Make room to read more into the buffer.
 	// First slide down data already consumed.
 	if dec.scanp > 0 {
+		dec.scanned += int64(dec.scanp)
 		n := copy(dec.buf, dec.buf[dec.scanp:])
 		dec.buf = dec.buf[:n]
 		dec.scanp = 0
@@ -306,7 +308,7 @@ func (dec *Decoder) tokenPrepareForDecode() error {
 			return err
 		}
 		if c != ',' {
-			return &SyntaxError{"expected comma after array element", 0}
+			return &SyntaxError{"expected comma after array element", dec.offset()}
 		}
 		dec.scanp++
 		dec.tokenState = tokenArrayValue
@@ -316,7 +318,7 @@ func (dec *Decoder) tokenPrepareForDecode() error {
 			return err
 		}
 		if c != ':' {
-			return &SyntaxError{"expected colon after object key", 0}
+			return &SyntaxError{"expected colon after object key", dec.offset()}
 		}
 		dec.scanp++
 		dec.tokenState = tokenObjectValue
@@ -433,7 +435,6 @@ func (dec *Decoder) Token() (Token, error) {
 				err := dec.Decode(&x)
 				dec.tokenState = old
 				if err != nil {
-					clearOffset(err)
 					return nil, err
 				}
 				dec.tokenState = tokenObjectColon
@@ -447,7 +448,6 @@ func (dec *Decoder) Token() (Token, error) {
 			}
 			var x interface{}
 			if err := dec.Decode(&x); err != nil {
-				clearOffset(err)
 				return nil, err
 			}
 			return x, nil
@@ -455,12 +455,6 @@ func (dec *Decoder) Token() (Token, error) {
 	}
 }
 
-func clearOffset(err error) {
-	if s, ok := err.(*SyntaxError); ok {
-		s.Offset = 0
-	}
-}
-
 func (dec *Decoder) tokenError(c byte) (Token, error) {
 	var context string
 	switch dec.tokenState {
@@ -477,7 +471,7 @@ func (dec *Decoder) tokenError(c byte) (Token, error) {
 	case tokenObjectComma:
 		context = " after object key:value pair"
 	}
-	return nil, &SyntaxError{"invalid character " + quoteChar(c) + " " + context, 0}
+	return nil, &SyntaxError{"invalid character " + quoteChar(c) + " " + context, dec.offset()}
 }
 
 // More reports whether there is another element in the
@@ -505,3 +499,7 @@ func (dec *Decoder) peek() (byte, error) {
 		err = dec.refill()
 	}
 }
+
+func (dec *Decoder) offset() int64 {
+	return dec.scanned + int64(dec.scanp)
+}

src/encoding/json/stream_test.go

@@ -342,11 +342,18 @@ var tokenStreamCases []tokenStreamCase = []tokenStreamCase{
 	{json: ` [{"a": 1} {"a": 2}] `, expTokens: []interface{}{
 		Delim('['),
 		decodeThis{map[string]interface{}{"a": float64(1)}},
-		decodeThis{&SyntaxError{"expected comma after array element", 0}},
+		decodeThis{&SyntaxError{"expected comma after array element", 11}},
 	}},
-	{json: `{ "a" 1 }`, expTokens: []interface{}{
-		Delim('{'), "a",
-		decodeThis{&SyntaxError{"expected colon after object key", 0}},
+	{json: `{ "` + strings.Repeat("a", 513) + `" 1 }`, expTokens: []interface{}{
+		Delim('{'), strings.Repeat("a", 513),
+		decodeThis{&SyntaxError{"expected colon after object key", 518}},
+	}},
+	{json: `{ "\a" }`, expTokens: []interface{}{
+		Delim('{'),
+		&SyntaxError{"invalid character 'a' in string escape code", 3},
+	}},
+	{json: ` \a`, expTokens: []interface{}{
+		&SyntaxError{"invalid character '\\\\' looking for beginning of value", 1},
 	}},
 }
 
@@ -367,15 +374,15 @@ func TestDecodeInStream(t *testing.T) {
 				tk, err = dec.Token()
 			}
 			if experr, ok := etk.(error); ok {
-				if err == nil || err.Error() != experr.Error() {
-					t.Errorf("case %v: Expected error %v in %q, but was %v", ci, experr, tcase.json, err)
+				if err == nil || !reflect.DeepEqual(err, experr) {
+					t.Errorf("case %v: Expected error %#v in %q, but was %#v", ci, experr, tcase.json, err)
 				}
 				break
 			} else if err == io.EOF {
 				t.Errorf("case %v: Unexpected EOF in %q", ci, tcase.json)
 				break
 			} else if err != nil {
-				t.Errorf("case %v: Unexpected error '%v' in %q", ci, err, tcase.json)
+				t.Errorf("case %v: Unexpected error '%#v' in %q", ci, err, tcase.json)
 				break
 			}
 			if !reflect.DeepEqual(tk, etk) {