encoding/xml: add InputOffset method to Decoder

Among other things, this allows users to match the decoded
pieces with the original XML, which can be necessary for
implementing standards like XML signatures.

Fixes #8484.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/122960043

1 files changed, 27 insertions, 4 deletions

diff --git a/src/pkg/encoding/xml/xml_test.go b/src/pkg/encoding/xml/xml_test.go
index 7723ab1c9f..be995c0d52 100644
--- a/src/pkg/encoding/xml/xml_test.go
+++ b/src/pkg/encoding/xml/xml_test.go
@@ -170,7 +170,7 @@ var xmlInput = []string{
 func TestRawToken(t *testing.T) {
 	d := NewDecoder(strings.NewReader(testInput))
 	d.Entity = testEntity
-	testRawToken(t, d, rawTokens)
+	testRawToken(t, d, testInput, rawTokens)
 }
 
 const nonStrictInput = `
@@ -225,7 +225,7 @@ var nonStrictTokens = []Token{
 func TestNonStrictRawToken(t *testing.T) {
 	d := NewDecoder(strings.NewReader(nonStrictInput))
 	d.Strict = false
-	testRawToken(t, d, nonStrictTokens)
+	testRawToken(t, d, nonStrictInput, nonStrictTokens)
 }
 
 type downCaser struct {
@@ -254,7 +254,7 @@ func TestRawTokenAltEncoding(t *testing.T) {
 		}
 		return &downCaser{t, input.(io.ByteReader)}, nil
 	}
-	testRawToken(t, d, rawTokensAltEncoding)
+	testRawToken(t, d, testInputAltEncoding, rawTokensAltEncoding)
 }
 
 func TestRawTokenAltEncodingNoConverter(t *testing.T) {
@@ -280,9 +280,12 @@ func TestRawTokenAltEncodingNoConverter(t *testing.T) {
 	}
 }
 
-func testRawToken(t *testing.T, d *Decoder, rawTokens []Token) {
+func testRawToken(t *testing.T, d *Decoder, raw string, rawTokens []Token) {
+	lastEnd := int64(0)
 	for i, want := range rawTokens {
+		start := d.InputOffset()
 		have, err := d.RawToken()
+		end := d.InputOffset()
 		if err != nil {
 			t.Fatalf("token %d: unexpected error: %s", i, err)
 		}
@@ -300,6 +303,26 @@ func testRawToken(t *testing.T, d *Decoder, rawTokens []Token) {
 			}
 			t.Errorf("token %d = %s, want %s", i, shave, swant)
 		}
+
+		// Check that InputOffset returned actual token.
+		switch {
+		case start < lastEnd:
+			t.Errorf("token %d: position [%d,%d) for %T is before previous token", i, start, end, have)
+		case start >= end:
+			// Special case: EndElement can be synthesized.
+			if start == end && end == lastEnd {
+				break
+			}
+			t.Errorf("token %d: position [%d,%d) for %T is empty", i, start, end, have)
+		case end > int64(len(raw)):
+			t.Errorf("token %d: position [%d,%d) for %T extends beyond input", i, start, end, have)
+		default:
+			text := raw[start:end]
+			if strings.ContainsAny(text, "<>") && (!strings.HasPrefix(text, "<") || !strings.HasSuffix(text, ">")) {
+				t.Errorf("token %d: misaligned raw token %#q for %T", i, text, have)
+			}
+		}
+		lastEnd = end
 	}
 }