bytes, strings: fix Reader.UnreadRune returning without error on a zero Reader

Fixes #28269

Change-Id: I878dff43c0b6bdb98702d8e73f2ecd984fb2350f
Reviewed-on: https://go-review.googlesource.com/c/145098
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 5 insertions, 1 deletions

diff --git a/src/bytes/reader.go b/src/bytes/reader.go
index 08464c2402..5946cf9780 100644
--- a/src/bytes/reader.go
+++ b/src/bytes/reader.go
@@ -14,6 +14,7 @@ import (
 // io.ByteScanner, and io.RuneScanner interfaces by reading from
 // a byte slice.
 // Unlike a Buffer, a Reader is read-only and supports seeking.
+// The zero value for Reader operates like a Reader of an empty slice.
 type Reader struct {
 	s        []byte
 	i        int64 // current reading index
@@ -75,10 +76,10 @@ func (r *Reader) ReadByte() (byte, error) {
 
 // UnreadByte complements ReadByte in implementing the io.ByteScanner interface.
 func (r *Reader) UnreadByte() error {
-	r.prevRune = -1
 	if r.i <= 0 {
 		return errors.New("bytes.Reader.UnreadByte: at beginning of slice")
 	}
+	r.prevRune = -1
 	r.i--
 	return nil
 }
@@ -101,6 +102,9 @@ func (r *Reader) ReadRune() (ch rune, size int, err error) {
 
 // UnreadRune complements ReadRune in implementing the io.RuneScanner interface.
 func (r *Reader) UnreadRune() error {
+	if r.i <= 0 {
+		return errors.New("bytes.Reader.UnreadRune: at beginning of slice")
+	}
 	if r.prevRune < 0 {
 		return errors.New("bytes.Reader.UnreadRune: previous operation was not ReadRune")
 	}