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/strings/reader.go b/src/strings/reader.go
index 6c1a5064c0..eb2fa1164c 100644
--- a/src/strings/reader.go
+++ b/src/strings/reader.go
@@ -13,6 +13,7 @@ import (
 // A Reader implements the io.Reader, io.ReaderAt, io.Seeker, io.WriterTo,
 // io.ByteScanner, and io.RuneScanner interfaces by reading
 // from a string.
+// The zero value for Reader operates like a Reader of an empty string.
 type Reader struct {
 	s        string
 	i        int64 // current reading index
@@ -70,10 +71,10 @@ func (r *Reader) ReadByte() (byte, error) {
 }
 
 func (r *Reader) UnreadByte() error {
-	r.prevRune = -1
 	if r.i <= 0 {
 		return errors.New("strings.Reader.UnreadByte: at beginning of string")
 	}
+	r.prevRune = -1
 	r.i--
 	return nil
 }
@@ -94,6 +95,9 @@ func (r *Reader) ReadRune() (ch rune, size int, err error) {
 }
 
 func (r *Reader) UnreadRune() error {
+	if r.i <= 0 {
+		return errors.New("strings.Reader.UnreadRune: at beginning of string")
+	}
 	if r.prevRune < 0 {
 		return errors.New("strings.Reader.UnreadRune: previous operation was not ReadRune")
 	}