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>

2 files changed, 47 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")
 	}
diff --git a/src/strings/reader_test.go b/src/strings/reader_test.go
index bf40eb1a31..a4c211d699 100644
--- a/src/strings/reader_test.go
+++ b/src/strings/reader_test.go
@@ -190,3 +190,45 @@ func TestReaderReset(t *testing.T) {
 		t.Errorf("ReadAll: got %q, want %q", got, want)
 	}
 }
+
+func TestReaderZero(t *testing.T) {
+	if l := (&strings.Reader{}).Len(); l != 0 {
+		t.Errorf("Len: got %d, want 0", l)
+	}
+
+	if n, err := (&strings.Reader{}).Read(nil); n != 0 || err != io.EOF {
+		t.Errorf("Read: got %d, %v; want 0, io.EOF", n, err)
+	}
+
+	if n, err := (&strings.Reader{}).ReadAt(nil, 11); n != 0 || err != io.EOF {
+		t.Errorf("ReadAt: got %d, %v; want 0, io.EOF", n, err)
+	}
+
+	if b, err := (&strings.Reader{}).ReadByte(); b != 0 || err != io.EOF {
+		t.Errorf("ReadByte: got %d, %v; want 0, io.EOF", b, err)
+	}
+
+	if ch, size, err := (&strings.Reader{}).ReadRune(); ch != 0 || size != 0 || err != io.EOF {
+		t.Errorf("ReadRune: got %d, %d, %v; want 0, 0, io.EOF", ch, size, err)
+	}
+
+	if offset, err := (&strings.Reader{}).Seek(11, io.SeekStart); offset != 11 || err != nil {
+		t.Errorf("Seek: got %d, %v; want 11, nil", offset, err)
+	}
+
+	if s := (&strings.Reader{}).Size(); s != 0 {
+		t.Errorf("Size: got %d, want 0", s)
+	}
+
+	if (&strings.Reader{}).UnreadByte() == nil {
+		t.Errorf("UnreadByte: got nil, want error")
+	}
+
+	if (&strings.Reader{}).UnreadRune() == nil {
+		t.Errorf("UnreadRune: got nil, want error")
+	}
+
+	if n, err := (&strings.Reader{}).WriteTo(ioutil.Discard); n != 0 || err != nil {
+		t.Errorf("WriteTo: got %d, %v; want 0, nil", n, err)
+	}
+}