strings, bytes: fix Reader.UnreadRune

UnreadRune should return an error if previous operation is not
ReadRune.

Fixes #7579.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/77580046

2 files changed, 32 insertions, 2 deletions

diff --git a/src/pkg/strings/reader.go b/src/pkg/strings/reader.go
index 11240efc07..df4d807120 100644
--- a/src/pkg/strings/reader.go
+++ b/src/pkg/strings/reader.go
@@ -29,6 +29,7 @@ func (r *Reader) Len() int {
 }
 
 func (r *Reader) Read(b []byte) (n int, err error) {
+	r.prevRune = -1
 	if len(b) == 0 {
 		return 0, nil
 	}
@@ -37,11 +38,11 @@ func (r *Reader) Read(b []byte) (n int, err error) {
 	}
 	n = copy(b, r.s[r.i:])
 	r.i += n
-	r.prevRune = -1
 	return
 }
 
 func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
+	r.prevRune = -1
 	if off < 0 {
 		return 0, errors.New("strings: invalid offset")
 	}
@@ -56,12 +57,12 @@ func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
 }
 
 func (r *Reader) ReadByte() (b byte, err error) {
+	r.prevRune = -1
 	if r.i >= len(r.s) {
 		return 0, io.EOF
 	}
 	b = r.s[r.i]
 	r.i++
-	r.prevRune = -1
 	return
 }
 
@@ -76,6 +77,7 @@ func (r *Reader) UnreadByte() error {
 
 func (r *Reader) ReadRune() (ch rune, size int, err error) {
 	if r.i >= len(r.s) {
+		r.prevRune = -1
 		return 0, 0, io.EOF
 	}
 	r.prevRune = r.i
@@ -99,6 +101,7 @@ func (r *Reader) UnreadRune() error {
 
 // Seek implements the io.Seeker interface.
 func (r *Reader) Seek(offset int64, whence int) (int64, error) {
+	r.prevRune = -1
 	var abs int64
 	switch whence {
 	case 0:
diff --git a/src/pkg/strings/strings_test.go b/src/pkg/strings/strings_test.go
index a5be2f9bed..5c38965775 100644
--- a/src/pkg/strings/strings_test.go
+++ b/src/pkg/strings/strings_test.go
@@ -858,6 +858,33 @@ func TestReadRune(t *testing.T) {
 	}
 }
 
+var UnreadRuneErrorTests = []struct {
+	name string
+	f    func(*Reader)
+}{
+	{"Read", func(r *Reader) { r.Read([]byte{}) }},
+	{"ReadAt", func(r *Reader) { r.ReadAt([]byte{}, 0) }},
+	{"ReadByte", func(r *Reader) { r.ReadByte() }},
+	{"UnreadRune", func(r *Reader) { r.UnreadRune() }},
+	{"Seek", func(r *Reader) { r.Seek(0, 1) }},
+	{"WriteTo", func(r *Reader) { r.WriteTo(&bytes.Buffer{}) }},
+}
+
+func TestUnreadRuneError(t *testing.T) {
+	for _, tt := range UnreadRuneErrorTests {
+		reader := NewReader("0123456789")
+		if _, _, err := reader.ReadRune(); err != nil {
+			// should not happen
+			t.Fatal(err)
+		}
+		tt.f(reader)
+		err := reader.UnreadRune()
+		if err == nil {
+			t.Errorf("Unreading after %s: expected error", tt.name)
+		}
+	}
+}
+
 var ReplaceTests = []struct {
 	in       string
 	old, new string