bytes, strings: more consistent error messages

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/86060044

src/pkg/bytes/reader.go

@@ -45,7 +45,7 @@ func (r *Reader) Read(b []byte) (n int, err error) {
 func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
 	// cannot modify state - see io.ReaderAt
 	if off < 0 {
-		return 0, errors.New("bytes: invalid offset")
+		return 0, errors.New("bytes.Reader.ReadAt: negative offset")
 	}
 	if off >= int64(len(r.s)) {
 		return 0, io.EOF
@@ -68,11 +68,11 @@ func (r *Reader) ReadByte() (b byte, err error) {
 }
 
 func (r *Reader) UnreadByte() error {
+	r.prevRune = -1
 	if r.i <= 0 {
-		return errors.New("bytes.Reader: at beginning of slice")
+		return errors.New("bytes.Reader.UnreadByte: at beginning of slice")
 	}
 	r.i--
-	r.prevRune = -1
 	return nil
 }
 
@@ -93,7 +93,7 @@ func (r *Reader) ReadRune() (ch rune, size int, err error) {
 
 func (r *Reader) UnreadRune() error {
 	if r.prevRune < 0 {
-		return errors.New("bytes.Reader: previous operation was not ReadRune")
+		return errors.New("bytes.Reader.UnreadRune: previous operation was not ReadRune")
 	}
 	r.i = int64(r.prevRune)
 	r.prevRune = -1
@@ -112,10 +112,10 @@ func (r *Reader) Seek(offset int64, whence int) (int64, error) {
 	case 2:
 		abs = int64(len(r.s)) + offset
 	default:
-		return 0, errors.New("bytes: invalid whence")
+		return 0, errors.New("bytes.Reader.Seek: invalid whence")
 	}
 	if abs < 0 {
-		return 0, errors.New("bytes: negative position")
+		return 0, errors.New("bytes.Reader.Seek: negative position")
 	}
 	r.i = abs
 	return abs, nil

src/pkg/bytes/reader_test.go

@@ -27,7 +27,7 @@ func TestReader(t *testing.T) {
 		{seek: os.SEEK_SET, off: 0, n: 20, want: "0123456789"},
 		{seek: os.SEEK_SET, off: 1, n: 1, want: "1"},
 		{seek: os.SEEK_CUR, off: 1, wantpos: 3, n: 2, want: "34"},
-		{seek: os.SEEK_SET, off: -1, seekerr: "bytes: negative position"},
+		{seek: os.SEEK_SET, off: -1, seekerr: "bytes.Reader.Seek: negative position"},
 		{seek: os.SEEK_SET, off: 1 << 33, wantpos: 1 << 33},
 		{seek: os.SEEK_CUR, off: 1, wantpos: 1<<33 + 1},
 		{seek: os.SEEK_SET, n: 5, want: "01234"},
@@ -84,7 +84,7 @@ func TestReaderAt(t *testing.T) {
 		{1, 9, "123456789", nil},
 		{11, 10, "", io.EOF},
 		{0, 0, "", nil},
-		{-1, 0, "", "bytes: invalid offset"},
+		{-1, 0, "", "bytes.Reader.ReadAt: negative offset"},
 	}
 	for i, tt := range tests {
 		b := make([]byte, tt.n)

src/pkg/strings/reader.go

@@ -44,7 +44,7 @@ func (r *Reader) Read(b []byte) (n int, err error) {
 func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
 	// cannot modify state - see io.ReaderAt
 	if off < 0 {
-		return 0, errors.New("strings: invalid offset")
+		return 0, errors.New("strings.Reader.ReadAt: negative offset")
 	}
 	if off >= int64(len(r.s)) {
 		return 0, io.EOF
@@ -67,11 +67,11 @@ func (r *Reader) ReadByte() (b byte, err error) {
 }
 
 func (r *Reader) UnreadByte() error {
+	r.prevRune = -1
 	if r.i <= 0 {
-		return errors.New("strings.Reader: at beginning of string")
+		return errors.New("strings.Reader.UnreadByte: at beginning of string")
 	}
 	r.i--
-	r.prevRune = -1
 	return nil
 }
 
@@ -92,7 +92,7 @@ func (r *Reader) ReadRune() (ch rune, size int, err error) {
 
 func (r *Reader) UnreadRune() error {
 	if r.prevRune < 0 {
-		return errors.New("strings.Reader: previous operation was not ReadRune")
+		return errors.New("strings.Reader.UnreadRune: previous operation was not ReadRune")
 	}
 	r.i = int64(r.prevRune)
 	r.prevRune = -1
@@ -111,10 +111,10 @@ func (r *Reader) Seek(offset int64, whence int) (int64, error) {
 	case 2:
 		abs = int64(len(r.s)) + offset
 	default:
-		return 0, errors.New("strings: invalid whence")
+		return 0, errors.New("strings.Reader.Seek: invalid whence")
 	}
 	if abs < 0 {
-		return 0, errors.New("strings: negative position")
+		return 0, errors.New("strings.Reader.Seek: negative position")
 	}
 	r.i = abs
 	return abs, nil

src/pkg/strings/reader_test.go

@@ -27,7 +27,7 @@ func TestReader(t *testing.T) {
 		{seek: os.SEEK_SET, off: 0, n: 20, want: "0123456789"},
 		{seek: os.SEEK_SET, off: 1, n: 1, want: "1"},
 		{seek: os.SEEK_CUR, off: 1, wantpos: 3, n: 2, want: "34"},
-		{seek: os.SEEK_SET, off: -1, seekerr: "strings: negative position"},
+		{seek: os.SEEK_SET, off: -1, seekerr: "strings.Reader.Seek: negative position"},
 		{seek: os.SEEK_SET, off: 1 << 33, wantpos: 1 << 33},
 		{seek: os.SEEK_CUR, off: 1, wantpos: 1<<33 + 1},
 		{seek: os.SEEK_SET, n: 5, want: "01234"},
@@ -84,7 +84,7 @@ func TestReaderAt(t *testing.T) {
 		{1, 9, "123456789", nil},
 		{11, 10, "", io.EOF},
 		{0, 0, "", nil},
-		{-1, 0, "", "strings: invalid offset"},
+		{-1, 0, "", "strings.Reader.ReadAt: negative offset"},
 	}
 	for i, tt := range tests {
 		b := make([]byte, tt.n)