bytes: clarify documentation for UnreadByte/Rune

Fixes #19522.

Change-Id: Ib3cf0336e0bf91580d533704ec1a9d45eb0bf62d
Reviewed-on: https://go-review.googlesource.com/42020Reviewed-by: Rob Pike <r@golang.org>

src/bytes/buffer.go

@@ -346,12 +346,12 @@ func (b *Buffer) ReadRune() (r rune, size int, err error) {
 
 // UnreadRune unreads the last rune returned by ReadRune.
 // If the most recent read or write operation on the buffer was
-// not a ReadRune, UnreadRune returns an error.  (In this regard
+// not a successful ReadRune, UnreadRune returns an error.  (In this regard
 // it is stricter than UnreadByte, which will unread the last byte
 // from any read operation.)
 func (b *Buffer) UnreadRune() error {
 	if b.lastRead <= opInvalid {
-		return errors.New("bytes.Buffer: UnreadRune: previous operation was not ReadRune")
+		return errors.New("bytes.Buffer: UnreadRune: previous operation was not a successful ReadRune")
 	}
 	if b.off >= int(b.lastRead) {
 		b.off -= int(b.lastRead)
@@ -360,12 +360,13 @@ func (b *Buffer) UnreadRune() error {
 	return nil
 }
 
-// UnreadByte unreads the last byte returned by the most recent
-// read operation. If write has happened since the last read, UnreadByte
-// returns an error.
+// UnreadByte unreads the last byte returned by the most recent successful
+// read operation that read at least one byte. If a write has happened since
+// the last read, if the last read returned an error, or if the read read zero
+// bytes, UnreadByte returns an error.
 func (b *Buffer) UnreadByte() error {
 	if b.lastRead == opInvalid {
-		return errors.New("bytes.Buffer: UnreadByte: previous operation was not a read")
+		return errors.New("bytes.Buffer: UnreadByte: previous operation was not a successful read")
 	}
 	b.lastRead = opInvalid
 	if b.off > 0 {

src/bytes/buffer_test.go

@@ -311,6 +311,19 @@ func TestRuneIO(t *testing.T) {
 
 	// Check that UnreadRune works
 	buf.Reset()
+
+	// check at EOF
+	if err := buf.UnreadRune(); err == nil {
+		t.Fatal("UnreadRune at EOF: got no error")
+	}
+	if _, _, err := buf.ReadRune(); err == nil {
+		t.Fatal("ReadRune at EOF: got no error")
+	}
+	if err := buf.UnreadRune(); err == nil {
+		t.Fatal("UnreadRune after ReadRune at EOF: got no error")
+	}
+
+	// check not at EOF
 	buf.Write(b)
 	for r := rune(0); r < NRune; r++ {
 		r1, size, _ := buf.ReadRune()
@@ -473,15 +486,34 @@ func TestReadEmptyAtEOF(t *testing.T) {
 
 func TestUnreadByte(t *testing.T) {
 	b := new(Buffer)
+
+	// check at EOF
+	if err := b.UnreadByte(); err == nil {
+		t.Fatal("UnreadByte at EOF: got no error")
+	}
+	if _, err := b.ReadByte(); err == nil {
+		t.Fatal("ReadByte at EOF: got no error")
+	}
+	if err := b.UnreadByte(); err == nil {
+		t.Fatal("UnreadByte after ReadByte at EOF: got no error")
+	}
+
+	// check not at EOF
 	b.WriteString("abcdefghijklmnopqrstuvwxyz")
 
-	_, err := b.ReadBytes('m')
-	if err != nil {
-		t.Fatalf("ReadBytes: %v", err)
+	// after unsuccessful read
+	if n, err := b.Read(nil); n != 0 || err != nil {
+		t.Fatalf("Read(nil) = %d,%v; want 0,nil", n, err)
+	}
+	if err := b.UnreadByte(); err == nil {
+		t.Fatal("UnreadByte after Read(nil): got no error")
 	}
 
-	err = b.UnreadByte()
-	if err != nil {
+	// after successful read
+	if _, err := b.ReadBytes('m'); err != nil {
+		t.Fatalf("ReadBytes: %v", err)
+	}
+	if err := b.UnreadByte(); err != nil {
 		t.Fatalf("UnreadByte: %v", err)
 	}
 	c, err := b.ReadByte()