bufio: fix UnreadByte

Also:
- fix error messages in tests
- make tests more symmetric

Fixes #7607.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/86180043

src/pkg/bufio/bufio.go

@@ -177,7 +177,7 @@ func (b *Reader) Read(p []byte) (n int, err error) {
 // If no byte is available, returns an error.
 func (b *Reader) ReadByte() (c byte, err error) {
 	b.lastRuneSize = -1
-	for b.w == b.r {
+	for b.r == b.w {
 		if b.err != nil {
 			return 0, b.readErr()
 		}
@@ -191,19 +191,19 @@ func (b *Reader) ReadByte() (c byte, err error) {
 
 // UnreadByte unreads the last byte.  Only the most recently read byte can be unread.
 func (b *Reader) UnreadByte() error {
-	b.lastRuneSize = -1
-	if b.r == b.w && b.lastByte >= 0 {
-		b.w = 1
-		b.r = 0
-		b.buf[0] = byte(b.lastByte)
-		b.lastByte = -1
-		return nil
-	}
-	if b.r <= 0 {
+	if b.lastByte < 0 || b.r == 0 && b.w > 0 {
 		return ErrInvalidUnreadByte
 	}
-	b.r--
+	// b.r > 0 || b.w == 0
+	if b.r > 0 {
+		b.r--
+	} else {
+		// b.r == 0 && b.w == 0
+		b.w = 1
+	}
+	b.buf[b.r] = byte(b.lastByte)
 	b.lastByte = -1
+	b.lastRuneSize = -1
 	return nil
 }
 
@@ -233,7 +233,7 @@ func (b *Reader) ReadRune() (r rune, size int, err error) {
 // regard it is stricter than UnreadByte, which will unread the last byte
 // from any read operation.)
 func (b *Reader) UnreadRune() error {
-	if b.lastRuneSize < 0 || b.r == 0 {
+	if b.lastRuneSize < 0 || b.r < b.lastRuneSize {
 		return ErrInvalidUnreadRune
 	}
 	b.r -= b.lastRuneSize

src/pkg/bufio/bufio_test.go

@@ -228,66 +228,94 @@ func TestReadRune(t *testing.T) {
 }
 
 func TestUnreadRune(t *testing.T) {
-	got := ""
 	segments := []string{"Hello, world:", "日本語"}
-	data := strings.Join(segments, "")
 	r := NewReader(&StringReader{data: segments})
+	got := ""
+	want := strings.Join(segments, "")
 	// Normal execution.
 	for {
 		r1, _, err := r.ReadRune()
 		if err != nil {
 			if err != io.EOF {
-				t.Error("unexpected EOF")
+				t.Error("unexpected error on ReadRune:", err)
 			}
 			break
 		}
 		got += string(r1)
-		// Put it back and read it again
+		// Put it back and read it again.
 		if err = r.UnreadRune(); err != nil {
-			t.Error("unexpected error on UnreadRune:", err)
+			t.Fatal("unexpected error on UnreadRune:", err)
 		}
 		r2, _, err := r.ReadRune()
 		if err != nil {
-			t.Error("unexpected error reading after unreading:", err)
+			t.Fatal("unexpected error reading after unreading:", err)
 		}
 		if r1 != r2 {
-			t.Errorf("incorrect rune after unread: got %c wanted %c", r1, r2)
+			t.Fatalf("incorrect rune after unread: got %c, want %c", r1, r2)
 		}
 	}
-	if got != data {
-		t.Errorf("want=%q got=%q", data, got)
+	if got != want {
+		t.Errorf("got %q, want %q", got, want)
 	}
 }
 
 func TestUnreadByte(t *testing.T) {
-	want := "Hello, world"
-	got := ""
 	segments := []string{"Hello, ", "world"}
 	r := NewReader(&StringReader{data: segments})
+	got := ""
+	want := strings.Join(segments, "")
 	// Normal execution.
 	for {
 		b1, err := r.ReadByte()
 		if err != nil {
 			if err != io.EOF {
-				t.Fatal("unexpected EOF")
+				t.Error("unexpected error on ReadByte:", err)
 			}
 			break
 		}
 		got += string(b1)
-		// Put it back and read it again
+		// Put it back and read it again.
 		if err = r.UnreadByte(); err != nil {
-			t.Fatalf("unexpected error on UnreadByte: %v", err)
+			t.Fatal("unexpected error on UnreadByte:", err)
 		}
 		b2, err := r.ReadByte()
 		if err != nil {
-			t.Fatalf("unexpected error reading after unreading: %v", err)
+			t.Fatal("unexpected error reading after unreading:", err)
 		}
 		if b1 != b2 {
-			t.Fatalf("incorrect byte after unread: got %c wanted %c", b1, b2)
+			t.Fatalf("incorrect byte after unread: got %q, want %q", b1, b2)
 		}
 	}
 	if got != want {
-		t.Errorf("got=%q want=%q", got, want)
+		t.Errorf("got %q, want %q", got, want)
+	}
+}
+
+func TestUnreadByteMultiple(t *testing.T) {
+	segments := []string{"Hello, ", "world"}
+	data := strings.Join(segments, "")
+	for n := 0; n <= len(data); n++ {
+		r := NewReader(&StringReader{data: segments})
+		// Read n bytes.
+		for i := 0; i < n; i++ {
+			b, err := r.ReadByte()
+			if err != nil {
+				t.Fatalf("n = %d: unexpected error on ReadByte: %v", n, err)
+			}
+			if b != data[i] {
+				t.Fatalf("n = %d: incorrect byte returned from ReadByte: got %q, want %q", n, b, data[i])
+			}
+		}
+		// Unread one byte if there is one.
+		if n > 0 {
+			if err := r.UnreadByte(); err != nil {
+				t.Errorf("n = %d: unexpected error on UnreadByte: %v", n, err)
+			}
+		}
+		// Test that we cannot unread any further.
+		if err := r.UnreadByte(); err == nil {
+			t.Errorf("n = %d: expected error on UnreadByte", n)
+		}
 	}
 }