strings, bytes: panic if Repeat overflows or if given a negative count

Panic if Repeat is given a negative count or
if the value of (len(*) * count) is detected
to overflow.
We panic because we cannot change the
signature of Repeat to return an error.

Fixes #16237

Change-Id: I9f5ba031a5b8533db0582d7a672ffb715143f3fb
Reviewed-on: https://go-review.googlesource.com/29954
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/bytes/bytes_test.go

@@ -903,6 +903,54 @@ func TestRepeat(t *testing.T) {
 	}
 }
 
+func repeat(b []byte, count int) (err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			switch v := r.(type) {
+			case error:
+				err = v
+			default:
+				err = fmt.Errorf("%s", v)
+			}
+		}
+	}()
+
+	Repeat(b, count)
+
+	return
+}
+
+// See Issue golang.org/issue/16237
+func TestRepeatCatchesOverflow(t *testing.T) {
+	tests := [...]struct {
+		s      string
+		count  int
+		errStr string
+	}{
+		0: {"--", -2147483647, "negative"},
+		1: {"", int(^uint(0) >> 1), ""},
+		2: {"-", 10, ""},
+		3: {"gopher", 0, ""},
+		4: {"-", -1, "negative"},
+		5: {"--", -102, "negative"},
+		6: {string(make([]byte, 255)), int((^uint(0))/255 + 1), "overflow"},
+	}
+
+	for i, tt := range tests {
+		err := repeat([]byte(tt.s), tt.count)
+		if tt.errStr == "" {
+			if err != nil {
+				t.Errorf("#%d panicked %v", i, err)
+			}
+			continue
+		}
+
+		if err == nil || !strings.Contains(err.Error(), tt.errStr) {
+			t.Errorf("#%d expected %q got %q", i, tt.errStr, err)
+		}
+	}
+}
+
 func runesEqual(a, b []rune) bool {
 	if len(a) != len(b) {
 		return false