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/strings/strings.go

@@ -418,7 +418,20 @@ func Map(mapping func(rune) rune, s string) string {
 }
 
 // Repeat returns a new string consisting of count copies of the string s.
+//
+// It panics if count is negative or if
+// the result of (len(s) * count) overflows.
 func Repeat(s string, count int) string {
+	// Since we cannot return an error on overflow,
+	// we should panic if the repeat will generate
+	// an overflow.
+	// See Issue golang.org/issue/16237
+	if count < 0 {
+		panic("strings: negative Repeat count")
+	} else if count > 0 && len(s)*count/count != len(s) {
+		panic("strings: Repeat count causes overflow")
+	}
+
 	b := make([]byte, len(s)*count)
 	bp := copy(b, s)
 	for bp < len(b) {