http2: panic on invalid WriteHeader status code

Tests are in net/http. (upcoming CL)

Updates golang/go#22880

Change-Id: Ie94693ad4e14f0c07926a0b6c7827caace94a0aa
Reviewed-on: https://go-review.googlesource.com/80076
Reviewed-by: Tom Bergan <tombergan@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

http2/server.go

@@ -2490,7 +2490,26 @@ func (w *responseWriter) Header() http.Header {
 	return rws.handlerHeader
 }
 
+// checkWriteHeaderCode is a copy of net/http's checkWriteHeaderCode.
+func checkWriteHeaderCode(code int) {
+	// Issue 22880: require valid WriteHeader status codes.
+	// For now we only enforce that it's three digits.
+	// In the future we might block things over 599 (600 and above aren't defined
+	// at http://httpwg.org/specs/rfc7231.html#status.codes)
+	// and we might block under 200 (once we have more mature 1xx support).
+	// But for now any three digits.
+	//
+	// We used to send "HTTP/1.1 000 0" on the wire in responses but there's
+	// no equivalent bogus thing we can realistically send in HTTP/2,
+	// so we'll consistently panic instead and help people find their bugs
+	// early. (We can't return an error from WriteHeader even if we wanted to.)
+	if code < 100 || code > 999 {
+		panic(fmt.Sprintf("invalid WriteHeader code %v", code))
+	}
+}
+
 func (w *responseWriter) WriteHeader(code int) {
+	checkWriteHeaderCode(code)
 	rws := w.rws
 	if rws == nil {
 		panic("WriteHeader called after Handler finished")