net/http/httptest: panic on non-3 digit (XXX) status code in Recorder.WriteHeader

This change conforms Recorder with net/http servers, to panic
when a handler writes a non-3 digit XXX status code.

Fixes #45353

Change-Id: Id5ed4af652e8c150ae86bf50402b800d935e2203
Reviewed-on: https://go-review.googlesource.com/c/go/+/308950
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>

src/net/http/httptest/recorder.go

@@ -122,11 +122,30 @@ func (rw *ResponseRecorder) WriteString(str string) (int, error) {
 	return len(str), nil
 }
 
+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 https://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))
+	}
+}
+
 // WriteHeader implements http.ResponseWriter.
 func (rw *ResponseRecorder) WriteHeader(code int) {
 	if rw.wroteHeader {
 		return
 	}
+
+	checkWriteHeaderCode(code)
 	rw.Code = code
 	rw.wroteHeader = true
 	if rw.HeaderMap == nil {

src/net/http/httptest/recorder_test.go

@@ -345,3 +345,28 @@ func TestParseContentLength(t *testing.T) {
 		}
 	}
 }
+
+// Ensure that httptest.Recorder panics when given a non-3 digit (XXX)
+// status HTTP code. See https://golang.org/issues/45353
+func TestRecorderPanicsOnNonXXXStatusCode(t *testing.T) {
+	badCodes := []int{
+		-100, 0, 99, 1000, 20000,
+	}
+	for _, badCode := range badCodes {
+		badCode := badCode
+		t.Run(fmt.Sprintf("Code=%d", badCode), func(t *testing.T) {
+			defer func() {
+				if r := recover(); r == nil {
+					t.Fatal("Expected a panic")
+				}
+			}()
+
+			handler := func(rw http.ResponseWriter, _ *http.Request) {
+				rw.WriteHeader(badCode)
+			}
+			r, _ := http.NewRequest("GET", "http://example.org/", nil)
+			rw := NewRecorder()
+			handler(rw, r)
+		})
+	}
+}