metrics: Fix panic when headers aren't written (#3737)

Signed-off-by: Dave Henderson <dhenderson@gmail.com>

modules/caddyhttp/metrics.go

@@ -108,7 +108,7 @@ func newMetricsInstrumentedHandler(handler string, mh MiddlewareHandler) *metric
 func (h *metricsInstrumentedHandler) ServeHTTP(w http.ResponseWriter, r *http.Request, next Handler) error {
 	server := serverNameFromContext(r.Context())
 	labels := prometheus.Labels{"server": server, "handler": h.handler}
-	statusLabels := prometheus.Labels{"server": server, "handler": h.handler, "method": r.Method}
+	statusLabels := prometheus.Labels{"server": server, "handler": h.handler, "method": r.Method, "code": ""}
 
 	inFlight := httpMetrics.requestInFlight.With(labels)
 	inFlight.Inc()
@@ -134,6 +134,14 @@ func (h *metricsInstrumentedHandler) ServeHTTP(w http.ResponseWriter, r *http.Re
 		return err
 	}
 
+	// If the code hasn't been set yet, and we didn't encounter an error, we're
+	// probably falling through with an empty handler.
+	if statusLabels["code"] == "" {
+		// we still sanitize it, even though it's likely to be 0. A 200 is
+		// returned on fallthrough so we want to reflect that.
+		statusLabels["code"] = sanitizeCode(wrec.Status())
+	}
+
 	httpMetrics.requestDuration.With(statusLabels).Observe(dur)
 	httpMetrics.requestSize.With(statusLabels).Observe(float64(computeApproximateRequestSize(r)))
 	httpMetrics.responseSize.With(statusLabels).Observe(float64(wrec.Size()))

modules/caddyhttp/metrics_test.go

@@ -57,6 +57,24 @@ func TestMetricsInstrumentedHandler(t *testing.T) {
 	if err := ih.ServeHTTP(w, r, h); err != nil {
 		t.Errorf("Received unexpected error: %w", err)
 	}
+
+	// an empty handler - no errors, no header written
+	mh = middlewareHandlerFunc(func(w http.ResponseWriter, r *http.Request, h Handler) error {
+		return nil
+	})
+	ih = newMetricsInstrumentedHandler("empty", mh)
+	r = httptest.NewRequest("GET", "/", nil)
+	w = httptest.NewRecorder()
+
+	if err := ih.ServeHTTP(w, r, h); err != nil {
+		t.Errorf("Received unexpected error: %w", err)
+	}
+	if actual := w.Result().StatusCode; actual != 200 {
+		t.Errorf("Not same: expected status code %#v, but got %#v", 200, actual)
+	}
+	if actual := w.Result().Header; len(actual) != 0 {
+		t.Errorf("Not empty: expected headers to be empty, but got %#v", actual)
+	}
 }
 
 type middlewareHandlerFunc func(http.ResponseWriter, *http.Request, Handler) error