fix(master): redirect a cancelled Check now instead of an empty response (#35)

internal/master/web/server.go

@@ -633,13 +633,15 @@ func (s *Server) pollService(w http.ResponseWriter, r *http.Request) {
 		s.renderServiceError(w, r, id, http.StatusConflict, "Cannot check: "+np.Reason+".")
 		return
 	case r.Context().Err() != nil:
-		return // the client is gone
+		// The client is gone or the master is shutting down. Redirect anyway:
+		// a browser still waiting gets the page, not an empty 200.
 	case err != nil:
 		s.failLookup(w, err)
 		return
+	default:
+		sess, _ := webauth.SessionFrom(r.Context())
+		s.log.Info("poll_requested", "service_id", id, "subject", sess.Subject)
 	}
-	sess, _ := webauth.SessionFrom(r.Context())
-	s.log.Info("poll_requested", "service_id", id, "subject", sess.Subject)
 	http.Redirect(w, r, "/services/"+strconv.FormatInt(id, 10), http.StatusSeeOther)
 }