code: static int mask(Class<? extends ChannelHandler> clazz) {
          // Try to obtain the mask from the cache first. If this fails calculate it and put it in the cache for fast
          // lookup in the future.
          Map<Class<? extends ChannelHandler>, Integer> cache = MASKS.get();
          Integer mask = cache.get(clazz);
          if (mask == null) {
              mask = mask0(clazz);
              cache.put(clazz, mask);
          }
          return mask;
      }
docs: Return the {@code executionMask}.