private void handlePendingPiggybacks(@Nullable PooledChannel value) {
            final Object pendingPiggybackHandlers = this.pendingPiggybackHandlers;
            if (pendingPiggybackHandlers == null) {
                return;
            }

            this.pendingPiggybackHandlers = null;

            if (!(pendingPiggybackHandlers instanceof List)) {
                // 1 handler
                @SuppressWarnings("unchecked")
                final Consumer<PooledChannel> handler =
                        (Consumer<PooledChannel>) pendingPiggybackHandlers;
                handler.accept(value);
                return;
            }

            // 2+ handlers
            @SuppressWarnings("unchecked")
            final List<Consumer<PooledChannel>> list =
                    (List<Consumer<PooledChannel>>) pendingPiggybackHandlers;
            for (Consumer<PooledChannel> handler : list) {
                handler.accept(value);
            }
        }
A {@code Consumer<PooledChannel>} if only 1 handler.
A {@code List<Consumer<PooledChannel>>} if there are 2+ handlers.
core/src/main/java/com/linecorp/armeria/client/HttpChannelPool.java