@Override
    public void close() {
        final boolean setClosing = setClosing();
        if (setClosing) {
            invokeCloseAction();
        }

        boolean interrupted = false;
        try {
            for (;;) {
                try {
                    closeFuture.get();
                    break;
                } catch (InterruptedException e) {
                    interrupted = true;
                } catch (CancellationException e) {
                    // Throw the exception only for the first.
                    if (setClosing) {
                        throw e;
                    } else {
                        break;
                    }
                } catch (ExecutionException e) {
                    // Throw the exception only for the first.
                    if (setClosing) {
                        throw new CompletionException(e.getCause());
                    } else {
                        break;
                    }
                }
            }
        } finally {
            if (interrupted) {
                Thread.currentThread().interrupt();
            }
        }
    }
Returns the {@link AsyncCloseableSupport} which has been closed already.
core/src/main/java/com/linecorp/armeria/common/util/AsyncCloseableSupport.java