void signal() {
            if (downstreamExecutor.inEventLoop()) {
                doSignal();
            } else {
                downstreamExecutor.execute(this::doSignal);
            }
        }
{@link CancelledSubscriptionException} if cancelled. {@link Throwable} if aborted.
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessageDuplicator.java