presto-main/src/main/java/io/prestosql/operator/SetBuilderOperator.java/SetBuilderOperator/setChannelSet
class SetBuilderOperator:        void setChannelSet(ChannelSet channelSet)
        {
            boolean wasSet = channelSetFuture.set(requireNonNull(channelSet, "channelSet is null"));
            checkState(wasSet, "ChannelSet already set");
        }