Process recursion considers topic channels for cardinality of outputs

[OlivierCoen]

Bug report

Expected behavior and actual behavior

Topic channels are counted as actual outputs for processes included in a recursion loop. This is a problem for topic channels, in particular for tool versioning.

Steps to reproduce the problem

Here is a little test workflow to reproduce the error:

nextflow.preview.recursion=true

process INIT {

  output:
    path 'result.txt'
  script:
    """
    echo "a" > result.txt
    """
}

process FOO {
  input:
    path 'input.txt'
  output:
    path 'result.txt', emit: text
    val("a"),          topic: test
  script:
    """
    cat input.txt > result.txt
    echo "Task ${task.index} was here" >> result.txt
    """
}

workflow {

  INIT()

  FOO
    .recurse(INIT.out)
    .times(10)

  FOO
    .out.text
    .view()
}

The script fails with the error:
Process FOOinputs and outputs do not have the same cardinality - Feedback loop is not supported but works well by commenting theval("a"), topic: test` line.

Environment

• Nextflow version: 26.04.03
• Java version: openjdk 17.0.10 2024-01-16
• Operating system: Ubuntu 24.04
• Bash version: zsh 5.9 (x86_64-ubuntu-linux-gnu)

Thanks a lot in advance!

[bentsherman]

This is addressed by typed processes , where topics are defined separately from outputs

I don't think we will solve this problem for legacy processes, especially since recursion is still an experimental feature

In fact, I've heard it is possible to implement recursion with a topic... might be something interesting to try 😄