Channel operations toList followed by merge hang

[glormph]

Bug report

Expected behavior and actual behavior

Converting a channel to a list with toList, followed by merge to another channel works as expected (it merges the channels), but actual behaviour is that this happens, but the pipeline hangs after all the steps have been processed. Not sure if I'm doing it wrong here.

Steps to reproduce the problem

q1 = Channel.from([0, 'a'])

process test {
  output:
  file('hello') into q2

  """
  touch hello
  """
}

q1
  .toList()  // replacing with .buffer(size: 2) works fine.
  .merge(q2)
  .println{it}

Program output

$ nextflow run  small_tolist.nf
N E X T F L O W  ~  version 18.10.1
Launching `/Z/jorrit/repos/testnfcddadeq/small_tolist.nf` [awesome_murdock
[nextflow.log](https://github.com/nextflow-io/nextflow/files/2987949/nextflow.log)
] - revision: 690b801a5c
[warm up] executor > local
[15/261c24] Submitted process > test
[0, a, /path/to/nfrepo/work/15/261c24a59553470addc29f88b57e5e/hello]

... minutes pass ...

^C
$ 

Environment

• Nextflow version: [18.10.1, 19.01.0.5050]
• Java version: [11]
• Operating system: [Ubuntu 18.04]

Additional context

[glormph]

If it's at all interesting, I can add that using collect instead of toList results in the same behaviour.

[pditommaso]

Found the problem, thanks for reporting it.

[glormph]

Thank you very much for fixing! :) 👍