Passing an empty value channel to a process causes nextflow to hang

[human9]

If a channel is created via channel.value() and passed directly to a process, nextflow hangs forever.

Expected behavior and actual behavior

I assumed that passing channel.value() would work for passing optional inputs that I always want to evaluate to false, as it seems to create a value channel with null as the value. But doing this will just hang the nextflow process with no error message.

I notice the docs specifically mention that channel.value(null) isn't allowed, which seems to result in identical behaviour to channel.value(). I also notice that passing null directly to a process results in an error message. So maybe the expected behaviour here is to throw the same error message instead of hanging?

Script to reproduce

process P {
 input:
 val empty

 exec:
 println empty
}

workflow {
 P(
  channel.value()
 )
}

Output (this doesn't error out, just hangs)

 N E X T F L O W   ~  version 24.03.0-edge
Launching `main.nf` [ecstatic_jepsen] DSL2 - revision: 0527d9bad8
[-        ] process > P -

.nextflow.log

Environment

• Nextflow version: 24.03.0-edge
• Java version: Temurin 21.0.2
• Operating system: Ubuntu 24.04
• Bash version: GNU bash, version 5.2.21(1)-release