Why do I get a `java.nio.file.ProviderMismatchException` when I access `isEmpty()` on a staged file - nextflow

Question

I am getting a java.nio.file.ProviderMismatchException when I run the following script:
process a {
output:
file _biosample_id optional true into biosample_id
script:
"""
touch _biosample_id
"""
}
process b {
input:
file _biosample_id from biosample_id.ifEmpty{file("_biosample_id")}
script:
def biosample_id_option = _biosample_id.isEmpty() ? '' : "--biosample_id \$(cat _biosample_id)"
"""
echo \$(cat ${_biosample_id})
"""
}
i'm using a slightly modified version of Optional Input pattern.
Any ideas on why I'm getting the java.nio.file.ProviderMismatchException?

Answer 1

In your script block, _biosample_id is actually an instance of the nextflow.processor.TaskPath class. So to check if the file (or directory) is empty you can just call it's .empty() method. For example:
script:
def biosample_id_option = _biosample_id.empty() ? '' : "--biosample_id \$(< _biosample_id)"
I like your solution - I think it's neat. And I think it should be robust (but I haven't tested it). The optional input pattern that is recommended will fail when attempting to stage missing input files to a remote filesystem/object store. There is a solution however, which is to keep an empty file in your $baseDir and point to it in your scripts. For example:
params.inputs = 'prots/*{1,2,3}.fa'
params.filter = "${baseDir}/assets/null/NO_FILE"
prots_ch = Channel.fromPath(params.inputs)
opt_file = file(params.filter)
process foo {
input:
file seq from prots_ch
file opt from opt_file
script:
def filter = opt.name != 'NO_FILE' ? "--filter $opt" : ''
"""
your_commad --input $seq $filter
"""
}