Jenkinsfile with email in loop fails on first iteration - groovy

Question

Latest Jenkins used.
edit: pastebin of full Java exception - https://pastebin.com/zZDNj18E
Goal: loop through all nodes, check for offline, email alert for each offline node.
(tried emailext alerts as well, could not use "offline")
Failed: My jenkinsfile runs perfectly with no email.
With email in the for loop or separately defined in a function, the job crashes after the first email is sent.
[Pipeline] End of Pipeline an exception which occurred: in field hudson.model.Slave.launcher in object hudson.slaves.DumbSlave#ae938e61 .... and many more
My jenkinsfile:
pipeline {
agent{
label 'master'
}
options {
// Enable timestamps in log
timestamps()
skipDefaultCheckout()
timeout(time: 4, unit: 'MINUTES')
}
stages {
stage('Monitor') {
steps{
script{
def offlineSlaves = []
for (aSlave in hudson.model.Hudson.instance.slaves) {
def thisSlave = aSlave.name
echo 'Name: ' + thisSlave + ' is being checked.'
if ( aSlave.getComputer().isOffline().toString() == 'true') {
slaveState = 'OFFLINE'
echo 'Name: ' + thisSlave + ' is ' + slaveState + ' !'
emailext (
mimeType: 'text/html',
body: "${env.JOB_NAME} found an OFFLINE node: ${name} ",
subject: "Jenkins ERROR: Build Node ${name} is OFFLINE " ,
to: 'jfisher#xxx')
}
}
}
}
}
}
post {
failure {
emailext (
body: 'Monitor Nodes Jenkins Job failed !',
presendScript: '$DEFAULT_PRESEND_SCRIPT',
recipientProviders: [requestor(),culprits()],
subject: 'Monitor Nodes Jenkins Failed',
to: 'jfisher#intouchhealth.com')
}
}
}

Answer 1

The problem with this code is the getComputer() part. In the pipeline you should only use Serializable and the SlaveComputer returned from getComputer() isn't.
https://javadoc.jenkins.io/hudson/slaves/SlaveComputer.html
What you should do is move this part to a function annotated with NonCPS
#NonCPS
def shallTrigger() {
for (aSlave in hudson.model.Hudson.instance.slaves) {
def thisSlave = aSlave.name
echo 'Name: ' + thisSlave + ' is being checked.'
if ( aSlave.getComputer().isOffline().toString() == 'true') {
slaveState = 'OFFLINE'
echo 'Name: ' + thisSlave + ' is ' + slaveState + ' !'
emailext (
mimeType: 'text/html',
body: "${env.JOB_NAME} found an OFFLINE node: ${name} ",
subject: "Jenkins ERROR: Build Node ${name} is OFFLINE " ,
to: 'jfisher#xxx')
}
}
}