Why does this Jenkins build script assign the same node ID twice? - jenkins

Question

I'm using Knapsack Pro to split up unit tests and run them in parallel, and I've recently noticed that sometimes, it assigns the same node index to multiple nodes.
The build script looks like this (trimmed for brevity):
def num_nodes = 10;
def nodes = [:]
env.total_nodes = num_nodes
for (i in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]) {
int index = i
nodes["ci_node_${i}"] = {
node('ec2-slave') {
...
env.new_index = index
stage('Build & run rspec') {
dir("$WORKSPACE") {
sh '''#!/bin/bash
cd $WORKSPACE
export NODE_ENV=production
export KNAPSACK_PRO_CI_NODE_TOTAL=${total_nodes}
export KNAPSACK_PRO_CI_NODE_BUILD_ID=${BUILD_TAG}
export KNAPSACK_PRO_CI_NODE_INDEX=${new_index}
export KNAPSACK_PRO_COMMIT_HASH=${commithash}
export KNAPSACK_PRO_BRANCH=${BRANCH_NAME}
export KNAPSACK_PRO_TEST_FILE_EXCLUDE_PATTERN="spec/smokes/*_spec.rb"
env
...
'''
}
}
}
}
}
try {
parallel nodes // run CI nodes in parallel
} catch (err) {
echo "Build Failed - ${err.getMessage()}"
} finally {
...
}
But when I grep the logs for all the values of KNAPSACK_PRO_CI_NODE_INDEX that get printed by env, I sometimes (but not always) see something like this:
KNAPSACK_PRO_CI_NODE_INDEX=0
KNAPSACK_PRO_CI_NODE_INDEX=2
KNAPSACK_PRO_CI_NODE_INDEX=1
KNAPSACK_PRO_CI_NODE_INDEX=5
KNAPSACK_PRO_CI_NODE_INDEX=3
KNAPSACK_PRO_CI_NODE_INDEX=8
KNAPSACK_PRO_CI_NODE_INDEX=6
KNAPSACK_PRO_CI_NODE_INDEX=6
KNAPSACK_PRO_CI_NODE_INDEX=7
KNAPSACK_PRO_CI_NODE_INDEX=9
Node 4 never gets assigned, and node 6 gets assigned twice. How is this happening?
note: That horrible hard-coded for-loop is an attempt to work around this, which may be relevant:
http://blog.freeside.co/2013/03/29/groovy-gotcha-for-loops-and-closure-scope
Passing loop variable to sh in Jenkinsfile doesn't scope correctly
Parallel jenkins job using a for loop

Answer 1

env.new_index = index
env is a global object that is part of the main script. env.new_index is a global property.
Example:
node {
env.VAR1 = "value"
}
println(env.VAR1)
will print
[Pipeline] Start of Pipeline
[Pipeline] node
[Pipeline] {
[Pipeline] }
[Pipeline] // node
[Pipeline] echo
value
[Pipeline] End of Pipeline
Finished: SUCCESS
If you want to define env vars per node then it's better to use withEnv
withEnv(["new_index=$index"]) {
stage('Build & run rspec') {
dir("$WORKSPACE") {
sh '''#!/bin/bash
cd $WORKSPACE
export NODE_ENV=production
export KNAPSACK_PRO_CI_NODE_TOTAL=${total_nodes}
export KNAPSACK_PRO_CI_NODE_BUILD_ID=${BUILD_TAG}
export KNAPSACK_PRO_CI_NODE_INDEX=${new_index}
export KNAPSACK_PRO_COMMIT_HASH=${commithash}
export KNAPSACK_PRO_BRANCH=${BRANCH_NAME}
export KNAPSACK_PRO_TEST_FILE_EXCLUDE_PATTERN="spec/smokes/*_spec.rb"
env
...
'''
}
}
}