FYI the workaround on JENKINS-25550 (appending /doDelete to the build URL) doesn't work either. It removes the build in that the URL for the build returns 404, but the "part of foo" build is still consuming an executor on the slave.

Also FYI, even deleting the Workflow itself doesn't fix it. Executors still in use (although that at least cleared any queued runs).

Ahh... deleting the Workflow job, and then stopping and starting Jenkins finally did the trick. All executors free, and the phantom slave is gone.

(FYI I initially tried to use the <jenkins>/restart URL to restart Jenkins, and the Java process went into a 100% CPU busyloop. But killing it and restarting Jenkins finally put things right.)

This is still a pretty severe consequence of a trivial bug in a workflow script.

You need to delete the build as mentioned in JENKINS-25550, then restart.

Good to know the work-around, but IMHO at least this is still a significant bug as the consequences are severe and the work-around is pretty hidden.

No argument there.

Although obvious for the Groovy hackers, the workaround is:

def possiblyEmptyArray = ...
if (possiblyEmptyArray) {
  parallel possiblyEmptyArray
}

Code changed in jenkins
User: Jesse Glick
Path:
CHANGES.md
aggregator/src/test/java/org/jenkinsci/plugins/workflow/steps/parallel/ParallelStepTest.java
cps/src/main/java/org/jenkinsci/plugins/workflow/cps/steps/ParallelStepExecution.java
http://jenkins-ci.org/commit/workflow-plugin/1ecc56e967940fbc6047aa432d5d5e556670eca2
Log:
[FIXED JENKINS-29413]: hung build when running the parallel step with an empty map.

Code changed in jenkins
User: Jesse Glick
Path:
CHANGES.md
aggregator/src/test/java/org/jenkinsci/plugins/workflow/steps/parallel/ParallelStepTest.java
cps/src/main/java/org/jenkinsci/plugins/workflow/cps/steps/ParallelStepExecution.java
http://jenkins-ci.org/commit/workflow-plugin/85330f60060d84c9d4d5b05a2062e9fc97063a74
Log:
Merge pull request #265 from jglick/parallel-JENKINS-29413

JENKINS-29413 Hung build when running the parallel step with an empty map

Compare: https://github.com/jenkinsci/workflow-plugin/compare/be558db587b9...85330f60060d

Code changed in jenkins
User: Jesse Glick
Path:
aggregator/src/test/java/org/jenkinsci/plugins/workflow/steps/parallel/ParallelStepTest.java
cps/src/main/java/org/jenkinsci/plugins/workflow/cps/steps/ParallelStepExecution.java
http://jenkins-ci.org/commit/workflow-cps-plugin/bd606c258e887f8b7f9fa4bab9de0d008541d524
Log:
[FIXED JENKINS-29413]: hung build when running the parallel step with an empty map.
Originally-Committed-As: 1ecc56e967940fbc6047aa432d5d5e556670eca2