As recorded in https://groups.google.com/d/msg/jenkinsci-dev/kgEc7vZQgi0/IC1rH_6KwNYJ:
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Our deploy process currently builds deploy artifacts and runs tests at the same time. I see how we could implement that using workflow via the 'parallel' step. However, we also want the behavior that if and when build-artifacts fails, it immediately cancels the running tests and fails the deploy job; and likewise if the tests fail, it immediately cancels the building of artifacts and fails the deploy job. If both succeed, then the deploy job continues.
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This is a feature request to implement this sibling-cancel: with a flag (or perhaps as the default, though that's kinda scary), if one of the jobs in parallel() fails, then parallel cancels the other jobs.

It sounds like parallel returns a map from job-label to job-status. I think under this behavior, the job-status would be "failed" for the job that failed, and "cancelled" for the sibling jobs that were cancelled.