It appears that the build trigger provided by the p4 plugin does not launch a new pipeline build if a new change is submitted. It seems to not be obeying the 'Do not allow concurrent builds' setting of pipelines. If that setting is disabled, multiple pipeline builds should be allowed simultaneously, and the pipeline script itself should take care to mediate locking of shared resources.

This effectively makes the p4 trigger useless on pipelines that have any sort of long running parallelized test processes, and requires falling back to polling (or using github.com).