PlaceholderExecutable needs an environment variable to add to the context to ensure that Launcher.kill, called when the block exits, will clean up any stray external processes run on this node.
Originally the name of this variable was chosen to be JENKINS_SERVER_COOKIE, which is set by Job (to a confidential hex key) and normally used by AbstractBuild for this purpose. Unlike in that case, for Workflow the value is overridden (to a random UUID) for each node block, since there may be several. I think the intent was to use the same variable name to suppress any unwanted kills coming from Jenkins core code, but there seem to be none. Anyway after the many refactorings of environment variable handling in Workflow, it turns out this is dead code!
- FileMonitoringTask sets its own value for JENKINS_SERVER_COOKIE, to durable-<workspaceHash>, which overrides any other value, and is used by stop. So within a shell step, $JENKINS_SERVER_COOKIE is the one from durable-task, not PlaceholderExecutable.
- env.JENKINS_SERVER_COOKIE picks up the value from Job, due to the precedence order in getEffectiveEnvironment.
So it seems that while the intention is still good (we would like a final check that any processes started inside node are killed when the block exits), the implementation is not right. Probably PlaceholderExecutable just needs to pick an unrelated environment variable for this purpose. And this logic needs to be tested; for example: