I'm trying to use the "requirements file" option.

Setting it to a non-empty value causes a test to run whether the file already exists and shows a warning if it doesn't (good). The warning is also shown if the file already exists (bad), and trying to save the configuration leads to an exception (very bad).

Setting the option through editing the job configuration XML and reloading the Jenkins configuration works, and the job correctly uses the contents of the file as additional packages to install (good), but editing the job from the UI is now impossible as saving the configuration fails.