spotify/luigi — pull request #444 by daveFNbuck (2014-09-12 00:35:07+00:00)
Title: Ensures that dependencies have at least as high priority as dependents

This recursively passes priorities down to dependents so if task A depends on task B, task B must have at least as high a priority as A. This avoids situations where extremely high priority tasks only end up running after all the low-priority tasks have finished because it depends on the one that runs last.