This dfs checks why tasks are still pending.

def _depth_first_search(set_tasks, current_task, visited):
    """
    This dfs checks why tasks are still pending.
    """
    visited.add(current_task)
    if current_task in set_tasks["still_pending_not_ext"]:
        upstream_failure = False
        upstream_missing_dependency = False
        upstream_run_by_other_worker = False
        upstream_scheduling_error = False
        for task in current_task._requires():
            if task not in visited:
                _depth_first_search(set_tasks, task, visited)
            if task in set_tasks["ever_failed"] or task in set_tasks["upstream_failure"]:
                set_tasks["upstream_failure"].add(current_task)
                upstream_failure = True
            if task in set_tasks["still_pending_ext"] or task in set_tasks["upstream_missing_dependency"]:
                set_tasks["upstream_missing_dependency"].add(current_task)
                upstream_missing_dependency = True
            if task in set_tasks["run_by_other_worker"] or task in set_tasks["upstream_run_by_other_worker"]:
                set_tasks["upstream_run_by_other_worker"].add(current_task)
                upstream_run_by_other_worker = True
            if task in set_tasks["scheduling_error"]:
                set_tasks["upstream_scheduling_error"].add(current_task)
                upstream_scheduling_error = True
        if not upstream_failure and not upstream_missing_dependency and \
                not upstream_run_by_other_worker and not upstream_scheduling_error and \
                current_task not in set_tasks["run_by_other_worker"]:
            set_tasks["not_run"].add(current_task)