airflow/utils/dag_cycle_tester.py

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"""DAG Cycle tester"""
from collections import defaultdict, deque

from airflow.exceptions import AirflowDagCycleException

CYCLE_NEW = 0
CYCLE_IN_PROGRESS = 1
CYCLE_DONE = 2


def test_cycle(dag):
    """
    Check to see if there are any cycles in the DAG. Returns False if no cycle found,
    otherwise raises exception.
    """
    # default of int is 0 which corresponds to CYCLE_NEW
    visited = defaultdict(int)
    path_stack = deque()
    task_dict = dag.task_dict

    def _check_adjacent_tasks(task_id, current_task):
        """Returns first untraversed child task, else None if all tasks traversed."""
        for adjacent_task in current_task.get_direct_relative_ids():
            if visited[adjacent_task] == CYCLE_IN_PROGRESS:
                msg = f"Cycle detected in DAG. Faulty task: {task_id}"
                raise AirflowDagCycleException(msg)
            elif visited[adjacent_task] == CYCLE_NEW:
                return adjacent_task
        return None

    for dag_task_id in dag.task_dict.keys():
        if visited[dag_task_id] == CYCLE_DONE:
            continue
        path_stack.append(dag_task_id)
        while path_stack:
            current_task_id = path_stack[-1]
            if visited[current_task_id] == CYCLE_NEW:
                visited[current_task_id] = CYCLE_IN_PROGRESS
            task = task_dict[current_task_id]
            child_to_check = _check_adjacent_tasks(current_task_id, task)
            if not child_to_check:
                visited[current_task_id] = CYCLE_DONE
                path_stack.pop()
            else:
                path_stack.append(child_to_check)