[AIRFLOW-2855] Check Cron Expression Validity in DagBag.process_file() (

#3698)

A DAG can be imported as a .py script properly,
but the Cron expression inside as "schedule_interval" may be
invalid, like "0 100 * * *".

This commit helps check the validity of Cron expression in DAG
files (.py) and packaged DAG files (.zip), and help show
exception messages in web UI by add these exceptions into
metadata "import_error".

airflow/models.py

@@ -62,7 +62,9 @@
 from sqlalchemy.orm import reconstructor, relationship, synonym
 from sqlalchemy_utc import UtcDateTime
 
-from croniter import croniter
+from croniter import (
+    croniter, CroniterBadCronError, CroniterBadDateError, CroniterNotAlphaError
+)
 import six
 
 from airflow import settings, utils
@@ -412,8 +414,18 @@ def process_file(self, filepath, only_if_updated=True, safe_mode=True):
                     try:
                         dag.is_subdag = False
                         self.bag_dag(dag, parent_dag=dag, root_dag=dag)
+                        if isinstance(dag._schedule_interval, six.string_types):
+                            croniter(dag._schedule_interval)
                         found_dags.append(dag)
                         found_dags += dag.subdags
+                    except (CroniterBadCronError,
+                            CroniterBadDateError,
+                            CroniterNotAlphaError) as cron_e:
+                        self.log.exception("Failed to bag_dag: %s", dag.full_filepath)
+                        self.import_errors[dag.full_filepath] = \
+                            "Invalid Cron expression: " + str(cron_e)
+                        self.file_last_changed[dag.full_filepath] = \
+                            file_last_changed_on_disk
                     except AirflowDagCycleException as cycle_exception:
                         self.log.exception("Failed to bag_dag: %s", dag.full_filepath)
                         self.import_errors[dag.full_filepath] = str(cycle_exception)

tests/dags/test_invalid_cron.py

@@ -0,0 +1,35 @@
+# -*- coding: utf-8 -*-
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+from airflow.models import DAG
+from airflow.operators.dummy_operator import DummyOperator
+from airflow.utils.timezone import datetime
+
+# The schedule_interval specified here is an INVALID
+# Cron expression. This invalid DAG will be used to
+# test whether dagbag.process_file() can identify
+# invalid Cron expression.
+dag1 = DAG(
+    dag_id='test_invalid_cron',
+    start_date=datetime(2015, 1, 1),
+    schedule_interval="0 100 * * *")
+dag1_task1 = DummyOperator(
+    task_id='task1',
+    dag=dag1,
+    owner='airflow')

tests/models.py

@@ -56,7 +56,7 @@
 from airflow.utils.trigger_rule import TriggerRule
 from mock import patch, ANY
 from parameterized import parameterized
-from tempfile import NamedTemporaryFile
+from tempfile import mkdtemp, NamedTemporaryFile
 
 DEFAULT_DATE = timezone.datetime(2016, 1, 1)
 TEST_DAGS_FOLDER = os.path.join(
@@ -1047,6 +1047,19 @@ def test_zip(self):
         dagbag.process_file(os.path.join(TEST_DAGS_FOLDER, "test_zip.zip"))
         self.assertTrue(dagbag.get_dag("test_zip_dag"))
 
+    def test_process_file_cron_validity_check(self):
+        """
+        test if an invalid cron expression
+        as schedule interval can be identified
+        """
+        invalid_dag_files = ["test_invalid_cron.py", "test_zip_invalid_cron.zip"]
+        dagbag = models.DagBag(dag_folder=mkdtemp())
+
+        self.assertEqual(len(dagbag.import_errors), 0)
+        for d in invalid_dag_files:
+            dagbag.process_file(os.path.join(TEST_DAGS_FOLDER, d))
+        self.assertEqual(len(dagbag.import_errors), len(invalid_dag_files))
+
     @patch.object(DagModel,'get_current')
     def test_get_dag_without_refresh(self, mock_dagmodel):
         """