[AIRFLOW-2888] Add deprecation path for task_runner config change

airflow/configuration.py

@@ -152,6 +152,19 @@ class AirflowConfigParser(ConfigParser):
         'setting has been used, but please update your config.'
     )
 
+    # A mapping of old default values that we want to change and warn the user
+    # about. Mapping of section -> setting -> { old, replace, by_version }
+    deprecated_values = {
+        'core': {
+            'task_runner': ('BashTaskRunner', 'StandardTaskRunner', '2.0'),
+        },
+    }
+    deprecation_value_format_string = (
+        'The {name} setting in [{section}] has the old default value of {old!r}. This '
+        'value has been changed to {new!r} in the running config, but please '
+        'update your config before Apache Airflow {version}.'
+    )
+
     def __init__(self, default_config=None, *args, **kwargs):
         super(AirflowConfigParser, self).__init__(*args, **kwargs)
 
@@ -169,11 +182,30 @@ def _validate(self):
                 "error: cannot use sqlite with the {}".format(
                     self.get('core', 'executor')))
 
+        for section, replacement in self.deprecated_values.items():
+            for name, info in replacement.items():
+                old, new, version = info
+                if self.get(section, name, fallback=None) == old:
+                    # Make sure the env var option is removed, otherwise it
+                    # would be read and used instead of the value we set
+                    env_var = self._env_var_name(section, name)
+                    os.environ.pop(env_var, None)
+
+                    self.set(section, name, new)
+                    warnings.warn(
+                        self.deprecation_value_format_string.format(**locals()),
+                        FutureWarning,
+                    )
+
         self.is_validated = True
 
+    @staticmethod
+    def _env_var_name(section, key):
+        return 'AIRFLOW__{S}__{K}'.format(S=section.upper(), K=key.upper())
+
     def _get_env_var_option(self, section, key):
         # must have format AIRFLOW__{SECTION}__{KEY} (note double underscore)
-        env_var = 'AIRFLOW__{S}__{K}'.format(S=section.upper(), K=key.upper())
+        env_var = self._env_var_name(section, key)
         if env_var in os.environ:
             return expand_env_var(os.environ[env_var])
 

tests/test_configuration.py

@@ -20,8 +20,9 @@
 from __future__ import print_function
 from __future__ import unicode_literals
 
-import os
 import contextlib
+import os
+import warnings
 from collections import OrderedDict
 
 import six
@@ -339,3 +340,40 @@ def test_deprecated_options_cmd(self):
             self.assertEqual(conf.getint('celery', 'result_backend'), 99)
             if tmp:
                 os.environ['AIRFLOW__CELERY__RESULT_BACKEND'] = tmp
+
+    def test_deprecated_values(self):
+        def make_config():
+            test_conf = AirflowConfigParser(default_config='')
+            # Guarantee we have a deprecated setting, so we test the deprecation
+            # lookup even if we remove this explicit fallback
+            test_conf.deprecated_values = {
+                'core': {
+                    'task_runner': ('BashTaskRunner', 'StandardTaskRunner', '2.0'),
+                },
+            }
+            test_conf.read_dict({
+                'core': {
+                    'executor': 'SequentialExecutor',
+                    'task_runner': 'BashTaskRunner',
+                    'sql_alchemy_conn': 'sqlite://',
+                },
+            })
+            return test_conf
+
+        with self.assertWarns(FutureWarning):
+            test_conf = make_config()
+            self.assertEqual(test_conf.get('core', 'task_runner'), 'StandardTaskRunner')
+
+        with self.assertWarns(FutureWarning):
+            with env_vars(AIRFLOW__CORE__TASK_RUNNER='BashTaskRunner'):
+                test_conf = make_config()
+
+                self.assertEqual(test_conf.get('core', 'task_runner'), 'StandardTaskRunner')
+
+        with warnings.catch_warnings(record=True) as w:
+            with env_vars(AIRFLOW__CORE__TASK_RUNNER='NotBashTaskRunner'):
+                test_conf = make_config()
+
+                self.assertEqual(test_conf.get('core', 'task_runner'), 'NotBashTaskRunner')
+
+                self.assertListEqual([], w)