Add support for migrating to Redis on PaaS

Similarly to https://github.com/alphagov/notifications-api/pull/3508

tests/app/test_cloudfoundry_config.py

@@ -1,3 +1,4 @@
+import json
 import os
 
 import pytest
@@ -6,12 +7,33 @@ from app.cloudfoundry_config import extract_cloudfoundry_config
 
 
 @pytest.fixture
-def cloudfoundry_environ(os_environ):
+def cloudfoundry_config():
+    return {
+        'redis': [{
+            'credentials': {
+                'uri': 'redis uri'
+            }
+        }],
+    }
+
+
+@pytest.fixture
+def vcap_application(os_environ):
     os.environ['VCAP_APPLICATION'] = '{"space_name":"🚀🌌"}'
 
 
-def test_extract_cloudfoundry_config_populates_other_vars(cloudfoundry_environ):
+def test_extract_cloudfoundry_config_populates_other_vars(vcap_application, cloudfoundry_config):
+    os.environ['VCAP_SERVICES'] = json.dumps(cloudfoundry_config)
     extract_cloudfoundry_config()
 
+    assert os.environ['REDIS_URL'] == 'redis uri'
     assert os.environ['NOTIFY_ENVIRONMENT'] == '🚀🌌'
     assert os.environ['NOTIFY_LOG_PATH'] == '/home/vcap/logs/app.log'
+
+
+def test_set_config_env_vars_copes_if_redis_not_set(vcap_application, cloudfoundry_config):
+    del cloudfoundry_config['redis']
+    os.environ['VCAP_SERVICES'] = json.dumps(cloudfoundry_config)
+
+    extract_cloudfoundry_config()
+    assert 'REDIS_URL' not in os.environ