Cache known domains in Redis

The domains lookup is a bit slow because it’s serialising all the
organisations in the database. Since we’re putting this in the sign up
flow it should feel snappy, so lets cache just the domain bit of it in
Redis.

tests/app/notify_client/test_organisation_client.py

@@ -0,0 +1,94 @@
+from unittest.mock import call
+
+import pytest
+
+from app import organisations_client
+
+
+@pytest.mark.parametrize(
+    (
+        'expected_cache_get_calls,'
+        'cache_value,'
+        'expected_api_calls,'
+        'expected_cache_set_calls,'
+        'expected_return_value,'
+    ),
+    [
+        (
+            [
+                call('domains')
+            ],
+            b"""
+                [
+                    {"domains": ["a", "b", "c"]},
+                    {"domains": ["c", "d", "e"]}
+                ]
+            """,
+            [],
+            [],
+            ['a', 'b', 'c', 'd', 'e'],
+        ),
+        (
+            [
+                call('domains')
+            ],
+            None,
+            [
+                call(url='/organisations')
+            ],
+            [
+                call(
+                    'domains',
+                    '["x", "y", "z"]',
+                    ex=604800
+                )
+            ],
+            'from api',
+        ),
+    ]
+)
+def test_returns_value_from_cache(
+    app_,
+    mocker,
+    expected_cache_get_calls,
+    cache_value,
+    expected_return_value,
+    expected_api_calls,
+    expected_cache_set_calls,
+):
+
+    mock_redis_get = mocker.patch(
+        'app.extensions.RedisClient.get',
+        return_value=cache_value,
+    )
+    mock_api_get = mocker.patch(
+        'app.notify_client.NotifyAdminAPIClient.get',
+        return_value=[
+            {'domains': ['x', 'y', 'z']}
+        ],
+    )
+    mock_redis_set = mocker.patch(
+        'app.extensions.RedisClient.set',
+    )
+
+    organisations_client.get_domains()
+
+    assert mock_redis_get.call_args_list == expected_cache_get_calls
+    assert mock_api_get.call_args_list == expected_api_calls
+    assert mock_redis_set.call_args_list == expected_cache_set_calls
+
+
+def test_deletes_domain_cache(
+    app_,
+    mock_get_user,
+    mocker,
+    fake_uuid,
+):
+    mocker.patch('app.notify_client.current_user', id='1')
+    mock_redis_delete = mocker.patch('app.extensions.RedisClient.delete')
+    mock_request = mocker.patch('notifications_python_client.base.BaseAPIClient.request')
+
+    organisations_client.update_organisation(fake_uuid, foo='bar')
+
+    assert call('domains') in mock_redis_delete.call_args_list
+    assert len(mock_request.call_args_list) == 1