Enable variable substitution in config.secrets

Signed-off-by: Joffrey F <joffrey@docker.com>

compose/config/config.py

@@ -218,6 +218,8 @@ class Config(namedtuple('_Config', 'version services volumes networks secrets'))
     :type  volumes: :class:`dict`
     :param networks: Dictionary mapping network names to description dictionaries
     :type  networks: :class:`dict`
+    :param secrets: Dictionary mapping secret names to description dictionaries
+    :type secrets: :class:`dict`
     """
 
 
@@ -491,6 +493,13 @@ def process_config_file(config_file, environment, service_name=None):
             config_file.get_networks(),
             'network',
             environment)
+        if config_file.version in (V3_1,):
+            processed_config['secrets'] = interpolate_config_section(
+                config_file,
+                config_file.get_secrets(),
+                'secrets',
+                environment
+            )
     elif config_file.version == V1:
         processed_config = services
     else:

tests/unit/config/config_test.py

@@ -1821,6 +1821,23 @@ class ConfigTest(unittest.TestCase):
             }
         }
 
+    def test_empty_environment_key_allowed(self):
+        service_dict = config.load(
+            build_config_details(
+                {
+                    'web': {
+                        'build': '.',
+                        'environment': {
+                            'POSTGRES_PASSWORD': ''
+                        },
+                    },
+                },
+                '.',
+                None,
+            )
+        ).services[0]
+        self.assertEqual(service_dict['environment']['POSTGRES_PASSWORD'], '')
+
     def test_merge_pid(self):
         # Regression: https://github.com/docker/compose/issues/4184
         base = {
@@ -2335,22 +2352,23 @@ class InterpolationTest(unittest.TestCase):
         self.assertIn('in service "web"', cm.exception.msg)
         self.assertIn('"${"', cm.exception.msg)
 
-    def test_empty_environment_key_allowed(self):
-        service_dict = config.load(
-            build_config_details(
-                {
-                    'web': {
-                        'build': '.',
-                        'environment': {
-                            'POSTGRES_PASSWORD': ''
-                        },
-                    },
-                },
-                '.',
-                None,
-            )
-        ).services[0]
-        self.assertEqual(service_dict['environment']['POSTGRES_PASSWORD'], '')
+    @mock.patch.dict(os.environ)
+    def test_interpolation_secrets_section(self):
+        os.environ['FOO'] = 'baz.bar'
+        config_dict = config.load(build_config_details({
+            'version': '3.1',
+            'secrets': {
+                'secretdata': {
+                    'external': {'name': '$FOO'}
+                }
+            }
+        }))
+        assert config_dict.secrets == {
+            'secretdata': {
+                'external': {'name': 'baz.bar'},
+                'external_name': 'baz.bar'
+            }
+        }
 
 
 class VolumeConfigTest(unittest.TestCase):
@@ -3663,4 +3681,4 @@ class SerializeTest(unittest.TestCase):
         serialized_config = yaml.load(serialize_config(config_dict))
         serialized_service = serialized_config['services']['web']
         assert secret_sort(serialized_service['secrets']) == secret_sort(service_dict['secrets'])
-        assert serialized_config['secrets'] == secrets_dict
+        assert 'secrets' in serialized_config

tests/unit/config/interpolation_test.py

@@ -75,7 +75,32 @@ def test_interpolate_environment_variables_in_volumes(mock_env):
         },
         'other': {},
     }
-    value = interpolate_environment_variables("2.0",  volumes, 'volume', mock_env)
+    value = interpolate_environment_variables("2.0", volumes, 'volume', mock_env)
+    assert value == expected
+
+
+def test_interpolate_environment_variables_in_secrets(mock_env):
+    secrets = {
+        'secretservice': {
+            'file': '$FOO',
+            'labels': {
+                'max': 2,
+                'user': '${USER}'
+            }
+        },
+        'other': None,
+    }
+    expected = {
+        'secretservice': {
+            'file': 'bar',
+            'labels': {
+                'max': 2,
+                'user': 'jenny'
+            }
+        },
+        'other': {},
+    }
+    value = interpolate_environment_variables("3.1", secrets, 'volume', mock_env)
     assert value == expected