Detect the service that causes the invalid service name error

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

compose/config/validation.py

@@ -211,9 +211,12 @@ def handle_error_for_schema_with_id(error, path):
 
     if is_service_dict_schema(schema_id) and error.validator == 'additionalProperties':
         return "Invalid service name '{}' - only {} characters are allowed".format(
-            # The service_name is the key to the json object
-            list(error.instance)[0],
-            VALID_NAME_CHARS)
+            # The service_name is one of the keys in the json object
+            [i for i in list(error.instance) if not i or any(filter(
+                lambda c: not re.match(VALID_NAME_CHARS, c), i
+            ))][0],
+            VALID_NAME_CHARS
+        )
 
     if error.validator == 'additionalProperties':
         if schema_id == '#/definitions/service':

tests/unit/config/config_test.py

@@ -554,6 +554,20 @@ class ConfigTest(unittest.TestCase):
             excinfo.exconly()
         )
 
+    def test_config_invalid_service_name_raise_validation_error(self):
+        with pytest.raises(ConfigurationError) as excinfo:
+            config.load(
+                build_config_details({
+                    'version': '2',
+                    'services': {
+                        'test_app': {'build': '.'},
+                        'mong\\o': {'image': 'mongo'},
+                    }
+                })
+            )
+
+            assert 'Invalid service name \'mong\\o\'' in excinfo.exconly()
+
     def test_load_with_multiple_files_v1(self):
         base_file = config.ConfigFile(
             'base.yaml',