Add support for extension fields in v2.x and v3.4

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

compose/config/config_schema_v2.0.json

@@ -41,6 +41,7 @@
     }
   },
 
+  "patternProperties": {"^x-": {}},
   "additionalProperties": false,
 
   "definitions": {

compose/config/config_schema_v2.1.json

@@ -41,6 +41,7 @@
     }
   },
 
+  "patternProperties": {"^x-": {}},
   "additionalProperties": false,
 
   "definitions": {

compose/config/config_schema_v2.2.json

@@ -41,6 +41,7 @@
     }
   },
 
+  "patternProperties": {"^x-": {}},
   "additionalProperties": false,
 
   "definitions": {

compose/config/config_schema_v2.3.json

@@ -41,6 +41,7 @@
     }
   },
 
+  "patternProperties": {"^x-": {}},
   "additionalProperties": false,
 
   "definitions": {

compose/config/config_schema_v3.4-beta.json

@@ -64,6 +64,7 @@
     }
   },
 
+  "patternProperties": {"^x-": {}},
   "additionalProperties": false,
 
   "definitions": {

compose/config/validation.py

@@ -239,6 +239,16 @@ def handle_error_for_schema_with_id(error, path):
             invalid_config_key = parse_key_from_error_msg(error)
             return get_unsupported_config_msg(path, invalid_config_key)
 
+        if schema_id.startswith('config_schema_v'):
+            invalid_config_key = parse_key_from_error_msg(error)
+            return ('Invalid top-level property "{key}". Valid top-level '
+                    'sections for this Compose file are: {properties}, and '
+                    'extensions starting with "x-".\n\n{explanation}').format(
+                key=invalid_config_key,
+                properties=', '.join(error.schema['properties'].keys()),
+                explanation=VERSION_EXPLANATION
+            )
+
         if not error.path:
             return '{}\n\n{}'.format(error.message, VERSION_EXPLANATION)
 

tests/unit/config/config_test.py

@@ -251,7 +251,7 @@ class ConfigTest(unittest.TestCase):
                 )
             )
 
-        assert 'Additional properties are not allowed' in excinfo.exconly()
+        assert 'Invalid top-level property "web"' in excinfo.exconly()
         assert VERSION_EXPLANATION in excinfo.exconly()
 
     def test_named_volume_config_empty(self):
@@ -773,6 +773,18 @@ class ConfigTest(unittest.TestCase):
         assert services[1]['name'] == 'db'
         assert services[2]['name'] == 'web'
 
+    def test_load_with_extensions(self):
+        config_details = build_config_details({
+            'version': '2.3',
+            'x-data': {
+                'lambda': 3,
+                'excess': [True, {}]
+            }
+        })
+
+        config_data = config.load(config_details)
+        assert config_data.services == []
+
     def test_config_build_configuration(self):
         service = config.load(
             build_config_details(