Add overwrite strategy options to config

  - Relates to docker#2260

compose/config/config.py

@@ -97,6 +97,7 @@
     'log_opt',
     'logging',
     'network_mode',
+    'override_strategy'
 ]
 
 DOCKER_VALID_URL_PREFIXES = (
@@ -323,7 +324,6 @@ def load(config_details):
     if main_file.version != V1:
         for service_dict in service_dicts:
             match_named_volumes(service_dict, volumes)
-
     return Config(main_file.version, service_dicts, volumes, networks)
 
 
@@ -753,7 +753,13 @@ def merge_service_dicts(base, override, version):
         'ports', 'cap_add', 'cap_drop', 'expose', 'external_links',
         'security_opt', 'volumes_from', 'depends_on',
     ]:
-        md.merge_field(field, merge_unique_items_lists, default=[])
+        if ('override_strategy' in base and
+                base['override_strategy'] == 'overwrite'):
+            md.merge_field(field, merge_overwrite_items_lists, default=[])
+        elif ('overwrite' in base and field in base['overwrite']):
+            md.merge_field(field, merge_overwrite_items_lists, default=[])
+        else:
+            md.merge_field(field, merge_unique_items_lists, default=[])
 
     for field in ['dns', 'dns_search', 'env_file', 'tmpfs']:
         md.merge_field(field, merge_list_or_string)
@@ -773,6 +779,10 @@ def merge_unique_items_lists(base, override):
     return sorted(set().union(base, override))
 
 
+def merge_overwrite_items_lists(base, override):
+    return sorted(set(override))
+
+
 def merge_build(output, base, override):
     def to_dict(service):
         build_config = service.get('build', {})

compose/config/config_schema_v2.0.json

@@ -211,7 +211,16 @@
         "volumes": {"type": "array", "items": {"type": "string"}, "uniqueItems": true},
         "volume_driver": {"type": "string"},
         "volumes_from": {"type": "array", "items": {"type": "string"}, "uniqueItems": true},
-        "working_dir": {"type": "string"}
+        "working_dir": {"type": "string"},
+        "overwrite": {
+          "type": "array",
+          "items": {
+            "type": ["string"],
+            "format": "overwrites"
+          },
+          "uniqueItems": true
+        },
+        "override_strategy": {"type": "string"}
       },
 
       "dependencies": {

compose/config/validation.py

@@ -42,6 +42,8 @@
 
 VALID_NAME_CHARS = '[a-zA-Z0-9\._\-]'
 VALID_EXPOSE_FORMAT = r'^\d+(\-\d+)?(\/[a-zA-Z]+)?$'
+VALID_OVERWRITE_FORMAT = r'^(ports)$'
+VALID_OVERRIDE_STRATEGY_FORMAT = r'^(default|overwrite)$'
 
 
 @FormatChecker.cls_checks(format="ports", raises=ValidationError)
@@ -63,6 +65,26 @@ def format_expose(instance):
     return True
 
 
+@FormatChecker.cls_checks(format="overwrite", raises=ValidationError)
+def format_overwrite(instance):
+    if isinstance(instance, six.string_types):
+            if not re.match(VALID_OVERWRITE_FORMAT, instance):
+                raise ValidationError(
+                    "should be of the format 'ports'")
+
+    return True
+
+
+@FormatChecker.cls_checks(format="override_strategy", raises=ValidationError)
+def format_override_strategy(instance):
+    if isinstance(instance, six.string_types):
+        if not re.match(VALID_OVERRIDE_STRATEGY_FORMAT, instance):
+            raise ValidationError(
+                "should be either 'default' or 'overwrite'")
+
+    return True
+
+
 def match_named_volumes(service_dict, project_volumes):
     service_volumes = service_dict.get('volumes', [])
     for volume_spec in service_volumes:
@@ -361,7 +383,7 @@ def process_config_schema_errors(error):
 
 def validate_against_config_schema(config_file):
     schema = load_jsonschema(config_file.version)
-    format_checker = FormatChecker(["ports", "expose"])
+    format_checker = FormatChecker(["overwrite", "override_strategy", "ports", "expose"])
     validator = Draft4Validator(
         schema,
         resolver=RefResolver(get_resolver_path(), schema),