Add extra_constraints, which doesn't override default constraints.

docs/source/yelpsoa_configs.rst

@@ -59,14 +59,24 @@ instance MAY have:
     drain_method. Valid parameters are any of the kwargs defined for the
     specified bounce_method in `drain_lib <drain_lib.html>`_.
 
-  * ``constraints``: Specifies placement constraints for services. Should be
-    defined as an array within an array (E.g ``[["habitat", "GROUP_BY"]]``).
-    Defaults to ``[["<discover_location_type>, "GROUP_BY"]]`` where
-    ``<discover_location_type>`` is defined by the ``discover`` attribute in
-    ``smartstack.yaml``. For more details and other constraint types, see the
+  * ``constraints``: Overrides the default placement constraints for services.
+    Should be defined as an array of arrays (E.g ``[["habitat", "GROUP_BY"]]``
+    or ``[["habitat", "GROUP_BY"], ["hostname", "UNIQUE"]]``). Defaults to
+    ``[["<discover_location_type>, "GROUP_BY"], ["pool", "LIKE", <pool>],
+    [<deploy_blacklist_type>, "UNLIKE", <deploy_blacklist_value>], ...]``
+    where ``<discover_location_type>`` is defined by the ``discover`` attribute
+    in ``smartstack.yaml``, ``<pool>`` is defined by the ``pool`` attribute in
+    ``marathon.yaml``, and ``deploy_blacklist_type`` and
+    ``deploy_blacklist_value`` are defined in the ``deploy_blacklist`` attribute
+    in marathon.yaml. For more details and other constraint types, see the
     official `Marathon constraint documentation
     <https://mesosphere.github.io/marathon/docs/constraints.html>`_.
 
+  * ``extra_constraints``: Adds to the default placement constraints for
+    services. This acts the same as ``constraints``, but adds to the default
+    constraints instead of replacing them. See ``constraints`` for details on
+    format and the default constraints.
+
   * ``cmd``: The command that is executed. Can be used as an alternative to
     args for containers without an `entrypoint
     <https://docs.docker.com/reference/builder/#entrypoint>`_. This value is

paasta_tools/marathon_tools.py

@@ -256,15 +256,26 @@ def get_constraints(self, service_namespace_config):
         if 'constraints' in self.config_dict:
             return self.config_dict.get('constraints')
         else:
-            discover_level = service_namespace_config.get_discover()
-            locations = get_mesos_slaves_grouped_by_attribute(
-                attribute=discover_level, blacklist=self.get_deploy_blacklist())
-            pool = self.get_pool()
-
-            deploy_constraints = deploy_blacklist_to_constraints(self.get_deploy_blacklist())
-            routing_constraints = [[discover_level, "GROUP_BY", str(len(locations))]]
-            pool_constraints = [["pool", "LIKE", pool]]
-            return routing_constraints + deploy_constraints + pool_constraints
+            constraints = self.config_dict.get('extra_constraints', [])
+            constraints.extend(self.get_routing_constraints(service_namespace_config))
+            constraints.extend(self.get_deploy_constraints())
+            constraints.extend(self.get_pool_constraints())
+            return constraints
+
+    def get_routing_constraints(self, service_namespace_config):
+        discover_level = service_namespace_config.get_discover()
+        locations = get_mesos_slaves_grouped_by_attribute(
+            attribute=discover_level, blacklist=self.get_deploy_blacklist())
+
+        routing_constraints = [[discover_level, "GROUP_BY", str(len(locations))]]
+        return routing_constraints
+
+    def get_deploy_constraints(self):
+        return deploy_blacklist_to_constraints(self.get_deploy_blacklist())
+
+    def get_pool_constraints(self):
+        pool = self.get_pool()
+        return [["pool", "LIKE", pool]]
 
     def format_marathon_app_dict(self, app_id, docker_url, docker_volumes, service_namespace_config):
         """Create the configuration that will be passed to the Marathon REST API.

tests/test_marathon_tools.py

@@ -1030,7 +1030,7 @@ def test_get_constraints_in_config(self):
             service='fake_name',
             cluster='fake_cluster',
             instance='fake_instance',
-            config_dict={'constraints': 'so_many_walls'},
+            config_dict={'constraints': 'so_many_walls', 'extra_constraints': [['ignore', 'this']]},
             branch_dict={},
         )
         with mock.patch(
@@ -1060,6 +1060,27 @@ def test_get_constraints_default(self):
             ]
             assert fake_conf.get_constraints(fake_service_namespace_config) == expected_constraints
 
+    def test_get_constraints_extra_constraints(self):
+        fake_service_namespace_config = marathon_tools.ServiceNamespaceConfig()
+        fake_conf = marathon_tools.MarathonServiceConfig(
+            service='fake_name',
+            cluster='fake_cluster',
+            instance='fake_instance',
+            config_dict={'extra_constraints': [['extra', 'constraint']]},
+            branch_dict={},
+        )
+        with mock.patch(
+            'paasta_tools.marathon_tools.get_mesos_slaves_grouped_by_attribute',
+            autospec=True,
+        ) as get_slaves_patch:
+            get_slaves_patch.return_value = {'fake_region': {}}
+            expected_constraints = [
+                ['extra', 'constraint'],
+                ["region", "GROUP_BY", "1"],
+                ["pool", "LIKE", "default"],
+            ]
+            assert fake_conf.get_constraints(fake_service_namespace_config) == expected_constraints
+
     def test_get_constraints_from_discover(self):
         fake_service_namespace_config = marathon_tools.ServiceNamespaceConfig({
             'mode': 'http',