Add extension "router-enable-snat"

This new extension exposes the "enable_snat" flag of a router. The
"enable_snat" flag can be updated using the external gateway API
commands. However this is a router global configuration property
that is stored directly on the router register. As described in the
``update-external-gateways-of-router`` API section [1]:

  "The enable_snat field does not have any effect for extra gateways
  except for the first external gateway in the list."

[1]https://docs.openstack.org/api-ref/network/v2/index.html#update-external-gateways-of-router

Related-Bug: #2124252
Signed-off-by: Rodolfo Alonso Hernandez <ralonsoh@redhat.com>
Change-Id: I0ab856f56ff4e03cfff30075de938ae7178ebbff

api-ref/source/v2/routers.inc

@@ -161,6 +161,13 @@ enable or disable automatic configuration of BFD for default routes of a router
 created based on the default gateways of subnets accessible from a router's
 gateway ports (see ``enable-default-route-ecmp`` extension).
 
+Router exposes the "enable_snat" flag extension (``router-enable-snat``)
+========================================================================
+
+The ``router-enable-snat`` extension exposes, via the API, the router property
+flag "enable_snat". This flag can be set during the router creation but can
+only be modified using the external gateway API commands.
+
 List routers
 ============
 
@@ -211,6 +218,7 @@ Response Parameters
    - description: description
    - admin_state_up: admin_state_up
    - status: router-status
+   - enable_snat: router-enable_snat
    - external_gateway_info: router-external_gateway_info
    - external_gateways: router-external_gateways
    - revision_number: revision_number
@@ -298,6 +306,7 @@ Response Parameters
    - description: description
    - admin_state_up: admin_state_up
    - status: router-status
+   - enable_snat: router-enable_snat
    - external_gateway_info: router-external_gateway_info
    - external_gateways: router-external_gateways
    - revision_number: revision_number
@@ -358,6 +367,7 @@ Response Parameters
    - description: description
    - admin_state_up: admin_state_up
    - status: router-status
+   - enable_snat: router-enable_snat
    - external_gateway_info: router-external_gateway_info
    - external_gateways: router-external_gateways
    - revision_number: revision_number
@@ -436,6 +446,7 @@ Response Parameters
    - description: description
    - admin_state_up: admin_state_up
    - status: router-status
+   - enable_snat: router-enable_snat
    - external_gateway_info: router-external_gateway_info
    - external_gateways: router-external_gateways
    - revision_number: revision_number

neutron_lib/api/definitions/__init__.py

@@ -131,6 +131,7 @@ from neutron_lib.api.definitions import rbac_subnetpool
 from neutron_lib.api.definitions import revisionifmatch
 from neutron_lib.api.definitions import router_admin_state_down_before_update
 from neutron_lib.api.definitions import router_availability_zone
+from neutron_lib.api.definitions import router_enable_snat
 from neutron_lib.api.definitions import router_interface_fip
 from neutron_lib.api.definitions import routerservicetype
 from neutron_lib.api.definitions import security_groups_normalized_cidr
@@ -295,6 +296,7 @@ _ALL_API_DEFINITIONS = {
     revisionifmatch,
     router_admin_state_down_before_update,
     router_availability_zone,
+    router_enable_snat,
     router_interface_fip,
     routerservicetype,
     security_groups_normalized_cidr,

neutron_lib/api/definitions/router_enable_snat.py

@@ -0,0 +1,47 @@
+# Copyright (c) 2025 Red Hat, Inc.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from neutron_lib.api import converters
+from neutron_lib.api.definitions import l3
+
+
+ENABLE_SNAT = 'enable_snat'
+
+ALIAS = 'router-enable-snat'
+IS_SHIM_EXTENSION = False
+IS_STANDARD_ATTR_EXTENSION = False
+NAME = 'Exposes the router "enable_snat" read-only configuration flag'
+API_PREFIX = ''
+DESCRIPTION = 'Exposes the router "enable_snat" read-only configuration flag'
+UPDATED_TIMESTAMP = '2025-09-18T18:00:00-00:00'
+RESOURCE_NAME = l3.ROUTER
+COLLECTION_NAME = l3.ROUTERS
+RESOURCE_ATTRIBUTE_MAP = {
+    COLLECTION_NAME: {
+        ENABLE_SNAT: {'allow_post': False,
+                      'allow_put': False,
+                      'default': True,
+                      'convert_to': converters.convert_to_boolean,
+                      'required_by_policy': True,
+                      'is_filter': True,
+                      'is_sort_key': True,
+                      'is_visible': True
+                      },
+    },
+}
+SUB_RESOURCE_ATTRIBUTE_MAP = {}
+ACTION_MAP = {}
+REQUIRED_EXTENSIONS = []
+OPTIONAL_EXTENSIONS = []
+ACTION_STATUS = {}

neutron_lib/tests/unit/api/definitions/test_router_enable_snat.py

@@ -0,0 +1,22 @@
+# Copyright 2025 Red Hat, Inc.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from neutron_lib.api.definitions import router_enable_snat
+from neutron_lib.tests.unit.api.definitions import base
+
+
+class RouterEnableSnatTestCase(base.DefinitionBaseTestCase):
+    extension_module = router_enable_snat
+    extension_resources = (router_enable_snat.COLLECTION_NAME,)
+    extension_attributes = (router_enable_snat.ENABLE_SNAT,)