Introduce overwrite functionality in ``osc subnet set``

The overwrite functionality allows user to overwrite
the dns-nameservers of a specific subnet.

Change-Id: I421808a3bdeb4565668f627b7929c4762cf40212
partially-implements: blueprint allow-overwrite-set-options
partially-implements: blueprint network-commands-options

4 files changed, 34 insertions, 2 deletions

diff --git a/doc/source/command-objects/subnet.rst b/doc/source/command-objects/subnet.rst
index e2059875..a88b8103 100644
--- a/doc/source/command-objects/subnet.rst
+++ b/doc/source/command-objects/subnet.rst
@@ -231,6 +231,7 @@ Set subnet properties
         [--no-allocation-pool]
         [--dhcp | --no-dhcp]
         [--dns-nameserver <dns-nameserver>]
+        [--no-dns-nameserver]
         [--gateway <gateway-ip>]
         [--host-route destination=<subnet>,gateway=<ip-address>]
         [--no-host-route]
@@ -263,6 +264,12 @@ Set subnet properties
 
      DNS server for this subnet (repeat option to set multiple DNS servers)
 
+.. option:: --no-dns-nameservers
+
+     Clear existing information of DNS servers.
+     Specify both --dns-nameserver and --no-dns-nameservers
+     to overwrite the current DNS server information.
+
 .. option:: --gateway <gateway>
 
      Specify a gateway for the subnet. The options are:
diff --git a/openstackclient/network/v2/subnet.py b/openstackclient/network/v2/subnet.py
index f1ecb5a7..dc504b32 100644
--- a/openstackclient/network/v2/subnet.py
+++ b/openstackclient/network/v2/subnet.py
@@ -84,6 +84,15 @@ def _get_common_parse_arguments(parser, is_create=True):
         help=_("DNS server for this subnet "
                "(repeat option to set multiple DNS servers)")
     )
+
+    if not is_create:
+        parser.add_argument(
+            '--no-dns-nameservers',
+            action='store_true',
+            help=_("Clear existing information of DNS Nameservers. "
+                   "Specify both --dns-nameserver and --no-dns-nameserver "
+                   "to overwrite the current DNS Nameserver information.")
+        )
     parser.add_argument(
         '--host-route',
         metavar='destination=<subnet>,gateway=<ip-address>',
@@ -532,7 +541,10 @@ class SetSubnet(command.Command):
         attrs = _get_attrs(self.app.client_manager, parsed_args,
                            is_create=False)
         if 'dns_nameservers' in attrs:
-            attrs['dns_nameservers'] += obj.dns_nameservers
+            if not parsed_args.no_dns_nameservers:
+                attrs['dns_nameservers'] += obj.dns_nameservers
+        elif parsed_args.no_dns_nameservers:
+            attrs['dns_nameservers'] = []
         if 'host_routes' in attrs:
             if not parsed_args.no_host_route:
                 attrs['host_routes'] += obj.host_routes
diff --git a/openstackclient/tests/unit/network/v2/test_subnet.py b/openstackclient/tests/unit/network/v2/test_subnet.py
index 2d51aa4a..e7406575 100644
--- a/openstackclient/tests/unit/network/v2/test_subnet.py
+++ b/openstackclient/tests/unit/network/v2/test_subnet.py
@@ -925,13 +925,16 @@ class TestSetSubnet(TestSubnet):
             {'host_routes': [{'destination': '10.20.20.0/24',
                               'nexthop': '10.20.20.1'}],
              'allocation_pools': [{'start': '8.8.8.200',
-                                   'end': '8.8.8.250'}], })
+                                   'end': '8.8.8.250'}],
+             'dns_nameservers': ["10.0.0.1"], })
         self.network.find_subnet = mock.Mock(return_value=_testsubnet)
         arglist = [
             '--host-route', 'destination=10.30.30.30/24,gateway=10.30.30.1',
             '--no-host-route',
             '--allocation-pool', 'start=8.8.8.100,end=8.8.8.150',
             '--no-allocation-pool',
+            '--dns-nameserver', '10.1.10.1',
+            '--no-dns-nameservers',
             _testsubnet.name,
         ]
         verifylist = [
@@ -939,6 +942,8 @@ class TestSetSubnet(TestSubnet):
                 "destination": "10.30.30.30/24", "gateway": "10.30.30.1"}]),
             ('allocation_pools', [{
                 'start': '8.8.8.100', 'end': '8.8.8.150'}]),
+            ('dns_nameservers', ['10.1.10.1']),
+            ('no_dns_nameservers', True),
             ('no_host_route', True),
             ('no_allocation_pool', True),
         ]
@@ -948,6 +953,7 @@ class TestSetSubnet(TestSubnet):
             'host_routes': [{
                 "destination": "10.30.30.30/24", "nexthop": "10.30.30.1"}],
             'allocation_pools': [{'start': '8.8.8.100', 'end': '8.8.8.150'}],
+            'dns_nameservers': ["10.1.10.1"],
         }
         self.network.update_subnet.assert_called_once_with(
             _testsubnet, **attrs)
diff --git a/releasenotes/notes/add-dns-nameserver-overwrite-option-b866baeae12f9460.yaml b/releasenotes/notes/add-dns-nameserver-overwrite-option-b866baeae12f9460.yaml
new file mode 100644
index 00000000..04f0638d
--- /dev/null
+++ b/releasenotes/notes/add-dns-nameserver-overwrite-option-b866baeae12f9460.yaml
@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    ``subnet set`` command now allows the user to overwrite/clear dns-nameserver information
+    of a subnet by using the option ``no-dns-nameserver``.
+    [ Blueprint  `allow-overwrite-set-options <https://blueprints.launchpad.net/python-openstackclient/+spec/allow-overwrite-set-options>` _]
+