Revert "Remove _process_existing_zone as it's not currently needed"

This reverts commit f1fd63205e.

octodns/provider/base.py

@@ -105,6 +105,37 @@ class BaseProvider(BaseSource):
 
         return desired
 
+    def _process_existing_zone(self, existing):
+        '''
+        An opportunity for providers to modify the existing zone records before
+        planning. `existing` is a "shallow" copy, see `Zone.copy` for more
+        information
+
+        - Must call `super` at an appropriate point for their work, generally
+          that means as the final step of the method, returning the result of
+          the `super` call.
+        - May modify `existing` directly.
+        - Must not modify records directly, `record.copy` should be called,
+          the results of which can be modified, and then `Zone.add_record` may
+          be used with `replace=True`.
+        - May call `Zone.remove_record` to remove records from `existing`.
+        - Must call supports_warn_or_except with information about any changes
+          that are made to have them logged or throw errors depending on the
+          provider configuration.
+        '''
+
+        for record in existing.records:
+            if record._type == 'NS' and record.name == '' and \
+                    not self.SUPPORTS_ROOT_NS:
+                # ignore, we can't manage root NS records
+                msg = \
+                    f'root NS record not supported for {record.fqdn}'
+                fallback = 'ignoring it'
+                self.supports_warn_or_except(msg, fallback)
+                existing.remove_record(record)
+
+        return existing
+
     def _include_change(self, change):
         '''
         An opportunity for providers to filter out false positives due to
@@ -143,6 +174,8 @@ class BaseProvider(BaseSource):
             self.log.warning('Provider %s used in target mode did not return '
                              'exists', self.id)
 
+        existing = self._process_existing_zone(existing)
+
         for processor in processors:
             existing = processor.process_target_zone(existing, target=self)
 

tests/test_octodns_provider_base.py

@@ -362,6 +362,27 @@ class TestBaseProvider(TestCase):
         self.assertEqual(1, len(zone2.records))
         self.assertEqual(record1, list(zone2.records)[0])
 
+    def test_process_existing_zone(self):
+        provider = HelperProvider('test')
+
+        # SUPPORTS_ROOT_NS
+        provider.SUPPORTS_ROOT_NS = False
+        zone1 = Zone('unit.tests.', [])
+        record1 = Record.new(zone1, '', {
+            'type': 'NS',
+            'ttl': 3600,
+            'values': ['foo.com.', 'bar.com.'],
+        })
+        zone1.add_record(record1)
+
+        zone2 = provider._process_existing_zone(zone1.copy())
+        self.assertEqual(0, len(zone2.records))
+
+        provider.SUPPORTS_ROOT_NS = True
+        zone2 = provider._process_existing_zone(zone1.copy())
+        self.assertEqual(1, len(zone2.records))
+        self.assertEqual(record1, list(zone2.records)[0])
+
     def test_safe_none(self):
         # No changes is safe
         Plan(None, None, [], True).raise_if_unsafe()