pdns4 backend: check if zone exists before attempting delete

This should prevent us from obsessing over zones that are in the
designate DB but not in the pdns backend due to races/incomplete
transactions/etc.

Bug: 1880230
Change-Id: I00ae6227edf21007c39a7d55221898ae5c10dda2
(cherry picked from commit 5acba9443e2fc1f6e78f9a32cd85eb168d2a7cb2)

designate/backend/impl_pdns4.py

@@ -106,10 +106,17 @@ class PDNS4Backend(base.Backend):
     def delete_zone(self, context, zone):
         """Delete a DNS zone"""
 
-        try:
-            requests.delete(
-                self._build_url(zone.name),
-                headers=self.headers
-            ).raise_for_status()
-        except requests.HTTPError as e:
-            raise exceptions.Backend(e)
+        # First verify that the zone exists -- If it's not present
+        #  in the backend then we can just declare victory.
+        if self._check_zone_exists(zone):
+            try:
+                requests.delete(
+                    self._build_url(zone.name),
+                    headers=self.headers
+                ).raise_for_status()
+            except requests.HTTPError as e:
+                raise exceptions.Backend(e)
+        else:
+            LOG.warning("Trying to delete zone %s but that zone is not "
+                        "present in the pdns backend. Assuming success.",
+                        zone)

designate/tests/unit/backend/test_pdns4.py

@@ -285,6 +285,28 @@ class PDNS4BackendTestCase(designate.tests.TestCase):
         req_mock.delete(
             '%s/localhost/zones/example.com.' % self.base_address,
         )
+        req_mock.get(
+            '%s/localhost/zones/%s' % (self.base_address, self.zone.name),
+            status_code=200,
+        )
+
+        self.backend.delete_zone(self.context, self.zone)
+
+        self.assertEqual(
+            req_mock.last_request.headers.get('X-API-Key'), 'api_key'
+        )
+
+    @requests_mock.mock()
+    def test_delete_zone_missing(self, req_mock):
+        req_mock.delete(
+            '%s/localhost/zones/example.com.' % self.base_address,
+        )
+
+        # pdns returns 422 if asked about a zone that doesn't exist.
+        req_mock.get(
+            '%s/localhost/zones/%s' % (self.base_address, self.zone.name),
+            status_code=422,
+        )
 
         self.backend.delete_zone(self.context, self.zone)
 
@@ -298,6 +320,10 @@ class PDNS4BackendTestCase(designate.tests.TestCase):
             '%s/localhost/zones/example.com.' % self.base_address,
             status_code=500,
         )
+        req_mock.get(
+            '%s/localhost/zones/%s' % (self.base_address, self.zone.name),
+            status_code=200,
+        )
 
         self.assertRaisesRegexp(
             exceptions.Backend,