Smarter Create/Delete in BIND9/Agent

* In BIND9, an rndc addzone fails if the zone already exists,
  similarly, a delzone fails if the zone doesn't exist. This
  patch catches that in the output from the RNDC call, and allows
  the Pool Manager to realize success in the above situation.

* In the Agent, we must take action (write a zone file, for example)
  before we execute the addzone/delzone. So we check with an SOA query
  before the backend is called.

Partial-Bug: 1430976
Change-Id: I0430c3a402ae30d5705bbfef1a4394772cbc12f9

designate/agent/handler.py

@@ -115,9 +115,10 @@ class RequestHandler(object):
         serial = self.backend.find_domain_serial(domain_name)
 
         if serial is not None:
-            LOG.warn(_LW("Refusing CREATE for %(name)s, zone already exists") %
+            LOG.warn(_LW("Not creating %(name)s, zone already exists") %
                  {'name': domain_name})
-            response.set_rcode(dns.rcode.from_text("REFUSED"))
+            # Provide an authoritative answer
+            response.flags |= dns.flags.AA
             return response
 
         LOG.debug("Received %(verb)s for %(name)s from %(host)s" %
@@ -206,6 +207,15 @@ class RequestHandler(object):
             response.set_rcode(dns.rcode.from_text("REFUSED"))
             return response
 
+        serial = self.backend.find_domain_serial(domain_name)
+
+        if serial is None:
+            LOG.warn(_LW("Not deleting %(name)s, zone doesn't exist") %
+                 {'name': domain_name})
+            # Provide an authoritative answer
+            response.flags |= dns.flags.AA
+            return response
+
         LOG.debug("Received DELETE for %(name)s from %(host)s" %
                  {'name': domain_name, 'host': requester})
 

designate/backend/impl_bind9.py

@@ -85,7 +85,13 @@ class Bind9Backend(base.PoolBackend):
             (domain['name'].rstrip('.'), '; '.join(masters), domain['name'],
              domain['id']),
         ]
-        self._execute_rndc(rndc_op)
+
+        try:
+            self._execute_rndc(rndc_op)
+        except exceptions.Backend as e:
+            # If create fails because the domain exists, don't reraise
+            if "already exists" not in str(e.message):
+                raise
 
     def delete_domain(self, context, domain):
         LOG.debug('Delete Domain')
@@ -93,7 +99,13 @@ class Bind9Backend(base.PoolBackend):
             'delzone',
             '%s' % domain['name'].rstrip('.'),
         ]
-        self._execute_rndc(rndc_op)
+
+        try:
+            self._execute_rndc(rndc_op)
+        except exceptions.Backend as e:
+            # If domain is already deleted, don't reraise
+            if "not found" not in str(e.message):
+                raise
 
     def _rndc_base(self):
         rndc_call = [