Skip ARP collection if ARP already collected

This ensures that the SNMP-based `arp` plugin exits early if it appears
that some other plugin has already collected Arp records before it.

This makes the `arp` plugin a sort of fallback in a scenario where
vendor specific ARP collection plugins run first, rather than having
multiple ARP plugin step on each others toes.

python/nav/ipdevpoll/plugins/arp.py

@@ -70,6 +70,9 @@ def can_handle(cls, netbox):
     @defer.inlineCallbacks
     def handle(self):
         self._check_and_update_prefix_cache()
+        if self._is_arp_already_collected():
+            self._logger.debug("ARP records already collected for this device")
+            return
         self._logger.debug("Collecting IP/MAC mappings")
 
         # Fetch standard MIBs
@@ -101,6 +104,10 @@ def _check_and_update_prefix_cache(self):
         if prefix_cache_age > self.prefix_cache_max_age:
             yield self._update_prefix_cache()
 
+    def _is_arp_already_collected(self):
+        """Returns True if ARP entries have already been collected in this run"""
+        return shadows.Arp in self.containers and bool(self.containers[shadows.Arp])
+
     @defer.inlineCallbacks
     def _get_ip_mib(self):
         if not self.is_arista():

tests/unittests/ipdevpoll/plugins_arp_test.py

@@ -1,3 +1,4 @@
+from nav.ipdevpoll import shadows
 from nav.ipdevpoll.storage import ContainerRepository
 from nav.ipdevpoll.plugins.arp import ipv6_address_in_mappings, Arp
 
@@ -11,3 +12,15 @@ def test_make_new_mappings_should_not_raise_on_empty_ip():
     a = Arp(None, None, ContainerRepository())
     mappings = [(None, '00:0b:ad:c0:ff:ee')]
     a._make_new_mappings(mappings)
+
+
+def test_when_arp_records_exist_is_arp_already_collected_should_return_true():
+    containers = ContainerRepository()
+    containers.factory(('192.168.0.1', '00:co:ff:ee:ba:be'), shadows.Arp)
+    plugin = Arp(None, None, containers)
+    assert plugin._is_arp_already_collected()
+
+
+def test_when_arp_records_do_not_exist_is_arp_already_collected_should_return_false():
+    plugin = Arp(None, None, ContainerRepository())
+    assert not plugin._is_arp_already_collected()