Ignore traps with invalid/missing ifIndex

It's not entirely clear what Zino 1 does here, but it's also not clear
that we should attempt to do anything "useful" with crazy traps.  This
adds early ignores and tests for them.

src/zino/trapobservers/link_traps.py

@@ -29,8 +29,14 @@ def handle_trap(self, trap: TrapMessage) -> Optional[bool]:
         if "ifIndex" in trap.variables:
             ifindex = trap.variables.get("ifIndex").value
         else:
-            ifindex = -1
+            _logger.warning("%s: %s trap contained no ifIndex value, ignoring", trap.agent.device.name, trap.name)
+            return False
         port = trap.agent.device.ports.get(ifindex) if ifindex > 0 else None
+        if not port:
+            _logger.warning(
+                "%s: %s trap referenced unknown port (ix %s), ignoring", trap.agent.device.name, trap.name, ifindex
+            )
+            return False
 
         # TODO: The trap *might* contain an ifDescr value.  If present, Zino uses that for trap processing.
         #  Otherwise, it fetches ifDescr from its own state and uses that for trap processing.  Either way,

tests/trapobservers/link_traps_test.py

@@ -1,5 +1,5 @@
 from datetime import timedelta
-from unittest.mock import Mock
+from unittest.mock import Mock, patch
 
 import pytest
 
@@ -101,6 +101,21 @@ def test_when_link_trap_is_redundant_policy_should_ignore_trap(self, state_with_
             localhost, localhost.ports[1], is_up=False
         ), "did not ignore redundant linkDown trap"
 
+    def test_when_link_trap_is_missing_ifindex_value_it_should_ignore_trap_early(self, state_with_localhost_with_port):
+        observer = LinkTrapObserver(state=state_with_localhost_with_port, polldevs=Mock())
+        trap = Mock(variables={})
+        with patch.object(observer, "handle_link_transition") as handle_link_transition:
+            assert not observer.handle_trap(trap)
+            assert not handle_link_transition.called, "handle_link_transition was called"
+
+    def test_when_link_trap_refers_to_unknown_port_it_should_ignore_trap_early(self, state_with_localhost_with_port):
+        observer = LinkTrapObserver(state=state_with_localhost_with_port, polldevs=Mock())
+        localhost = state_with_localhost_with_port.devices.devices["localhost"]
+        trap = Mock(agent=Mock(device=localhost), variables={"ifIndex": Mock(value=99)})
+        with patch.object(observer, "handle_link_transition") as handle_link_transition:
+            assert not observer.handle_trap(trap)
+            assert not handle_link_transition.called, "handle_link_transition was called"
+
 
 @pytest.fixture
 def state_with_localhost_with_port(state_with_localhost):