Fixes #3782: Prevent power loops

docs/release-notes/version-2.6.md

@@ -19,6 +19,7 @@
 ## Bug Fixes
 
 * [#3589](https://github.com/netbox-community/netbox/issues/3589) - Fix validation on tagged VLANs of an interface
+* [#3782](https://github.com/netbox-community/netbox/issues/3782) - Prevent power loops
 * [#3849](https://github.com/netbox-community/netbox/issues/3849) - Fix ordering of models when dumping data to JSON
 * [#3853](https://github.com/netbox-community/netbox/issues/3853) - Fix device role link on config context view
 * [#3856](https://github.com/netbox-community/netbox/issues/3856) - Allow filtering VM interfaces by multiple MAC addresses

netbox/dcim/models.py

@@ -3099,6 +3099,43 @@ class Cable(ChangeLoggedModel):
         if self.termination_a == self.termination_b:
             raise ValidationError("Cannot connect {} to itself".format(self.termination_a_type))
 
+        # A power loop is prohibited. Power feeds cannot create loops as they're downstream-only. Cannot use power
+        # ports as one end may not have a device, like a power feed. This leaves power outlets which are always a part
+        # of a loop since they exist on a device and connect to another.
+        if 'poweroutlet' in [type_a, type_b]:
+            poweroutlet_type = ContentType.objects.get_for_model(PowerOutlet)
+
+            # Using PK as the use of `.values` on the queryset will return IDs
+            destination = self.termination_b.device.pk
+            nodes = None
+            next_nodes = set([self.termination_a.device.pk])
+
+            # Pseudo-implementation of the Bellman-Ford algorithm
+            while nodes != next_nodes:
+                nodes = next_nodes
+
+                # Destination found in the nodes; this cable would create a loop
+                if destination in nodes:
+                    raise ValidationError('Creating this cable will introduce a power loop')
+
+                # All power-outlet cables. Only the devices at each end of the cable are needed
+                poweroutlet_cables = Cable.objects.filter(
+                    Q(termination_a_type=poweroutlet_type) | Q(termination_b_type=poweroutlet_type)
+                ).values('_termination_a_device', '_termination_b_device')
+
+                # The next ring in the search will include the nodes of the previous ring. Otherwise, an existing
+                # loop (i.e. before this was implemented) will ping-pong the search forever as current and next rings
+                # will almost never match in order to break the loop (the exception is when the devices causing the
+                # loop are connected to the same exact set of devices).
+                next_nodes = nodes.copy()
+
+                # Node is temination a
+                for cable in poweroutlet_cables.filter(_termination_a_device__in=nodes):
+                    next_nodes.add(cable['_termination_b_device'])
+                # Node is termination b
+                for cable in poweroutlet_cables.filter(_termination_b_device__in=nodes):
+                    next_nodes.add(cable['_termination_a_device'])
+
         # A front port cannot be connected to its corresponding rear port
         if (
             type_a in ['frontport', 'rearport'] and

netbox/dcim/tests/test_models.py

@@ -300,12 +300,23 @@ class CableTestCase(TestCase):
         self.device2 = Device.objects.create(
             device_type=devicetype, device_role=devicerole, name='TestDevice2', site=site
         )
+        self.device3 = Device.objects.create(
+            device_type=devicetype, device_role=devicerole, name='TestDevice3', site=site
+        )
         self.interface1 = Interface.objects.create(device=self.device1, name='eth0')
         self.interface2 = Interface.objects.create(device=self.device2, name='eth0')
         self.cable = Cable(termination_a=self.interface1, termination_b=self.interface2)
         self.cable.save()
 
         self.power_port1 = PowerPort.objects.create(device=self.device2, name='psu1')
+        self.power_port11 = PowerPort.objects.create(device=self.device1, name='psu11')
+        self.power_port21 = PowerPort.objects.create(device=self.device2, name='psu21')
+        self.power_port31 = PowerPort.objects.create(device=self.device3, name='psu31')
+        self.power_port32 = PowerPort.objects.create(device=self.device3, name='psu32')
+        self.power_outlet11 = PowerOutlet.objects.create(device=self.device1, name='outlet11')
+        self.power_outlet21 = PowerOutlet.objects.create(device=self.device2, name='outlet21')
+        self.power_outlet22 = PowerOutlet.objects.create(device=self.device2, name='outlet22')
+        self.power_outlet31 = PowerOutlet.objects.create(device=self.device3, name='outlet31')
         self.patch_pannel = Device.objects.create(
             device_type=devicetype, device_role=devicerole, name='TestPatchPannel', site=site
         )
@@ -358,6 +369,25 @@ class CableTestCase(TestCase):
         with self.assertRaises(ValidationError):
             cable.clean()
 
+    def test_cable_cannot_create_power_loop(self):
+        """
+        A power loop is prohibited, be it direct (device to itself) or indirect (spanning multiple devices)
+        """
+        # Direct loop
+        with self.assertRaises(ValidationError):
+            Cable(termination_a=self.power_outlet11, termination_b=self.power_port11).clean()
+
+        # Intentionally reversing the type on the terminations to ensure queryset functionality
+        Cable.objects.create(termination_a=self.power_outlet11, termination_b=self.power_port21)
+        Cable.objects.create(termination_b=self.power_outlet21, termination_a=self.power_port31)
+
+        # Redundant connections are acceptable
+        Cable.objects.create(termination_a=self.power_outlet22, termination_b=self.power_port32)
+
+        # Indirect loop
+        with self.assertRaises(ValidationError):
+            Cable(termination_a=self.power_outlet31, termination_b=self.power_port11).clean()
+
     def test_cable_front_port_cannot_connect_to_corresponding_rear_port(self):
         """
         A cable cannot connect a front port to its corresponding rear port