fix(instances): skip IPv6 ExternalIP for Robot servers without a subnet

Robot servers do not necessarily have an IPv6 subnet assigned. In that
case the Robot API reports an empty `server_ipv6_net`, and appending "1"
to it added the literal address "1" as an ExternalIP to the Node object.

Guard the concatenation with an emptiness check and validate the result
before using it, emitting a warning event when the reported subnet does
not yield a valid address.

Co-authored-by: Claude <noreply@anthropic.com>

Author: Jakob3xD

hcloud/instances.go

@@ -41,6 +41,7 @@ import (
 const (
 	ProvidedBy              = "instance.hetzner.cloud/provided-by"
 	MisconfiguredInternalIP = "MisconfiguredInternalIP"
+	InvalidIPv6Net          = "InvalidIPv6Net"
 	instancesV2Subsystem    = "instances_v2"
 )
 
@@ -281,10 +282,23 @@ func robotNodeAddresses(
 	ipv4 := cfg.Instance.AddressFamily == config.AddressFamilyIPv4 || dualStack
 	ipv6 := cfg.Instance.AddressFamily == config.AddressFamilyIPv6 || dualStack
 
-	if ipv6 {
+	// Robot servers do not necessarily have an IPv6 subnet assigned, in which case the field is empty.
+	if ipv6 && server.ServerIPv6Net != "" {
 		// For a given IPv6 network of 2a01:f48:111:4221::, the instance address is 2a01:f48:111:4221::1
 		hostAddress := server.ServerIPv6Net + "1"
-		addresses = append(addresses, corev1.NodeAddress{Type: corev1.NodeExternalIP, Address: hostAddress})
+
+		if net.ParseIP(hostAddress) == nil {
+			utils.WarnEventLogf(
+				recorder,
+				node,
+				InvalidIPv6Net,
+				"Robot server %q reports the IPv6 subnet %q, which does not yield a valid address. As a result, no IPv6 ExternalIP is added",
+				server.Name,
+				server.ServerIPv6Net,
+			)
+		} else {
+			addresses = append(addresses, corev1.NodeAddress{Type: corev1.NodeExternalIP, Address: hostAddress})
+		}
 	}
 
 	if ipv4 {

hcloud/instances_test.go

@@ -712,6 +712,41 @@ func TestNodeAddressesRobotServer(t *testing.T) {
 				{Type: corev1.NodeExternalIP, Address: "203.0.113.7"},
 			},
 		},
+		{
+			name:          "public ipv6 without IPv6 subnet",
+			addressFamily: config.AddressFamilyIPv6,
+			server: &hrobotmodels.Server{
+				Name:     "foobar",
+				ServerIP: "203.0.113.7",
+			},
+			expected: []corev1.NodeAddress{
+				{Type: corev1.NodeHostName, Address: "foobar"},
+			},
+		},
+		{
+			name:          "public dual stack without IPv6 subnet",
+			addressFamily: config.AddressFamilyDualStack,
+			server: &hrobotmodels.Server{
+				Name:     "foobar",
+				ServerIP: "203.0.113.7",
+			},
+			expected: []corev1.NodeAddress{
+				{Type: corev1.NodeHostName, Address: "foobar"},
+				{Type: corev1.NodeExternalIP, Address: "203.0.113.7"},
+			},
+		},
+		{
+			name:          "public ipv6 with malformed IPv6 subnet",
+			addressFamily: config.AddressFamilyIPv6,
+			server: &hrobotmodels.Server{
+				Name:          "foobar",
+				ServerIP:      "203.0.113.7",
+				ServerIPv6Net: "2001:db8:1234::/64",
+			},
+			expected: []corev1.NodeAddress{
+				{Type: corev1.NodeHostName, Address: "foobar"},
+			},
+		},
 	}
 
 	for _, test := range tests {