fix: fall back to an IP-derived name for backend addresses with no cached node name

pkg/provider/azure_loadbalancer_backendpool.go

@@ -707,6 +707,13 @@ func (az *Cloud) addNodeIPAddressesToBackendPool(backendPool *armnetwork.Backend
 	for _, ipAddress := range nodeIPAddresses {
 		if !hasIPAddressInBackendPool(backendPool, ipAddress) {
 			name := az.nodePrivateIPToNodeNameMap[ipAddress]
+			if name == "" {
+				// The node name in cache could be empty for unknown reasons. Fall back to a name
+				// derived from the IP address, prefixed to avoid colliding with a real node name,
+				// so the backend address is not created with an empty name.
+				name = "ip-" + strings.NewReplacer(".", "-", ":", "-").Replace(ipAddress)
+				logger.V(2).Info("Node name not found in cache for IP address, generated a name for the backend address", "ip", ipAddress, "generatedName", name)
+			}
 			logger.V(4).Info("adding node to the backend pool", "ip", ipAddress, "backendPoolName", ptr.Deref(backendPool.Name, ""))
 			addresses = append(addresses, &armnetwork.LoadBalancerBackendAddress{
 				Name: ptr.To(name),

pkg/provider/azure_loadbalancer_backendpool_test.go

@@ -958,6 +958,24 @@ func TestAddNodeIPAddressesToBackendPoolDeduplicates(t *testing.T) {
 	}
 }
 
+func TestAddNodeIPAddressesToBackendPoolGeneratesNameWhenNodeNameMissing(t *testing.T) {
+	ctrl := gomock.NewController(t)
+	defer ctrl.Finish()
+
+	cloud := GetTestCloud(ctrl)
+	cloud.nodePrivateIPToNodeNameMap = map[string]string{}
+
+	bp := buildTestLoadBalancerBackendPoolWithIPs("test-pool", nil)
+	changed := cloud.addNodeIPAddressesToBackendPool(bp, []string{"10.0.0.1"})
+	assert.True(t, changed)
+
+	if assert.Len(t, bp.Properties.LoadBalancerBackendAddresses, 1) {
+		addr := bp.Properties.LoadBalancerBackendAddresses[0]
+		assert.Equal(t, "10.0.0.1", ptr.Deref(addr.Properties.IPAddress, ""))
+		assert.Equal(t, "ip-10-0-0-1", ptr.Deref(addr.Name, ""))
+	}
+}
+
 func TestGetBackendPrivateIPsNodeIPConfig(t *testing.T) {
 	ctrl := gomock.NewController(t)
 	defer ctrl.Finish()

pkg/provider/azure_local_services_test.go

@@ -20,6 +20,7 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"testing"
@@ -522,8 +523,10 @@ func getTestBackendAddressPoolWithIPs(lbName, bpName string, ips []string) *armn
 	}
 	for _, ip := range ips {
 		if len(ip) > 0 {
+			// The local service reconciliation path never populates nodePrivateIPToNodeNameMap,
+			// so addNodeIPAddressesToBackendPool falls back to a name derived from the IP address.
 			bp.Properties.LoadBalancerBackendAddresses = append(bp.Properties.LoadBalancerBackendAddresses, &armnetwork.LoadBalancerBackendAddress{
-				Name: ptr.To(""),
+				Name: ptr.To("ip-" + strings.NewReplacer(".", "-", ":", "-").Replace(ip)),
 				Properties: &armnetwork.LoadBalancerBackendAddressPropertiesFormat{
 					IPAddress: ptr.To(ip),
 				},