cni: ensure to setup CNI addresses in deterministic order

.changelog/17766.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+cni: Ensure to setup CNI addresses in deterministic order
+```

client/allocrunner/networking_cni.go

@@ -130,46 +130,52 @@ func (c *cniNetworkConfigurator) Setup(ctx context.Context, alloc *structs.Alloc
 // cniToAllocNet converts a CNIResult to an AllocNetworkStatus or returns an
 // error. The first interface and IP with a sandbox and address set are
 // preferred. Failing that the first interface with an IP is selected.
-//
-// Unfortunately the go-cni library returns interfaces in an unordered map so
-// the results may be nondeterministic depending on CNI plugin output.
 func (c *cniNetworkConfigurator) cniToAllocNet(res *cni.CNIResult) (*structs.AllocNetworkStatus, error) {
+	if len(res.Interfaces) == 0 {
+		return nil, fmt.Errorf("failed to configure network: no interfaces found")
+	}
+
 	netStatus := new(structs.AllocNetworkStatus)
 
+	// Unfortunately the go-cni library returns interfaces in an unordered map meaning
+	// the results may be nondeterministic depending on CNI plugin output so make
+	// sure we sort them by interface name.
+	names := make([]string, 0, len(res.Interfaces))
+	for k := range res.Interfaces {
+		names = append(names, k)
+	}
+	sort.Strings(names)
+
 	// Use the first sandbox interface with an IP address
-	if len(res.Interfaces) > 0 {
-		for name, iface := range res.Interfaces {
-			if iface == nil {
-				// this should never happen but this value is coming from external
-				// plugins so we should guard against it
-				delete(res.Interfaces, name)
-			}
+	for _, name := range names {
+		iface := res.Interfaces[name]
+		if iface == nil {
+			// this should never happen but this value is coming from external
+			// plugins so we should guard against it
+			delete(res.Interfaces, name)
+			continue
+		}
 
-			if iface.Sandbox != "" && len(iface.IPConfigs) > 0 {
-				netStatus.Address = iface.IPConfigs[0].IP.String()
-				netStatus.InterfaceName = name
-				break
-			}
+		if iface.Sandbox != "" && len(iface.IPConfigs) > 0 {
+			netStatus.Address = iface.IPConfigs[0].IP.String()
+			netStatus.InterfaceName = name
+			break
 		}
 	}
 
 	// If no IP address was found, use the first interface with an address
 	// found as a fallback
 	if netStatus.Address == "" {
-		var found bool
-		for name, iface := range res.Interfaces {
+		for _, name := range names {
+			iface := res.Interfaces[name]
 			if len(iface.IPConfigs) > 0 {
 				ip := iface.IPConfigs[0].IP.String()
 				c.logger.Debug("no sandbox interface with an address found CNI result, using first available", "interface", name, "ip", ip)
 				netStatus.Address = ip
 				netStatus.InterfaceName = name
-				found = true
 				break
 			}
 		}
-		if !found {
-			c.logger.Warn("no address could be found from CNI result")
-		}
 	}
 
 	// If no IP address could be found, return an error

client/allocrunner/networking_cni_test.go

@@ -127,6 +127,22 @@ func TestCNI_forceCleanup(t *testing.T) {
 	})
 }
 
+// TestCNI_cniToAllocNet_NoInterfaces asserts an error is returned if CNIResult
+// contains no interfaces.
+func TestCNI_cniToAllocNet_NoInterfaces(t *testing.T) {
+	ci.Parallel(t)
+
+	cniResult := &cni.CNIResult{}
+
+	// Only need a logger
+	c := &cniNetworkConfigurator{
+		logger: testlog.HCLogger(t),
+	}
+	allocNet, err := c.cniToAllocNet(cniResult)
+	require.Error(t, err)
+	require.Nil(t, allocNet)
+}
+
 // TestCNI_cniToAllocNet_Fallback asserts if a CNI plugin result lacks an IP on
 // its sandbox interface, the first IP found is used.
 func TestCNI_cniToAllocNet_Fallback(t *testing.T) {