cni: ensure to setup CNI addresses in deterministic order (#17766)

* cni: ensure to setup CNI addresses in deterministic order

  Currently as commented in the code the go-cni library returns an unordered map
  of interfaces. In cases where there are multiple CNI interfaces being created this
  creates a problem with service registration and healthchecking because the first
  address in the map is being used.

  The use case we have where this is an issue is that we run CNI with the macvlan
  plugin to isolate workloads, but they still need to be able to access the host on
  a static address to be able to perform local resolving and hit host services like
  the Consul agent API. To make this work there are 2 options, you either add a
  macvlan interface on the host with an assigned address for each VLAN you have or
  you create an additional veth bridged interface in the container namespace.
  We chose the latter option through a custom CNI plugin but the ordering issue
  leaves us with incorrect service registration.

* Updates after feedback

 * First check for the CNIResult interfaces length, if it's zero we don't need to proceed
   at all.
 * Use sorted interfaces list for the address fallback scenario as well.
 * Remove "found" log message logic, when an address isn't found an error is returned stating
   the allocation could not be configured as an address was missing from the CNIResult. If we
   still need a Warn message then we can add it to the condition that returns the error if no
   address could be found instead of using the "found" bool logic.

.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)
 
-	// 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)
-			}
+	// 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)
 
-			if iface.Sandbox != "" && len(iface.IPConfigs) > 0 {
-				netStatus.Address = iface.IPConfigs[0].IP.String()
-				netStatus.InterfaceName = name
-				break
-			}
+	// Use the first sandbox interface with an IP address
+	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 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) {