write empty json object to empty cni statefile if exists

cns/cnireconciler/podinfoprovider_test.go

@@ -36,6 +36,14 @@ func TestNewCNIPodInfoProvider(t *testing.T) {
 			},
 			wantErr: false,
 		},
+		{
+			name: "empty CNI response",
+			exec: newCNIStateFakeExec(
+				`{}`,
+			),
+			want:    map[string]cns.PodInfo{},
+			wantErr: false,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

cns/cnireconciler/statefile.go

@@ -0,0 +1,41 @@
+package cnireconciler
+
+import (
+	"encoding/json"
+	"errors"
+	"os"
+
+	"github.com/Azure/azure-container-networking/cns/logger"
+)
+
+// WriteObjectToCNIStatefile checks for a file at the CNI statefile path,
+// and checks if it is empty. If it is empty, writes an empty JSON object to
+// it so older CNI can execute. Does nothing and returns no error if the
+// file does not exist.
+//
+// This is a hack to get older CNI to run when CNS has mounted the statefile
+// path, but the statefile wasn't written by CNI yet. Kubelet will stub an
+// empty file on the host filesystem, crashing older CNI because it doesn't know
+// how to handle empty statefiles.
+func WriteObjectToCNIStatefile() error {
+	filename := "/var/run/azure-vnet.json"
+	return writeObjectToFile(filename)
+}
+
+func writeObjectToFile(filename string) error {
+	fi, err := os.Stat(filename)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return nil
+		}
+		return err
+	}
+
+	if fi.Size() != 0 {
+		return nil
+	}
+
+	logger.Printf("Writing {} to CNI statefile")
+	b, _ := json.Marshal(map[string]string{})
+	return os.WriteFile(filename, b, os.FileMode(0666))
+}

cns/cnireconciler/statefile_test.go

@@ -0,0 +1,41 @@
+package cnireconciler
+
+import (
+	"os"
+	"path"
+	"testing"
+
+	"github.com/Azure/azure-container-networking/cns/logger"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestWriteObjectToFile(t *testing.T) {
+	name := "testdata/test"
+	err := os.MkdirAll(path.Dir(name), 0666)
+	require.NoError(t, err)
+
+	_, err = os.Stat(name)
+	require.ErrorIs(t, err, os.ErrNotExist)
+
+	// create empty file
+	_, err = os.Create(name)
+	require.NoError(t, err)
+	defer os.Remove(name)
+
+	// check it's empty
+	fi, _ := os.Stat(name)
+	assert.Equal(t, fi.Size(), int64(0))
+
+	// populate
+	require.NoError(t, writeObjectToFile(name))
+
+	// read
+	b, err := os.ReadFile(name)
+	require.NoError(t, err)
+	assert.Equal(t, string(b), "{}")
+}
+
+func init() {
+	logger.InitLogger("testlogs", 0, 0, "./")
+}

cns/service/main.go

@@ -21,6 +21,7 @@ import (
 	"github.com/Azure/azure-container-networking/cnm/ipam"
 	"github.com/Azure/azure-container-networking/cnm/network"
 	"github.com/Azure/azure-container-networking/cns"
+	"github.com/Azure/azure-container-networking/cns/cnireconciler"
 	cni "github.com/Azure/azure-container-networking/cns/cnireconciler"
 	"github.com/Azure/azure-container-networking/cns/cnsclient"
 	"github.com/Azure/azure-container-networking/cns/common"
@@ -521,6 +522,13 @@ func main() {
 	// Initialze state in if CNS is running in CRD mode
 	// State must be initialized before we start HTTPRestService
 	if config.ChannelMode == cns.CRD {
+		// Check the CNI statefile mount, and if the file is empty
+		// stub an empty JSON object
+		if err := cnireconciler.WriteObjectToCNIStatefile(); err != nil {
+			logger.Errorf("Failed to write empty object to CNI state: %v", err)
+			return
+		}
+
 		// We might be configured to reinitialize state from the CNI instead of the apiserver.
 		// If so, we should check that the the CNI is new enough to support the state commands,
 		// otherwise we fall back to the existing behavior.