thin plugin: Handle --multus-master-cni-file-name flag

Multus v3.9.3 has `--multus-master-cni-file-name` flag to specify the
name of a primary CNI config file.
https://github.com/k8snetworkplumbingwg/multus-cni/blob/v3.9.3/images/entrypoint.sh#L22

In Multus v4.0.2, the thin plugin has the flag defined, but it is not
read and so does not have effect.

This pull request fixes the problem by making the thin plugin correctly
handles `--multus-master-cni-file-name` flag.

Fixes #1226

Signed-off-by: Hidehito Yabuuchi <hyab@preferred.jp>

cmd/thin_entrypoint/main.go

@@ -310,22 +310,32 @@ const multusConfTemplate = `{
 }
 `
 
-func (o *Options) createMultusConfig(prevMasterConfigFileHash []byte) (string, []byte, error) {
-	// find master file from MultusAutoconfigDir
-	files, err := libcni.ConfFiles(o.MultusAutoconfigDir, []string{".conf", ".conflist"})
-	if err != nil {
-		return "", nil, fmt.Errorf("cannot find master CNI config in %q: %v", o.MultusAutoconfigDir, err)
+func (o *Options) getMasterConfigPath() (string, error) {
+	// Master config file is specified
+	if o.MultusMasterCNIFileName != "" {
+		return filepath.Join(o.MultusAutoconfigDir, o.MultusMasterCNIFileName), nil
+	}
+
+	// Pick the alphabetically first config file from MultusAutoconfigDir
+	files, err := libcni.ConfFiles(o.MultusAutoconfigDir, []string{".conf", ".conflist"})
+	if err != nil {
+		return "", fmt.Errorf("cannot find master CNI config in %q: %v", o.MultusAutoconfigDir, err)
 	}
 
-	masterConfigPath := ""
 	for _, filename := range files {
 		if !strings.HasPrefix(filepath.Base(filename), "00-multus.conf") {
-			masterConfigPath = filename
-			break
+			return filename, nil
 		}
 	}
-	if masterConfigPath == "" {
-		return "", nil, fmt.Errorf("cannot find valid master CNI config in %q", o.MultusAutoconfigDir)
+
+	// No config file found
+	return "", fmt.Errorf("cannot find valid master CNI config in %q", o.MultusAutoconfigDir)
+}
+
+func (o *Options) createMultusConfig(prevMasterConfigFileHash []byte) (string, []byte, error) {
+	masterConfigPath, err := o.getMasterConfigPath()
+	if err != nil {
+		return "", nil, err
 	}
 
 	masterConfigBytes, masterConfigFileHash, err := getFileAndHash(masterConfigPath)

cmd/thin_entrypoint/main_test.go

@@ -411,4 +411,65 @@ var _ = Describe("thin entrypoint testing", func() {
 		Expect(os.RemoveAll(tmpDir)).To(Succeed())
 	})
 
+	It("Run createMultusConfig(), with options, conflist", func() {
+		// create directory and files
+		tmpDir, err := os.MkdirTemp("", "multus_thin_entrypoint_tmp")
+		Expect(err).NotTo(HaveOccurred())
+
+		multusAutoConfigDir := fmt.Sprintf("%s/auto_conf", tmpDir)
+		cniConfDir := fmt.Sprintf("%s/cni_conf", tmpDir)
+
+		Expect(os.Mkdir(multusAutoConfigDir, 0755)).To(Succeed())
+		Expect(os.Mkdir(cniConfDir, 0755)).To(Succeed())
+
+		// create master CNI config
+		masterCNIConfigFileName := "10-testcni.conf"
+		masterCNIConfig := `
+		{
+			"cniVersion": "1.0.0",
+			"name": "test1",
+			"type": "cnitesttype"
+		}`
+		Expect(os.WriteFile(fmt.Sprintf("%s/%s", multusAutoConfigDir, masterCNIConfigFileName), []byte(masterCNIConfig), 0755)).To(Succeed())
+
+		// create another CNI config
+		anotherCNIConfigFileName := "09-test2cni.conf" // Alphabetically before masterCNIConfigFileName
+		anotherCNIConfig := `
+		{
+			"cniVersion": "1.0.0",
+			"name": "test2",
+			"type": "cnitest2type"
+		}`
+		Expect(os.WriteFile(fmt.Sprintf("%s/%s", multusAutoConfigDir, anotherCNIConfigFileName), []byte(anotherCNIConfig), 0755)).To(Succeed())
+
+		masterConfigPath, masterConfigHash, err := (&Options{
+			MultusAutoconfigDir:      multusAutoConfigDir,
+			MultusMasterCNIFileName:  masterCNIConfigFileName,
+			CNIConfDir:               cniConfDir,
+			MultusKubeConfigFileHost: "/etc/foobar_kubeconfig",
+		}).createMultusConfig(nil)
+		Expect(err).NotTo(HaveOccurred())
+
+		Expect(masterConfigPath).NotTo(Equal(""))
+		Expect(masterConfigHash).NotTo(Equal(""))
+
+		expectedResult :=
+			`{
+    "cniVersion": "1.0.0",
+    "name": "multus-cni-network",
+    "plugins": [ {
+        "type": "multus",
+        "logToStderr": false,
+        "kubeconfig": "/etc/foobar_kubeconfig",
+        "delegates": [
+            {"cniVersion":"1.0.0","name":"test1","type":"cnitesttype"}
+        ]
+    }]
+}
+`
+		conf, err := os.ReadFile(fmt.Sprintf("%s/00-multus.conflist", cniConfDir))
+		Expect(string(conf)).To(Equal(expectedResult))
+
+		Expect(os.RemoveAll(tmpDir)).To(Succeed())
+	})
 })