Add overlay configuration via config file

This allows to provide per node default
configuraiton for nv-ipam cni plugin

Signed-off-by: adrianc <adrianc@nvidia.com>

pkg/cni/types/types.go

@@ -27,15 +27,24 @@ import (
 )
 
 const (
+	// DefaultConfDir is the default dir where configurations are found
 	DefaultConfDir = "/etc/cni/net.d/nv-ipam.d"
+	// DefaultDataDir is the default dir where cni stores data and binaries
 	DefaultDataDir = "/var/lib/cni/nv-ipam"
+	// DefaultLogFile default log file path to be used for logging
 	DefaultLogFile = "/var/log/nv-ipam-cni.log"
 
-	HostLocalDataDir          = "state/host-local"
-	K8sNodeNameFile           = "k8s-node-name"
+	// HostLocalDataDir is the relative path within the data dir for host-local state data
+	HostLocalDataDir = "state/host-local"
+	// K8sNodeNameFile is the file name containing k8s node name
+	K8sNodeNameFile = "k8s-node-name"
+	// DefaultKubeConfigFileName is the default name of kubeconfig file
 	DefaultKubeConfigFileName = "nv-ipam.kubeconfig"
+	// ConfFileName is the name of CNI configuration file found in conf dir
+	ConfFileName = "nv-ipam.conf"
 )
 
+// IPAMConf is the configuration supported by our CNI plugin
 type IPAMConf struct {
 	types.IPAM
 
@@ -52,12 +61,14 @@ type IPAMConf struct {
 	K8sClient *kubernetes.Clientset
 }
 
+// NetConf is CNI network config
 type NetConf struct {
 	Name       string    `json:"name"`
 	CNIVersion string    `json:"cniVersion"`
 	IPAM       *IPAMConf `json:"ipam"`
 }
 
+// Load NetConf from json string provided as []byte
 func LoadConf(bytes []byte) (*NetConf, error) {
 	n := &NetConf{}
 
@@ -73,6 +84,10 @@ func LoadConf(bytes []byte) (*NetConf, error) {
 		n.IPAM.ConfDir = DefaultConfDir
 	}
 
+	// NOTE: ignore errors, we treat this as best effort. we might fail here if file does not exist or
+	// its malformed. in both cases we can ignore.
+	_ = LoadFromConfFile(n.IPAM, filepath.Join(n.IPAM.ConfDir, ConfFileName))
+
 	if n.IPAM.DataDir == "" {
 		n.IPAM.DataDir = DefaultDataDir
 	}
@@ -106,3 +121,37 @@ func LoadConf(bytes []byte) (*NetConf, error) {
 
 	return n, nil
 }
+
+// LoadFromConfFile updates IPAMConf with values from config file located in filePath
+// for empty entries.
+// if error occurs no updates will occur and error will be returned
+func LoadFromConfFile(conf *IPAMConf, filePath string) error {
+	data, err := os.ReadFile(filePath)
+	if err != nil {
+		return err
+	}
+
+	confFromFile := IPAMConf{}
+	err = json.Unmarshal(data, &confFromFile)
+	if err != nil {
+		return err
+	}
+
+	if confFromFile.DataDir != "" {
+		conf.DataDir = confFromFile.DataDir
+	}
+
+	if confFromFile.Kubeconfig != "" {
+		conf.Kubeconfig = confFromFile.Kubeconfig
+	}
+
+	if confFromFile.LogFile != "" {
+		conf.LogFile = confFromFile.LogFile
+	}
+
+	if confFromFile.LogLevel != "" {
+		conf.LogLevel = confFromFile.LogLevel
+	}
+
+	return nil
+}