dhcp: add --pidfile option to DHCP client daemon

plugins/ipam/dhcp/README.md

@@ -16,6 +16,9 @@ $ rm -f /run/cni/dhcp.sock
 $ ./dhcp daemon
 ```
 
+If given `-pidfile <path>` arguments after 'daemon', the dhcp plugin will write
+its PID to the given file.
+
 Alternatively, you can use systemd socket activation protocol.
 Be sure that the .socket file uses /run/cni/dhcp.sock as the socket path.
 

plugins/ipam/dhcp/daemon.go

@@ -18,7 +18,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"log"
+	"io/ioutil"
 	"net"
 	"net/http"
 	"net/rpc"
@@ -141,19 +141,29 @@ func getListener() (net.Listener, error) {
 	}
 }
 
-func runDaemon() {
+func runDaemon(pidfilePath string) error {
 	// since other goroutines (on separate threads) will change namespaces,
 	// ensure the RPC server does not get scheduled onto those
 	runtime.LockOSThread()
 
+	// Write the pidfile
+	if pidfilePath != "" {
+		if !filepath.IsAbs(pidfilePath) {
+			return fmt.Errorf("Error writing pidfile %q: path not absolute", pidfilePath)
+		}
+		if err := ioutil.WriteFile(pidfilePath, []byte(fmt.Sprintf("%d", os.Getpid())), 0644); err != nil {
+			return fmt.Errorf("Error writing pidfile %q: %v", pidfilePath, err)
+		}
+	}
+
 	l, err := getListener()
 	if err != nil {
-		log.Printf("Error getting listener: %v", err)
-		return
+		return fmt.Errorf("Error getting listener: %v", err)
 	}
 
 	dhcp := newDHCP()
 	rpc.Register(dhcp)
 	rpc.HandleHTTP()
 	http.Serve(l, nil)
+	return nil
 }

plugins/ipam/dhcp/main.go

@@ -15,7 +15,9 @@
 package main
 
 import (
+	"flag"
 	"fmt"
+	"log"
 	"net/rpc"
 	"os"
 	"path/filepath"
@@ -30,7 +32,15 @@ const socketPath = "/run/cni/dhcp.sock"
 
 func main() {
 	if len(os.Args) > 1 && os.Args[1] == "daemon" {
-		runDaemon()
+		var pidfilePath string
+		daemonFlags := flag.NewFlagSet("daemon", flag.ExitOnError)
+		daemonFlags.StringVar(&pidfilePath, "pidfile", "", "optional path to write daemon PID to")
+		daemonFlags.Parse(os.Args[2:])
+
+		if err := runDaemon(pidfilePath); err != nil {
+			log.Printf(err.Error())
+			os.Exit(1)
+		}
 	} else {
 		skel.PluginMain(cmdAdd, cmdDel, version.All)
 	}