daemon: terminate on parent process ending; added --daemonize flag

This is useful when the cli daemon is launched from an external process
to avoid leaving zombie process if the parent process unexpectedly dies.

If daemonization (so no stdin/stdout) is the intended behaviour the
--daemonize flag must be used.

Author: cmaglie

cli/daemon/daemon.go

@@ -19,6 +19,8 @@ package daemon
 
 import (
 	"fmt"
+	"io"
+	"io/ioutil"
 	"log"
 	"net"
 	"net/http"
@@ -39,16 +41,20 @@ const (
 
 // NewCommand created a new `daemon` command
 func NewCommand() *cobra.Command {
-	return &cobra.Command{
+	cmd := &cobra.Command{
 		Use:     "daemon",
 		Short:   fmt.Sprintf("Run as a daemon on port %s", port),
 		Long:    "Running as a daemon the initialization of cores and libraries is done only once.",
 		Example: "  " + os.Args[0] + " daemon",
 		Args:    cobra.NoArgs,
 		Run:     runDaemonCommand,
 	}
+	cmd.Flags().BoolVar(&daemonize, "daemonize", false, "Do not terminate daemon process if the parent process dies")
+	return cmd
 }
 
+var daemonize bool
+
 func runDaemonCommand(cmd *cobra.Command, args []string) {
 	s := grpc.NewServer()
 
@@ -68,13 +74,20 @@ func runDaemonCommand(cmd *cobra.Command, args []string) {
 	// register the monitors service
 	srv_monitor.RegisterMonitorServer(s, &daemon.MonitorService{})
 
+	if !daemonize {
+		// When parent process ends terminate also the daemon
+		go func() {
+			// stdin is closed when the controlling parent process ends
+			_, _ = io.Copy(ioutil.Discard, os.Stdin)
+			os.Exit(0)
+		}()
+	}
+
 	lis, err := net.Listen("tcp", port)
 	if err != nil {
 		log.Fatalf("failed to listen: %v", err)
 	}
 	if err := s.Serve(lis); err != nil {
 		log.Fatalf("failed to serve: %v", err)
 	}
-
-	fmt.Println("Done serving")
 }