Systemd integration with runc, for on-demand socket activation

Signed-off-by: Shishir Mahajan <shishir.mahajan@redhat.com>

start.go

@@ -6,13 +6,16 @@ import (
 	"fmt"
 	"os"
 	"runtime"
+	"strconv"
 
 	"github.com/Sirupsen/logrus"
 	"github.com/codegangsta/cli"
 	"github.com/opencontainers/runc/libcontainer"
 	"github.com/opencontainers/specs"
 )
 
+const SD_LISTEN_FDS_START = 3
+
 var startCommand = cli.Command{
 	Name:  "start",
 	Usage: "create and run a container",
@@ -27,6 +30,13 @@ var startCommand = cli.Command{
 			setupSdNotify(spec, notifySocket)
 		}
 
+		listenFds := os.Getenv("LISTEN_FDS")
+		listenPid := os.Getenv("LISTEN_PID")
+
+		if listenFds != "" && listenPid == strconv.Itoa(os.Getpid()) {
+			setupSocketActivation(spec, listenFds)
+		}
+
 		if os.Geteuid() != 0 {
 			logrus.Fatal("runc should be run as root")
 		}
@@ -79,6 +89,19 @@ func startContainer(context *cli.Context, spec *specs.LinuxSpec) (int, error) {
 	// that created it.
 	defer destroy(container)
 	process := newProcess(spec.Process)
+
+	//support for on-demand socket activation: leak file descriptors into the container.
+	if os.Getenv("LISTEN_FDS") != "" {
+		listenFdsInt, err := strconv.Atoi(os.Getenv("LISTEN_FDS"))
+		if err != nil {
+			return -1, err
+		}
+
+		for i := SD_LISTEN_FDS_START; i < (listenFdsInt + SD_LISTEN_FDS_START); i++ {
+			process.ExtraFiles = append(process.ExtraFiles, os.NewFile(uintptr(i), ""))
+		}
+	}
+
 	tty, err := newTty(spec.Process.Terminal, process, rootuid)
 	if err != nil {
 		return -1, err
@@ -100,6 +123,13 @@ func setupSdNotify(spec *specs.LinuxSpec, notifySocket string) {
 	spec.Process.Env = append(spec.Process.Env, fmt.Sprintf("NOTIFY_SOCKET=%s", notifySocket))
 }
 
+// If systemd is supporting on-demand socket activation, this function will add support
+// for on-demand socket activation for the containerized service.
+func setupSocketActivation(spec *specs.LinuxSpec, listenFds string) {
+	spec.Process.Env = append(spec.Process.Env, fmt.Sprintf("LISTEN_FDS=%s", listenFds))
+	spec.Process.Env = append(spec.Process.Env, "LISTEN_PID=1")
+}
+
 func destroy(container libcontainer.Container) {
 	status, err := container.Status()
 	if err != nil {