child: createCmd: propogate systemd files to actual process within na…

…mespace

The child will inheret the file descriptors from the parent, but it needs to
pass them along to its child, the actual target process within the environment.

pkg/child/child.go

@@ -34,6 +34,23 @@ var propagationStates = map[string]uintptr{
 	"rslave":   uintptr(unix.MS_REC | unix.MS_SLAVE),
 }
 
+func setupFiles(cmd *exec.Cmd) {
+	// 0 1 and 2  are used for stdin. stdout, and stderr
+	const firstExtraFD = 3
+	systemdActivationFDs := 0
+	// check for systemd socket activation sockets
+	if v := os.Getenv("LISTEN_FDS"); v != "" {
+		if num, err := strconv.Atoi(v); err == nil {
+			systemdActivationFDs = num
+			cmd.ExtraFiles = make([]*os.File, systemdActivationFDs)
+		}
+	}
+	for fd := 0; fd < systemdActivationFDs; fd++ {
+		cmd.ExtraFiles[fd] = os.NewFile(uintptr(firstExtraFD + fd), "")
+	}
+}
+
+
 func createCmd(targetCmd []string) (*exec.Cmd, error) {
 	var args []string
 	if len(targetCmd) > 1 {
@@ -47,6 +64,7 @@ func createCmd(targetCmd []string) (*exec.Cmd, error) {
 	cmd.SysProcAttr = &syscall.SysProcAttr{
 		Pdeathsig: syscall.SIGKILL,
 	}
+	setupFiles(cmd)
 	return cmd, nil
 }