Add --preserve-fds=N to create and run

This preserves the given number of file descriptors on top of the 3 stdio and
the socket activation ($LISTEN_FDS=M) fds.

If LISTEN_FDS is not set then [3..3+N) would be preserved by --preserve-fds=N.

Given LISTEN_FDS=3 and --preserve-fds=5 then we would preserve fds [3, 11) (in
addition to stdio).  That's 3, 4 & 5 from LISTEN_FDS=3 and 6, 7, 8, 9 & 10 from
--preserve-fds=5.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

utils_linux.go

@@ -180,6 +180,7 @@ type runner struct {
 	shouldDestroy   bool
 	detach          bool
 	listenFDs       []*os.File
+	preserveFDs     int
 	pidFile         string
 	consoleSocket   string
 	container       libcontainer.Container
@@ -196,11 +197,17 @@ func (r *runner) run(config *specs.Process) (int, error) {
 		r.destroy()
 		return -1, err
 	}
+
 	if len(r.listenFDs) > 0 {
 		process.Env = append(process.Env, fmt.Sprintf("LISTEN_FDS=%d", len(r.listenFDs)), "LISTEN_PID=1")
 		process.ExtraFiles = append(process.ExtraFiles, r.listenFDs...)
 	}
 
+	baseFd := 3 + len(process.ExtraFiles)
+	for i := baseFd; i < baseFd+r.preserveFDs; i++ {
+		process.ExtraFiles = append(process.ExtraFiles, os.NewFile(uintptr(i), "PreserveFD:"+strconv.Itoa(i)))
+	}
+
 	rootuid, err := r.container.Config().HostUID()
 	if err != nil {
 		r.destroy()
@@ -353,6 +360,7 @@ func startContainer(context *cli.Context, spec *specs.Spec, create bool) (int, e
 		consoleSocket:   context.String("console-socket"),
 		detach:          context.Bool("detach"),
 		pidFile:         context.String("pid-file"),
+		preserveFDs:     context.Int("preserve-fds"),
 		create:          create,
 	}
 	return r.run(&spec.Process)