Merge pull request #1320 from ijc25/preserve-fds

Add --preserve-file-descriptors=N to create

create.go

@@ -46,6 +46,10 @@ command(s) that get executed on start, edit the args parameter of the spec. See
 			Name:  "no-new-keyring",
 			Usage: "do not create a new session keyring for the container.  This will cause the container to inherit the calling processes session key",
 		},
+		cli.IntFlag{
+			Name:  "preserve-fds",
+			Usage: "Pass N additional file descriptors to the container (stdio + $LISTEN_FDS + N in total)",
+		},
 	},
 	Action: func(context *cli.Context) error {
 		if err := checkArgs(context, 1, exactArgs); err != nil {

run.go

@@ -57,6 +57,10 @@ command(s) that get executed on start, edit the args parameter of the spec. See
 			Name:  "no-new-keyring",
 			Usage: "do not create a new session keyring for the container.  This will cause the container to inherit the calling processes session key",
 		},
+		cli.IntFlag{
+			Name:  "preserve-fds",
+			Usage: "Pass N additional file descriptors to the container (stdio + $LISTEN_FDS + N in total)",
+		},
 	},
 	Action: func(context *cli.Context) error {
 		if err := checkArgs(context, 1, exactArgs); err != nil {

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)