Adding RunIn to run a user specified command in an existing container, wait for it to exit and return the exit code.

RunIn will connect to a user specified Terminal before running the command.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)

namespaces/execin.go

@@ -5,31 +5,84 @@ package namespaces
 import (
 	"encoding/json"
 	"os"
+	"os/exec"
 	"strconv"
+	"syscall"
 
 	"github.com/docker/libcontainer"
 	"github.com/docker/libcontainer/label"
 	"github.com/docker/libcontainer/system"
 )
 
+// Runs the command under 'args' inside an existing container referred to by 'container'. This API currently does not support a Tty based 'term'.
+// Returns the exitcode of the command upon success and appropriate error on failure.
+func RunIn(container *libcontainer.Config, state *libcontainer.State, args []string, nsinitPath string, term Terminal, startCallback func()) (int, error) {
+	containerJson, err := getContainerJson(container)
+	if err != nil {
+		return -1, err
+	}
+
+	var cmd exec.Cmd
+
+	cmd.Path = nsinitPath
+	cmd.Args = getNsEnterCommand(nsinitPath, strconv.Itoa(state.InitPid), containerJson, args)
+
+	if err := term.Attach(&cmd); err != nil {
+		return -1, err
+	}
+	defer term.Close()
+
+	if err := cmd.Start(); err != nil {
+		return -1, err
+	}
+	if startCallback != nil {
+		startCallback()
+	}
+
+	if err := cmd.Wait(); err != nil {
+		if _, ok := err.(*exec.ExitError); !ok {
+			return -1, err
+		}
+	}
+
+	return cmd.ProcessState.Sys().(syscall.WaitStatus).ExitStatus(), nil
+}
+
 // ExecIn uses an existing pid and joins the pid's namespaces with the new command.
 func ExecIn(container *libcontainer.Config, state *libcontainer.State, args []string) error {
-	// TODO(vmarmol): If this gets too long, send it over a pipe to the child.
-	// Marshall the container into JSON since it won't be available in the namespace.
-	containerJson, err := json.Marshal(container)
+	containerJson, err := getContainerJson(container)
 	if err != nil {
 		return err
 	}
-
 	// Enter the namespace and then finish setup
-	finalArgs := []string{os.Args[0], "nsenter", "--nspid", strconv.Itoa(state.InitPid), "--containerjson", string(containerJson), "--"}
-	finalArgs = append(finalArgs, args...)
+	finalArgs := getNsEnterCommand(os.Args[0], strconv.Itoa(state.InitPid), containerJson, args)
+
 	if err := system.Execv(finalArgs[0], finalArgs[0:], os.Environ()); err != nil {
 		return err
 	}
 	panic("unreachable")
 }
 
+func getContainerJson(container *libcontainer.Config) (string, error) {
+	// TODO(vmarmol): If this gets too long, send it over a pipe to the child.
+	// Marshall the container into JSON since it won't be available in the namespace.
+	containerJson, err := json.Marshal(container)
+	if err != nil {
+		return "", err
+	}
+	return string(containerJson), nil
+}
+
+func getNsEnterCommand(nsinitPath, initPid, containerJson string, args []string) []string {
+	return append([]string{
+		nsinitPath,
+		"nsenter",
+		"--nspid", initPid,
+		"--containerjson", containerJson,
+		"--",
+	}, args...)
+}
+
 // Run a command in a container after entering the namespace.
 func NsEnter(container *libcontainer.Config, args []string) error {
 	// clear the current processes env and replace it with the environment