Destroy container along with processes before stdio

We need to make sure the container is destroyed before closing the stdio
for the container.  This becomes a big issues when running in the host's
pid namespace because the other processes could have inherited the stdio
of the initial process.  The call to close will just block as they still
have the io open.

Calling destroy before closing io, especially in the host pid namespace
will cause all additional processes to be killed in the container's
cgroup.  This will allow the io to be closed successfuly.

This change makes sure the order for destroy and close is correct as
well as ensuring that if any errors encoutered during start or exec will
be handled by terminating the process and destroying the container.  We
cannot use defers here because we need to enforce the correct ordering
on destroy.

This also sets the subreaper setting for runc so that when running in
pid host, runc can wait on the addiontal processes launched by the
container, useful on destroy, but also good for reaping the additional
processes that were launched.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

signals.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/Sirupsen/logrus"
 	"github.com/opencontainers/runc/libcontainer"
+	"github.com/opencontainers/runc/libcontainer/system"
 	"github.com/opencontainers/runc/libcontainer/utils"
 )
 
@@ -16,7 +17,13 @@ const signalBufferSize = 2048
 
 // newSignalHandler returns a signal handler for processing SIGCHLD and SIGWINCH signals
 // while still forwarding all other signals to the process.
-func newSignalHandler(tty *tty) *signalHandler {
+func newSignalHandler(tty *tty, enableSubreaper bool) *signalHandler {
+	if enableSubreaper {
+		// set us as the subreaper before registering the signal handler for the container
+		if err := system.SetSubreaper(1); err != nil {
+			logrus.Warn(err)
+		}
+	}
 	// ensure that we have a large buffer size so that we do not miss any signals
 	// incase we are not processing them fast enough.
 	s := make(chan os.Signal, signalBufferSize)
@@ -107,10 +114,3 @@ func (h *signalHandler) reap() (exits []exit, err error) {
 		})
 	}
 }
-
-func (h *signalHandler) Close() error {
-	if h.tty != nil {
-		return h.tty.Close()
-	}
-	return nil
-}