*: console rewrite

This implements {createTTY, detach} and all of the combinations and
negations of the two that were previously implemented. There are some
valid questions about out-of-OCI-scope topics like !createTTY and how
things should be handled (why do we dup the current stdio to the
process, and how is that not a security issue). However, these will be
dealt with in a separate patchset.

In order to allow for late console setup, split setupRootfs into the
"preparation" section where all of the mounts are created and the
"finalize" section where we pivot_root and set things as ro. In between
the two we can set up all of the console mountpoints and symlinks we
need.

We use two-stage synchronisation to ensures that when the syscalls are
reordered in a suboptimal way, an out-of-place read() on the parentPipe
will not gobble the ancilliary information.

This patch is part of the console rewrite patchset.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

signals.go

@@ -17,7 +17,7 @@ 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, enableSubreaper bool) *signalHandler {
+func newSignalHandler(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 {
@@ -30,7 +30,6 @@ func newSignalHandler(tty *tty, enableSubreaper bool) *signalHandler {
 	// handle all signals for the process.
 	signal.Notify(s)
 	return &signalHandler{
-		tty:     tty,
 		signals: s,
 	}
 }
@@ -44,12 +43,11 @@ type exit struct {
 
 type signalHandler struct {
 	signals chan os.Signal
-	tty     *tty
 }
 
 // forward handles the main signal event loop forwarding, resizing, or reaping depending
 // on the signal received.
-func (h *signalHandler) forward(process *libcontainer.Process) (int, error) {
+func (h *signalHandler) forward(process *libcontainer.Process, tty *tty) (int, error) {
 	// make sure we know the pid of our main process so that we can return
 	// after it dies.
 	pid1, err := process.Pid()
@@ -57,11 +55,11 @@ func (h *signalHandler) forward(process *libcontainer.Process) (int, error) {
 		return -1, err
 	}
 	// perform the initial tty resize.
-	h.tty.resize()
+	tty.resize()
 	for s := range h.signals {
 		switch s {
 		case syscall.SIGWINCH:
-			h.tty.resize()
+			tty.resize()
 		case syscall.SIGCHLD:
 			exits, err := h.reap()
 			if err != nil {