Resize a Console

LXD
1

I’ve been playing with the demo server code (https://github.com/lxc/lxd-demo-server) as a way to give clients access to a terminal via web socket (I’m writing an iOS native terminal front-end to view this). While that effort appears to be working, I’m hitting a wall trying to send a resize console command over the control web socket. My Go-fu is weak (er, non-existent might be better said), so it’s not entirely clear how I might tackle sending a command correctly.

If you don’t mind looking at the whole method, here’s what I’ve written to make this attempt:

func restConsoleResizeHandler(w http.ResponseWriter, r *http.Request) {
if r.Method != "POST" {
	http.Error(w, "Not implemented", 501)
	return
}

name := r.FormValue("id")
width := r.FormValue("width")
height := r.FormValue("height")
widthInt, err := strconv.Atoi(width)
heightInt, err := strconv.Atoi(height)

w.Header().Set("Access-Control-Allow-Origin", "*")

req := api.ContainerConsolePost{
	Width:  widthInt,
	Height: heightInt,
}

// control socket handler
control := func(conn *websocket.Conn) {
	for {
		_, _, err = conn.ReadMessage()
		if err != nil {
			break
		}
	}
}

consoleDisconnect := make(chan bool)
sendDisconnect := make(chan bool)
defer close(sendDisconnect)
consoleArgs := lxd.ContainerConsoleArgs{
	Terminal: &ReadWriteCloser{inRead, outWrite},
	Control:           control,
	ConsoleDisconnect: consoleDisconnect,
}

go func() {
	<-sendDisconnect
	close(consoleDisconnect)
}()

// Run the command in the container
op, err := lxdDaemon.ConsoleContainer(name, req, &consoleArgs)
fmt.Fprintln(os.Stdout, "Resize op:", op)
fmt.Fprintln(os.Stdout, "Resize err:", err)

return
}

I believe the problem is the Terminal argument in ContainerConsoleArgs; ultimately it’s not going to the same terminal instance as the one created in the console handler. But a definitive dearth of documentation around these methods is stopping me.

Any suggestions for how to proceed from here would be most welcome!

2

Ah, interesting so you’re trying to get something like lxd-demo-server but using the interactive console of the container rather than an exec session.

@brauner may be able to help you there. I’m not sure exactly how much testing was done on the width/height (SIGWINCH) handling for consoles given how new that feature is.

3

Ahhh, as it happens, I’m not using the interactive console! This is a clue, isn’t it? I’m naive enough in my understanding to know that there’s a difference between the two, but not about what to implement!

So I’m actually using the exec session, as it exists in the demo server. What would be technique for implementing a resize behaviour using that, rather than the interactive console?

Or, alternatively, should I implement the console interactively vs the current exec session?

4

For the exec session, you’ll want to send a resize-window message over the control websocket.

As the code we’re using in the interactive lxc exec tool.

5

This is a great lead, thank you! I hope I can impose upon you one more time, because this gets to the central question that I feel has stymied me during this adventure: how do I get a reference to the control socket in the context of this code?

6

You can find the control socket there:

So you could inside that handler function, prepare a control message and send it before entering that for loop.

7

This reads like it is settled? :slight_smile:

8

I’m hoping so. I’ll have a chance to try this tonight.

9

Thanks again for all your help so far. As I look into this more it appears that sending the resize message over the control socket may work (assuming I can figure out what I should be sending), but only for the initial setup request. Is there a way to do this if the window size changes during a session?

10

For during the session, you’d most likely need to keep a reference to the control socket in some kind of global map, then when you get a REST API call to resize a particular session, you can look up what control socket to send the new size to.

11

I’ve made some progress here: I have the control socket sending a resize message when I first create the new console! But I’ve gotten stuck on updating the socket. If you’ll permit a dissection of the code I’ve written to explain?

In restConsoleHandler I’ve added a function call within the control socket handler to resize the console:

// control socket handler
	handler := func(conn *websocket.Conn) {
		sendControlResize(conn, widthInt, heightInt)
		for {
			_, _, err = conn.ReadMessage()
			if err != nil {
				break
			}
		}
	}

This relies on the in-scope values for widthInt and heightInt. Here’s that function:

func sendControlResize(conn *websocket.Conn, width int, height int) {
	w, _ := conn.NextWriter(websocket.TextMessage)

	msg := api.ContainerExecControl{}
	msg.Command = "window-resize"
	msg.Args = make(map[string]string)
	msg.Args["width"] = strconv.Itoa(width)
	msg.Args["height"] = strconv.Itoa(height)

	buf, _ := json.Marshal(msg)
	_, _ = w.Write(buf)

	w.Close()
}

This works! Values that I send on the console start are passed through and the resulting console is sized correctly.

It’s the update attempt that fails. Following your advice, I tried creating a map of… well ha ha… both handlers and web socket connections (I’m like that dog in those “I have no idea what I’m doing” pictures).

type ControlSocketHandler func(conn *websocket.Conn)
var handlerMap = map[string]ControlSocketHandler{}
var connMap = map[string]*websocket.Conn{}

Maybe you can tell me which one you have in mind! I populate both maps with calls like these:

handlerMap[containerName] = handler
connMap[containerName] = conn

And this is where I run into WTF Land:

func restConsoleResizeHandler(w http.ResponseWriter, r *http.Request) {
	if r.Method != "POST" {
		http.Error(w, "Not implemented", 501)
		return
	}
	
	w.Header().Set("Access-Control-Allow-Origin", "*")

	name := r.FormValue("id")
	width := r.FormValue("width")
	height := r.FormValue("height")
	widthInt, _ := strconv.Atoi(width)
	heightInt, _ := strconv.Atoi(height)

	conn := connMap[name]
	handler := handlerMap[name]
	handler(conn)
}

Naw, that’s all wrong. But how the heck do I get that handler to fire, and with the right int values? :smiley:

Thanks Stephane. Thank you so much.

A.