package main

import (
	"flag"
	"io"
	"log"
	"net/http"
	"os"
	"os/exec"
	"time"

	"github.com/creack/pty"
	"github.com/gorilla/websocket"
)

var (
	addr = flag.String("addr", ":8080", "http service address")
)

const (
	// Time allowed to write a message to the peer.
	writeWait = 10 * time.Second

	// Maximum message size allowed from peer.
	maxMessageSize = 8192

	// Time allowed to read the next pong message from the peer.
	pongWait = 60 * time.Second

	// Send pings to peer with this period. Must be less than pongWait.
	pingPeriod = (pongWait * 9) / 10

	// Time to wait before force close on connection.
	closeGracePeriod = 10 * time.Second
)

var upgrader = websocket.Upgrader{
	ReadBufferSize:  1024,
	WriteBufferSize: 1024,
	// Allow all origins for now - we'll restrict this in production
	CheckOrigin: func(r *http.Request) bool {
		return true
	},
}

type terminal struct {
	cmd *exec.Cmd
	pty *os.File
}

func newTerminal() (*terminal, error) {
	// Start a new shell
	cmd := exec.Command("bash")
	ptmx, err := pty.Start(cmd)
	if err != nil {
		return nil, err
	}

	return &terminal{
		cmd: cmd,
		pty: ptmx,
	}, nil
}

func (t *terminal) Close() error {
	if t.pty != nil {
		t.pty.Close()
	}
	if t.cmd != nil {
		t.cmd.Process.Kill()
		t.cmd.Wait()
	}
	return nil
}

func pumpStdin(ws *websocket.Conn, w io.Writer) {
	defer ws.Close()
	ws.SetReadLimit(maxMessageSize)
	ws.SetReadDeadline(time.Now().Add(pongWait))
	ws.SetPongHandler(func(string) error { ws.SetReadDeadline(time.Now().Add(pongWait)); return nil })
	for {
		_, message, err := ws.ReadMessage()
		if err != nil {
			break
		}
		if _, err := w.Write(message); err != nil {
			break
		}
	}
}

func pumpStdout(ws *websocket.Conn, r io.Reader, done chan struct{}) {
	defer func() {
	}()
	buf := make([]byte, 1024)
	for {
		n, err := r.Read(buf)
		if err != nil {
			break
		}
		ws.SetWriteDeadline(time.Now().Add(writeWait))
		if err := ws.WriteMessage(websocket.TextMessage, buf[:n]); err != nil {
			ws.Close()
			break
		}
	}
	close(done)

	ws.SetWriteDeadline(time.Now().Add(writeWait))
	ws.WriteMessage(websocket.CloseMessage, websocket.FormatCloseMessage(websocket.CloseNormalClosure, ""))
	time.Sleep(closeGracePeriod)
	ws.Close()
}

func ping(ws *websocket.Conn, done chan struct{}) {
	ticker := time.NewTicker(pingPeriod)
	defer ticker.Stop()
	for {
		select {
		case <-ticker.C:
			if err := ws.WriteControl(websocket.PingMessage, []byte{}, time.Now().Add(writeWait)); err != nil {
				log.Println("ping:", err)
			}
		case <-done:
			return
		}
	}
}

func internalError(ws *websocket.Conn, msg string, err error) {
	log.Println(msg, err)
	ws.WriteMessage(websocket.TextMessage, []byte("Internal server error."))
}

func serveWs(w http.ResponseWriter, r *http.Request) {
	ws, err := upgrader.Upgrade(w, r, nil)
	if err != nil {
		log.Println("upgrade:", err)
		return
	}
	defer ws.Close()

	// Create new terminal
	term, err := newTerminal()
	if err != nil {
		internalError(ws, "start:", err)
		return
	}
	defer term.Close()

	stdoutDone := make(chan struct{})
	go pumpStdout(ws, term.pty, stdoutDone)
	go ping(ws, stdoutDone)

	pumpStdin(ws, term.pty)

	// Wait for stdout to finish
	<-stdoutDone
}

func serveHome(w http.ResponseWriter, r *http.Request) {
	if r.URL.Path != "/" {
		http.Error(w, "Not found", http.StatusNotFound)
		return
	}
	if r.Method != http.MethodGet {
		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
		return
	}
	http.ServeFile(w, r, "index.html")
}

func main() {
	flag.Parse()
	log.SetFlags(0)
	http.HandleFunc("/", serveHome)
	http.HandleFunc("/ws", serveWs)
	log.Fatal(http.ListenAndServe(*addr, nil))
}