lobby: replace lock with channel based semaphore

previously a race condition when starting a game
allowed the lobby loop to run into a draw call
trying to acquire the lobby lock, while a different
routine was holding the lock and waiting for this loop
to terminate.

only pass the termination check if the lobby lock for
the draw call can be acquired.

gameLobby/gamelobby.go

@@ -4,7 +4,6 @@ import (
 	tg "fahrradwurstmond2.0/telnet_gui"
 	common "fahrradwurstmond2.0/common"
 	game "fahrradwurstmond2.0/game"
-	"sync"
 	"fmt"
 )
 
@@ -24,7 +23,7 @@ type Player struct {
 }
 
 type Lobby struct {
-	lock    *sync.Mutex
+	lock    chan struct{}
 	agent   *Player
 	Players map[string]*Player
 	playerOrder []string
@@ -32,18 +31,18 @@ type Lobby struct {
 
 func NewLobby() (l *Lobby) {
 	return &Lobby{
-		lock:    &sync.Mutex{},
+		lock:    make(chan struct{}, 1),
 		Players: make(map[string]*Player),
 		playerOrder: make([]string, 0),
 	}
 }
 
 func (l *Lobby) Lock() {
-	l.lock.Lock()
+	l.lock<-struct{}{}
 }
 
 func (l *Lobby) Unlock() {
-	l.lock.Unlock()
+	<-l.lock
 }
 
 func (l *Lobby) Register(glp *common.Player) {
@@ -164,19 +163,18 @@ func (l *Lobby) Loop(p *Player) {
 		select { // check kill before doing anything that may lock
 		case <-p.Kill:
 			return
-		default:
+		case l.lock<-struct{}{}:
+			p.drawLobby(l)
+			l.Unlock()
+			p.Scr.Update()
 		}
-		p.DrawLobby(l)
-		p.Scr.Update()
 	}
 }
 
-func (p *Player) DrawLobby(l *Lobby) {
+func (p *Player) drawLobby(l *Lobby) {
 	line := 0
 	fg := tg.ColorDefault
 	bg := tg.ColorDefault
-	l.Lock()
-	defer l.Unlock()
 	v := p.playerListv
 	v.Clear()
 	v.DrawTextClearLine(fg, bg, []byte(fmt.Sprintf("   Seats:")), 0, line)