Some more logic to handle missing sub files and active file transfers when starting a round:

- server waits for transfers to finish before starting the round (up to a max 20 seconds, can be skipped by the host)
- clients enable the spectate button when the round starts (in case they fail to start the round due to a missing sub file or an error)
- clients notify the server if a transfer is cancelled

+ FileReceivers can't be instantiated if a server is running

Subsurface/Source/Networking/FileTransfer/FileReceiver.cs

@@ -154,6 +154,11 @@ namespace Barotrauma.Networking
 
         public FileReceiver(string downloadFolder)
         {
+            if (GameMain.Server != null)
+            {
+                throw new InvalidOperationException("Creating a file receiver is not allowed when a server is running.");
+            }
+
             activeTransfers = new List<FileTransferIn>();
 
             this.downloadFolder = downloadFolder;
@@ -161,6 +166,11 @@ namespace Barotrauma.Networking
         
         public void ReadMessage(NetIncomingMessage inc)
         {
+            if (GameMain.Server != null)
+            {
+                throw new InvalidOperationException("Receiving files when a server is running is not allowed");
+            }
+
             System.Diagnostics.Debug.Assert(!activeTransfers.Any(t => 
                 t.Status == FileTransferStatus.Error ||
                 t.Status == FileTransferStatus.Canceled ||

Subsurface/Source/Networking/FileTransfer/FileSender.cs

@@ -109,6 +109,11 @@ namespace Barotrauma.Networking
 
         private NetPeer peer;
 
+        public List<FileTransferOut> ActiveTransfers
+        {
+            get { return activeTransfers; }
+        }
+
         public FileSender(NetworkMember networkMember)
         {
             peer = networkMember.netPeer;

Subsurface/Source/Networking/GameClient.cs

@@ -629,6 +629,10 @@ namespace Barotrauma.Networking
         {
             if (Character != null) Character.Remove();
 
+            //enable spectate button in case we fail to start the round now
+            //(for example, due to a missing sub file or an error)
+            GameMain.NetLobbyScreen.ShowSpectateButton();
+
             Entity.Spawner.Clear();
             entityEventManager.Clear();
             LastSentEntityEventID = 0;
@@ -999,6 +1003,15 @@ namespace Barotrauma.Networking
             client.SendMessage(msg, NetDeliveryMethod.ReliableUnordered);
         }
 
+        public void CancelFileTransfer(FileReceiver.FileTransferIn transfer)
+        {
+            NetOutgoingMessage msg = client.CreateMessage();
+            msg.Write((byte)ClientPacketHeader.FILE_REQUEST);
+            msg.Write((byte)FileTransferMessageType.Cancel);
+            msg.Write((byte)transfer.SequenceChannel);
+            client.SendMessage(msg, NetDeliveryMethod.ReliableUnordered);
+        }
+
         private void OnFileReceived(FileReceiver.FileTransferIn transfer)
         {
             new GUIMessageBox("Download finished", "File \"" + transfer.FileName + "\" was downloaded succesfully.");
@@ -1062,8 +1075,9 @@ namespace Barotrauma.Networking
                         MathUtils.GetBytesReadable((long)transfer.Received) + " / " + MathUtils.GetBytesReadable((long)transfer.FileSize),
                         Color.White, null, 0, GUI.SmallFont);
 
-                    if (GUI.DrawButton(spriteBatch, new Rectangle((int)pos.X + 140, (int)pos.Y + 15, 60, 15), "Cancel", new Color(0.47f, 0.13f, 0.15f, 0.08f)))
+                    if (GUI.DrawButton(spriteBatch, new Rectangle((int)pos.X + 140, (int)pos.Y + 18, 60, 15), "Cancel", new Color(0.47f, 0.13f, 0.15f, 0.08f)))
                     {
+                        CancelFileTransfer(transfer);
                         fileReceiver.StopTransfer(transfer);
                     }
 
@@ -1210,12 +1224,12 @@ namespace Barotrauma.Networking
         public bool SpectateClicked(GUIButton button, object userData)
         {
             if (button != null) button.Enabled = false;
-
+            
             NetOutgoingMessage readyToStartMsg = client.CreateMessage();
             readyToStartMsg.Write((byte)ClientPacketHeader.RESPONSE_STARTGAME);
 
-            //correct sub & shuttle files found
-            //TODO: check if they're actually found
+            //assume we have the required sub files to start the round
+            //(if not, we'll find out when the server sends the STARTGAME message and can initiate a file transfer)
             readyToStartMsg.Write(true); 
 
             WriteCharacterInfo(readyToStartMsg);

Subsurface/Source/Networking/GameServer.cs

@@ -982,14 +982,32 @@ namespace Barotrauma.Networking
                 server.SendMessage(msg, connectedClients.Select(c => c.Connection).ToList(), NetDeliveryMethod.ReliableUnordered, 0);
 
                 //give the clients a few seconds to request missing sub/shuttle files before starting the round
-                float waitForResponseTimer = 3.0f;
+                float waitForResponseTimer = 5.0f;
                 while (connectedClients.Any(c => !c.ReadyToStart) && waitForResponseTimer > 0.0f)
                 {
                     waitForResponseTimer -= CoroutineManager.UnscaledDeltaTime;
                     yield return CoroutineStatus.Running;
                 }
 
-                //todo: wait until file transfers are finished/cancelled
+                if (fileSender.ActiveTransfers.Count > 0)
+                {
+                    var msgBox = new GUIMessageBox("", "Waiting for file transfers to finish before starting the round...", new string[] { "Start now" });
+                    msgBox.Buttons[0].OnClicked += msgBox.Close;
+
+                    float waitForTransfersTimer = 20.0f;
+                    while (fileSender.ActiveTransfers.Count > 0 && waitForTransfersTimer > 0.0f)
+                    {
+                        waitForTransfersTimer -= CoroutineManager.UnscaledDeltaTime;
+
+                        //message box close, break and start the round immediately
+                        if (!GUIMessageBox.MessageBoxes.Contains(msgBox))
+                        {
+                            break;
+                        }
+
+                        yield return CoroutineStatus.Running;
+                    }
+                }
             }
 
             GameMain.ShowLoading(StartGame(selectedSub, selectedShuttle, selectedMode), false);