Co-op: handle a dropped peer on the session tick, not the transport thread

src/SiegeFX.Core/Net/MpSession.cs

@@ -290,7 +290,7 @@ public void SendPvpHit(int target, float damage)
     public MpSession(ISessionTransport transport, bool isHost, string localName)
     {
         _t = transport; IsHost = isHost; _localName = localName ?? "Player";
-        _t.PeerDisconnected += OnPeerGone;
+        _t.PeerDisconnected += QueuePeerGone;
         if (IsHost) _roster[0] = new MpRosterEntry(0, _localName, true, 0);
     }
 
@@ -336,6 +336,13 @@ void RosterChanged()
         Broadcast(BuildRosterFrame());
     }
 
+    // Transports raise PeerDisconnected from their own threads (the LAN
+    // keepalive task, EOS callbacks). OnPeerGone fires OnPlayerLeft, whose
+    // handlers edit the render host's actor list, so it must run on the
+    // thread that ticks the session — queue it and drain it in Tick.
+    readonly System.Collections.Concurrent.ConcurrentQueue<int> _gonePeers = new();
+    void QueuePeerGone(int peer) => _gonePeers.Enqueue(peer);
+
     void OnPeerGone(int peer)
     {
         if (IsHost)
@@ -546,6 +553,8 @@ public void SendRecruitGrant(uint scid, int owningPlayer)
     /// Call once per engine frame with the frame dt.</summary>
     public void Tick(double dt)
     {
+        while (_gonePeers.TryDequeue(out int gone))
+            OnPeerGone(gone);
         SecondsSinceHost += dt;
         // The game clock runs while a game is under way and not paused.
         if (!Paused && (IsHost ? InProgress is not null : _joined)) GameSeconds += dt;
@@ -1226,7 +1235,7 @@ void RelayChat(int fromPlayer, string text)
 
     public void Dispose()
     {
-        _t.PeerDisconnected -= OnPeerGone;
+        _t.PeerDisconnected -= QueuePeerGone;
     }
 }
 

src/SiegeFX.Runtime/NetSelfTest.cs

@@ -198,6 +198,7 @@ void Assert(bool cond, string what)
                 "in-region: GameStart delivered region + difficulty to the client");
 
             c2.Dispose(); // client drops — host must be told player 1 left
+            hs.Tick(0);   // (on its next tick: transports report leaves off-thread)
             Assert(hostSawLeave == 1, $"in-region: host fired OnPlayerLeft on disconnect (got {hostSawLeave})");
             hs.Dispose(); cs.Dispose(); h2.Dispose();
         }
@@ -230,6 +231,7 @@ void Assert(bool cond, string what)
             hs.OnPlayerLeft = id => leftName = hostRoster?.FirstOrDefault(e => e.Player == id).Name;
             var before = hostRoster;
             c3.Dispose();
+            hs.Tick(0);   // leaves are handled on the next tick (transports report them off-thread)
             Assert(hostRoster is { Count: 1 } && before!.Count == 2 && hs.Roster.Count == 1 && leftName == "Joiner",
                 $"staging: a player who leaves drops out of the host's roster (left: '{leftName}')");
             hs.Dispose(); cs.Dispose(); h3.Dispose();

tests/SiegeFX.Tests/MpDisconnectTests.cs

@@ -0,0 +1,31 @@
+using SiegeFX.Core.Net;
+using Xunit;
+
+namespace SiegeFX.Tests;
+
+public class MpDisconnectTests
+{
+    [Fact]
+    public void ADisconnectFromATransportThreadIsHandledOnTick()
+    {
+        // The LAN keepalive drops a silent peer from a pool thread; the
+        // render host's OnPlayerLeft edits its actor list, so the session
+        // must hold the leave until the main thread ticks it.
+        var (h, c) = LoopbackTransport.CreatePair();
+        var host = new MpSession(h, isHost: true, "Host") { BuildSnapshot = () => [] };
+        var client = new MpSession(c, isHost: false, "Client");
+        client.SendJoinRequest();
+        for (int i = 0; i < 4; i++) { host.Tick(0); client.Tick(0); }
+
+        int left = -1, leftOnThread = -1, mainThread = Environment.CurrentManagedThreadId;
+        host.OnPlayerLeft = p => { left = p; leftOnThread = Environment.CurrentManagedThreadId; };
+
+        var t = new Thread(c.Dispose);
+        t.Start(); t.Join();
+        Assert.Equal(-1, left);
+
+        host.Tick(0);
+        Assert.Equal(1, left);
+        Assert.Equal(mainThread, leftOnThread);
+    }
+}