fix(net): survive transient socket errors in the receive loop (2026-07-24 audit review)

WorldSession.NetReceiveLoop wrapped its entire while loop in a single
try/catch, so ANY non-timeout SocketException permanently killed the
background receive thread: the catch block at the loop's end was empty
(misattributed the error to "socket closed during shutdown"), and the
finally called _inboundQueue.Writer.TryComplete(), which silently and
irrecoverably stopped all inbound processing for the rest of the
session — no log line, no recovery path, and LiveSessionHost.Reconnect
has zero production callers to notice.

The realistic trigger is a well-known Windows UdpClient quirk: an ICMP
"port unreachable" reply to an EARLIER Send (e.g. against a stale ACE
session that already tore down its socket) surfaces as a
WSAECONNRESET SocketException on this socket's NEXT, completely
unrelated Receive call. NetClient.Receive already swallows the
expected SocketError.TimedOut heartbeat case; anything else reaching
WorldSession was a real, transient, per-datagram error being treated
as session-fatal.

Three changes, root-cause not a band-aid:

- NetClient's constructor now disables SIO_UDP_CONNRESET reporting on
  Windows, so a delayed ICMP error can't poison receives at all.
- NetReceiveLoop now catches SocketException PER ITERATION, logs it,
  and continues polling instead of exiting. The existing clean-shutdown
  paths (cancellation, ObjectDisposedException during Dispose) are
  unchanged — only the non-timeout-socket-error case that used to kill
  the loop is now recoverable.
- NetClient.Receive no longer calls the ReceiveTimeout setter (a
  setsockopt syscall) on every single call — only when the requested
  timeout differs from the last-applied value, cached in a new field.
  This was an unrelated but adjacent finding (4x/sec syscall churn at
  the 250ms heartbeat cadence) in the same audit.

No change to outbound wire behavior, ack cadence, heartbeat interval,
or datagram ordering — this is purely receive-loop resilience.

Tests: NetClientTests gained a SIO_UDP_CONNRESET construction smoke
test and two ReceiveTimeout-caching tests. A new
WorldSessionNetReceiveLoopResilienceTests drives the actual private
NetReceiveLoop method (via the existing internal
IWorldSessionTransport seam + reflection) with a scripted transport
that throws a non-timeout SocketException on the first call, proving
the loop survives it and keeps enqueueing subsequent datagrams — fully
deterministic, no real sockets. Full solution suite green: 3204/3206
Core, 3462/3465 App, 552/552 Core.Net (skips pre-existing).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
(cherry picked from commit c72ce028a927e15b8a54a86bbd0661a723dc84ca)

src/AcDream.Core.Net/NetClient.cs

@@ -21,11 +21,28 @@ public sealed class NetClient : IDisposable
     private readonly UdpClient _udp;
     private readonly IPEndPoint _remote;
 
+    /// <summary>Last <see cref="Socket.ReceiveTimeout"/> value applied to the
+    /// underlying socket, so repeated <see cref="Receive"/> calls with the
+    /// same timeout (every 250ms heartbeat tick) skip the setsockopt syscall.</summary>
+    private int _lastAppliedReceiveTimeoutMs = int.MinValue;
+
     public NetClient(IPEndPoint remote)
     {
         _remote = remote;
         // Bind to an OS-assigned local port; server will reply to it.
         _udp = new UdpClient(new IPEndPoint(IPAddress.Any, 0));
+
+        if (OperatingSystem.IsWindows())
+        {
+            // SIO_UDP_CONNRESET: Windows' UDP stack otherwise surfaces an
+            // ICMP "port unreachable" reply to an EARLIER Send (e.g. a stale
+            // server session that already tore down its socket) as a
+            // WSAECONNRESET SocketException on this socket's NEXT unrelated
+            // Receive call. An ICMP error from one send must not poison
+            // receiving for the rest of the session — disable that reporting.
+            const int SioUdpConnReset = -1744830452; // 0x9800000C
+            _udp.Client.IOControl((IOControlCode)SioUdpConnReset, new byte[] { 0 }, null);
+        }
     }
 
     /// <summary>The local endpoint the OS assigned us.</summary>
@@ -62,7 +79,12 @@ public sealed class NetClient : IDisposable
     /// </summary>
     public byte[]? Receive(TimeSpan timeout, out IPEndPoint? from)
     {
-        _udp.Client.ReceiveTimeout = (int)timeout.TotalMilliseconds;
+        int timeoutMs = (int)timeout.TotalMilliseconds;
+        if (timeoutMs != _lastAppliedReceiveTimeoutMs)
+        {
+            _udp.Client.ReceiveTimeout = timeoutMs;
+            _lastAppliedReceiveTimeoutMs = timeoutMs;
+        }
         try
         {
             IPEndPoint any = new(IPAddress.Any, 0);

src/AcDream.Core.Net/WorldSession.cs

@@ -973,6 +973,21 @@ public sealed class WorldSession : IDisposable
     /// tries again. On shutdown, <see cref="Dispose"/> cancels the token
     /// and joins the thread.
     /// </para>
+    ///
+    /// <para>
+    /// 2026-07-24 audit fix: a per-iteration <see cref="SocketException"/>
+    /// (anything other than the expected <see cref="SocketError.TimedOut"/>,
+    /// which <see cref="NetClient.Receive"/> already swallows) used to be
+    /// caught by an OUTER try/catch that wrapped the entire while loop,
+    /// so a single transient error — classically Windows' WSAECONNRESET,
+    /// delivered on this socket's next Receive after an earlier Send hit
+    /// an ICMP port-unreachable — exited the loop for good. The
+    /// <c>finally</c> then completed <see cref="_inboundQueue"/>'s writer,
+    /// permanently killing inbound processing for the rest of the session
+    /// with no log line. Non-timeout socket errors are recoverable,
+    /// per-datagram events, not session-fatal, so they're now caught
+    /// per-iteration, logged, and the loop keeps polling.
+    /// </para>
     /// </summary>
     private void NetReceiveLoop()
     {
@@ -980,13 +995,26 @@ public sealed class WorldSession : IDisposable
         {
             while (!_netCancel.Token.IsCancellationRequested)
             {
-                var bytes = _net.Receive(TimeSpan.FromMilliseconds(250), out _);
+                byte[]? bytes;
+                try
+                {
+                    bytes = _net.Receive(TimeSpan.FromMilliseconds(250), out _);
+                }
+                catch (System.Net.Sockets.SocketException ex)
+                {
+                    // Transient, recoverable socket error (e.g. WSAECONNRESET
+                    // from a stale peer's ICMP port-unreachable) — log and
+                    // keep receiving. Does NOT tear down _inboundQueue.
+                    Console.Error.WriteLine(
+                        $"[net] receive error (continuing): {ex.SocketErrorCode} {ex.Message}");
+                    continue;
+                }
+
                 if (bytes is not null)
                     _inboundQueue.Writer.TryWrite(bytes);
             }
         }
         catch (OperationCanceledException) { /* graceful shutdown */ }
-        catch (System.Net.Sockets.SocketException) { /* socket closed during shutdown */ }
         catch (ObjectDisposedException) { /* NetClient disposed before thread noticed */ }
         finally
         {