[QUIC] Fixed Assert (#72022)

src/libraries/System.Net.Quic/src/System/Net/Quic/Implementations/MsQuic/MsQuicConnection.cs

@@ -47,6 +47,7 @@ internal sealed class State
 
             // These exists to prevent GC of the MsQuicConnection in the middle of an async op (Connect or Shutdown).
             public MsQuicConnection? Connection;
+            public bool ShutdownInProgress;
 
             public readonly ValueTaskSource ConnectTcs = new ValueTaskSource();
             // TODO: only allocate these when there is an outstanding shutdown.
@@ -215,7 +216,10 @@ private static unsafe int HandleEventConnected(State state, ref QUIC_CONNECTION_
             //state.Connection._remoteEndPoint = MsQuicParameterHelpers.GetIPEndPointParam(MsQuicApi.Api, state.Handle, QUIC_PARAM_CONN_REMOTE_ADDRESS);
             state.Connection._localEndPoint = MsQuicParameterHelpers.GetIPEndPointParam(MsQuicApi.Api, state.Handle, QUIC_PARAM_CONN_LOCAL_ADDRESS);
             state.Connection._negotiatedAlpnProtocol = new SslApplicationProtocol(new Span<byte>(connectionEvent.CONNECTED.NegotiatedAlpn, connectionEvent.CONNECTED.NegotiatedAlpnLength).ToArray());
-            state.Connection = null;
+            if (!state.ShutdownInProgress)
+            {
+                state.Connection = null;
+            }
 
             state.ConnectTcs.TrySetResult();
 
@@ -227,7 +231,10 @@ private static int HandleEventShutdownInitiatedByTransport(State state, ref QUIC
             if (!state.ConnectTcs.IsCompleted)
             {
                 Debug.Assert(state.Connection != null);
-                state.Connection = null;
+                if (!state.ShutdownInProgress)
+                {
+                    state.Connection = null;
+                }
 
                 state.ConnectTcs.TrySetException(new MsQuicException(connectionEvent.SHUTDOWN_INITIATED_BY_TRANSPORT.Status, "Connection has been shutdown by transport"));
             }
@@ -254,6 +261,7 @@ private static int HandleEventShutdownComplete(State state, ref QUIC_CONNECTION_
             state.StateGCHandle.Free();
 
             state.Connection = null;
+            state.ShutdownInProgress = false;
 
             state.ShutdownTcs.SetResult(QUIC_STATUS_SUCCESS);
 
@@ -595,7 +603,7 @@ internal unsafe ValueTask FinishHandshakeAsync(QuicServerConnectionOptions optio
             long ErrorCode)
         {
             // Store the connection into the GCHandle'd state to prevent GC if user calls ShutdownAsync and gets rid of all references to the MsQuicConnection.
-            Debug.Assert(_state.Connection == null);
+            _state.ShutdownInProgress = true;
             _state.Connection = this;
 
             try
@@ -608,6 +616,7 @@ internal unsafe ValueTask FinishHandshakeAsync(QuicServerConnectionOptions optio
             }
             catch
             {
+                _state.ShutdownInProgress = false;
                 _state.Connection = null;
                 throw;
             }