Fix WebSocket close single-consumer violation

src/libraries/System.Net.WebSockets.Client/tests/CloseTest.cs

@@ -422,9 +422,13 @@ protected async Task RunClient_CloseAsync_DuringConcurrentReceiveAsync_ExpectedS
 
                 await cws.CloseAsync(WebSocketCloseStatus.NormalClosure, "", CancellationToken.None);
 
-                // There is a race condition in the above.  If the ReceiveAsync receives the sent close message from the server,
-                // then it will complete successfully and the socket will close successfully.  If the CloseAsync receive the sent
-                // close message from the server, then the receive async will end up getting aborted along with the socket.
+                // The outcome depends on timing. Two outcomes are acceptable:
+                //   1. The pending ReceiveAsync picks up the server's close frame cleanly: it completes with a Close
+                //      message and the socket transitions to Closed.
+                //   2. The close handshake triggers an internal Abort (e.g. WaitForServerToCloseConnectionAsync
+                //      times out because the server doesn't close TCP within its 1s window): state becomes Aborted,
+                //      the parked receive's stream read fails, and ReceiveAsyncPrivate translates the exception to
+                //      OperationCanceledException because of the Aborted state.
                 try
                 {
                     await t;

src/libraries/System.Net.WebSockets/src/System/Net/WebSockets/ManagedWebSocket.cs

@@ -82,6 +82,8 @@ internal sealed partial class ManagedWebSocket : WebSocket
         private bool _sentCloseFrame;
         /// <summary>Whether we've ever received a close frame.</summary>
         private bool _receivedCloseFrame;
+        /// <summary>0 if <see cref="WaitForServerToCloseConnectionAsync"/> has not been invoked yet; 1 otherwise.</summary>
+        private int _waitedForServerClose;
         /// <summary>The reason for the close, as sent by the server, or null if not yet closed.</summary>
         private WebSocketCloseStatus? _closeStatus;
         /// <summary>A description of the close reason as sent by the server, or null if not yet closed.</summary>
@@ -1123,9 +1125,18 @@ private async ValueTask HandleReceivedCloseAsync(MessageHeader header, Cancellat
             }
         }
 
-        /// <summary>Issues a read on the stream to wait for EOF.</summary>
+        /// <summary>Issues a read on the stream to wait for EOF. Idempotent across call sites.</summary>
         private async ValueTask WaitForServerToCloseConnectionAsync(CancellationToken cancellationToken)
         {
+            // Called from both the receive and the send path. In rare concurrent Close + Receive
+            // scenarios both paths can observe the close handshake as complete and would otherwise
+            // both issue a stream read (violating most Stream implementations' single-consumer invariant).
+            // The flag guarantees that only the first invocation performs the wait; the other is a no-op.
+            if (Interlocked.CompareExchange(ref _waitedForServerClose, 1, 0) != 0)
+            {
+                return;
+            }
+
             if (NetEventSource.Log.IsEnabled()) NetEventSource.Trace(this);
 
             // Per RFC 6455 7.1.1, try to let the server close the connection.  We give it up to a second.