Fix canceling flush of CancelInvocation when canceling server-to-client stream (#46870)

Author: BrennanConroy

src/SignalR/clients/csharp/Client.Core/src/HubConnection.cs

@@ -641,7 +641,8 @@ async Task OnStreamCanceled(InvocationRequest irq)
                 {
                     Log.SendingCancellation(_logger, irq.InvocationId);
 
-                    await SendHubMessage(_state.CurrentConnectionStateUnsynchronized, new CancelInvocationMessage(irq.InvocationId), irq.CancellationToken).ConfigureAwait(false);
+                    // Don't pass irq.CancellationToken, that would result in canceling the Flush and a delayed CancelInvocationMessage being sent.
+                    await SendHubMessage(_state.CurrentConnectionStateUnsynchronized, new CancelInvocationMessage(irq.InvocationId), cancellationToken: default).ConfigureAwait(false);
                 }
                 else
                 {

src/SignalR/clients/csharp/Client/test/UnitTests/HubConnectionTests.cs

@@ -307,6 +307,40 @@ await connection.ReceiveJsonMessage(
         }
     }
 
+    [Fact]
+    public async Task CanCancelTokenDuringStream_SendsCancelInvocation()
+    {
+        using (StartVerifiableLog())
+        {
+            var connection = new TestConnection();
+            var hubConnection = CreateHubConnection(connection, loggerFactory: LoggerFactory);
+
+            await hubConnection.StartAsync().DefaultTimeout();
+
+            using var cts = new CancellationTokenSource();
+            var asyncEnumerable = hubConnection.StreamAsync<int>("Stream", 1, cts.Token);
+
+            await using var e = asyncEnumerable.GetAsyncEnumerator(cts.Token);
+            var task = e.MoveNextAsync();
+
+            var item = await connection.ReadSentJsonAsync().DefaultTimeout();
+            var invocationId = item["invocationId"];
+            await connection.ReceiveJsonMessage(
+                new { type = HubProtocolConstants.StreamItemMessageType, invocationId, item = 1 }
+                ).DefaultTimeout();
+
+            await task.DefaultTimeout();
+            cts.Cancel();
+
+            item = await connection.ReadSentJsonAsync().DefaultTimeout();
+            Assert.Equal(HubProtocolConstants.CancelInvocationMessageType, item["type"]);
+            Assert.Equal(invocationId, item["invocationId"]);
+
+            // Stream on client-side completes on cancellation
+            await Assert.ThrowsAsync<TaskCanceledException>(async () => await e.MoveNextAsync()).DefaultTimeout();
+        }
+    }
+
     [Fact]
     public async Task ConnectionTerminatedIfServerTimeoutIntervalElapsesWithNoMessages()
     {