Use a fresh execution context for application logic

src/SignalR/common/Http.Connections/src/Internal/HttpConnectionContext.cs

@@ -42,6 +42,7 @@ internal sealed partial class HttpConnectionContext : ConnectionContext,
     private IDictionary<object, object?>? _items;
     private readonly CancellationTokenSource _connectionClosedTokenSource;
     private readonly CancellationTokenSource _connectionCloseRequested;
+    private ConnectionDelegate? _connectionDelegate;
 
     private CancellationTokenSource? _sendCts;
     private bool _activeSend;
@@ -551,12 +552,50 @@ private async Task ExecuteApplication(ConnectionDelegate connectionDelegate)
         // Verify some initialization invariants
         Debug.Assert(TransportType != HttpTransportType.None, "Transport has not been initialized yet");
 
-        // Jump onto the thread pool thread so blocking user code doesn't block the setup of the
+        // - Jump onto the thread pool thread so blocking user code doesn't block the setup of the
         // connection and transport
-        await Task.Yield();
+        // - Discard the current ExecutionContext and SynchronizationContext if any
 
-        // Running this in an async method turns sync exceptions into async ones
-        await connectionDelegate(this);
+        // Set the connection delegate here so we can avoid the closure allocation in Task.Run.
+        // We're still paying for the delegate allocation but this happens once per connection so there's no point caching.
+        _connectionDelegate = connectionDelegate;
+
+        if (!ExecutionContext.IsFlowSuppressed())
+        {
+            ExecutionContext.SuppressFlow();
+        }
+
+        await Task.Run(ExecuteApplication);
+
+        // No need to undo the flow suppression since we're in an async method.
+        // The suppression won't be observed outside of this call.
+    }
+
+    private async Task ExecuteApplication()
+    {
+        var connectionDelegate = _connectionDelegate!;
+
+        Debug.Assert(connectionDelegate is not null, "Connection delegate was not set!");
+
+        // Clear the field
+        _connectionDelegate = null;
+
+        if (_logger.IsEnabled(LogLevel.Critical))
+        {
+            // Preserve the connection id logging scope when we execute the connection delegate
+            var logScope = new ConnectionLogScope(ConnectionId);
+
+            // REVIEW: Should we create also create an activity here
+
+            using (_logger.BeginScope(logScope))
+            {
+                await connectionDelegate(this);
+            }
+        }
+        else
+        {
+            await connectionDelegate(this);
+        }
     }
 
     internal void StartSendCancellation()