Components: don't block the SignalR loop during init. Fixes #8274 (#8863)

src/Components/Server/src/Circuits/CircuitHost.cs

@@ -108,20 +108,31 @@ namespace Microsoft.AspNetCore.Components.Server.Circuits
         {
             await Renderer.InvokeAsync(async () =>
             {
-                SetCurrentCircuitHost(this);
+                try
-
-                for (var i = 0; i < Descriptors.Count; i++)
                 {
-                    var (componentType, domElementSelector) = Descriptors[i];
+                    SetCurrentCircuitHost(this);
-                    await Renderer.AddComponentAsync(componentType, domElementSelector);
+                    _initialized = true; // We're ready to accept incoming JSInterop calls from here on
+
+                    await OnCircuitOpenedAsync(cancellationToken);
+                    await OnConnectionUpAsync(cancellationToken);
+
+                    // We add the root components *after* the circuit is flagged as open.
+                    // That's because AddComponentAsync waits for quiescence, which can take
+                    // arbitrarily long. In the meantime we might need to be receiving and
+                    // processing incoming JSInterop calls or similar.
+                    for (var i = 0; i < Descriptors.Count; i++)
+                    {
+                        var (componentType, domElementSelector) = Descriptors[i];
+                        await Renderer.AddComponentAsync(componentType, domElementSelector);
+                    }
+                }
+                catch (Exception ex)
+                {
+                    // We have to handle all our own errors here, because the upstream caller
+                    // has to fire-and-forget this
+                    Renderer_UnhandledException(this, ex);
                 }
-
-                await OnCircuitOpenedAsync(cancellationToken);
-
-                await OnConnectionUpAsync(cancellationToken);
             });
-
-            _initialized = true;
         }
 
         public async void BeginInvokeDotNetFromJS(string callId, string assemblyName, string methodIdentifier, long dotNetObjectId, string argsJson)

src/Components/Server/src/ComponentHub.cs

@@ -64,7 +64,7 @@ namespace Microsoft.AspNetCore.Components.Server
         /// <summary>
         /// Intended for framework use only. Applications should not call this method directly.
         /// </summary>
-        public async Task<string> StartCircuit(string uriAbsolute, string baseUriAbsolute)
+        public string StartCircuit(string uriAbsolute, string baseUriAbsolute)
         {
             var circuitClient = new CircuitClientProxy(Clients.Caller, Context.ConnectionId);
 
@@ -76,8 +76,11 @@ namespace Microsoft.AspNetCore.Components.Server
 
             circuitHost.UnhandledException += CircuitHost_UnhandledException;
 
-            // If initialization fails, this will throw. The caller will fail if they try to call into any interop API.
+            // Fire-and-forget the initialization process, because we can't block the
-            await circuitHost.InitializeAsync(Context.ConnectionAborted);
+            // SignalR message loop (we'd get a deadlock if any of the initialization
+            // logic relied on receiving a subsequent message from SignalR), and it will
+            // take care of its own errors anyway.
+            _ = circuitHost.InitializeAsync(Context.ConnectionAborted);
 
             _circuitRegistry.Register(circuitHost);
 

src/Components/Server/test/Circuits/CircuitHostTest.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Text.Encodings.Web;
 using System.Threading;
 using System.Threading.Tasks;
@@ -81,6 +82,46 @@ namespace Microsoft.AspNetCore.Components.Server.Circuits
             handler2.VerifyAll();
         }
 
+        [Fact]
+        public async Task InitializeAsync_ReportsOwnAsyncExceptions()
+        {
+            // Arrange
+            var handler = new Mock<CircuitHandler>(MockBehavior.Strict);
+            var tcs = new TaskCompletionSource<object>();
+            var reportedErrors = new List<UnhandledExceptionEventArgs>();
+
+            handler
+                .Setup(h => h.OnCircuitOpenedAsync(It.IsAny<Circuit>(), It.IsAny<CancellationToken>()))
+                .Returns(tcs.Task)
+                .Verifiable();
+
+            var circuitHost = TestCircuitHost.Create(handlers: new[] { handler.Object });
+            circuitHost.UnhandledException += (sender, errorInfo) =>
+            {
+                Assert.Same(circuitHost, sender);
+                reportedErrors.Add(errorInfo);
+            };
+
+            // Act
+            var initializeAsyncTask = circuitHost.InitializeAsync(new CancellationToken());
+
+            // Assert: No synchronous exceptions
+            handler.VerifyAll();
+            Assert.Empty(reportedErrors);
+
+            // Act: Trigger async exception
+            var ex = new InvalidTimeZoneException();
+            tcs.SetException(ex);
+
+            // Assert: The top-level task still succeeds, because the intended usage
+            // pattern is fire-and-forget.
+            await initializeAsyncTask;
+
+            // Assert: The async exception was reported via the side-channel
+            Assert.Same(ex, reportedErrors.Single().ExceptionObject);
+            Assert.False(reportedErrors.Single().IsTerminating);
+        }
+
         [Fact]
         public async Task DisposeAsync_InvokesCircuitHandler()
         {