Throw unhandled exceptions during prerendering

Fixes: #8609

Currently exceptions thrown during prerendering are simply logged. This
change uses the existing *unhandled exception* mechanism of the
renderer/circuit to throw these. The result is that the developer
exception page just works for prerendering.

Author: rynowak

src/Components/Server/src/Circuits/CircuitPrerenderer.cs

@@ -1,7 +1,9 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
 using System.Collections.Generic;
+using System.Runtime.ExceptionServices;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Extensions;
@@ -26,8 +28,12 @@ public async Task<IEnumerable<string>> PrerenderComponentAsync(ComponentPrerende
                 GetFullUri(context.Request),
                 GetFullBaseUri(context.Request));
 
+            // We don't need to unsubscribe because the circuit host object is scoped to this call.
+            circuitHost.UnhandledException += CircuitHost_UnhandledException;
+
             // For right now we just do prerendering and dispose the circuit. In the future we will keep the circuit around and
-            // reconnect to it from the ComponentsHub.
+            // reconnect to it from the ComponentsHub. If we keep the circuit/renderer we also need to unsubscribe this error
+            // handler.
             try
             {
                 return await circuitHost.PrerenderComponentAsync(
@@ -40,6 +46,13 @@ public async Task<IEnumerable<string>> PrerenderComponentAsync(ComponentPrerende
             }
         }
 
+        private void CircuitHost_UnhandledException(object sender, UnhandledExceptionEventArgs e)
+        {
+            // Throw all exceptions encountered during pre-rendering so the default developer
+            // error page can respond.
+            ExceptionDispatchInfo.Capture((Exception)e.ExceptionObject).Throw();
+        }
+
         private string GetFullUri(HttpRequest request)
         {
             return UriHelper.BuildAbsolute(

src/Components/Server/src/Circuits/RemoteRenderer.cs

@@ -94,6 +94,8 @@ protected override void HandleException(Exception exception)
             {
                 Log.UnhandledExceptionRenderingComponent(_logger, exception);
             }
+
+            UnhandledException?.Invoke(this, exception);
         }
 
         /// <inheritdoc />

src/Mvc/test/Mvc.FunctionalTests/ComponentRenderingFunctionalTests.cs

@@ -1,12 +1,14 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
 using System.Net;
 using System.Net.Http;
 using System.Threading.Tasks;
 using AngleSharp.Parser.Html;
 using BasicWebSite;
 using BasicWebSite.Services;
+using Microsoft.AspNetCore.Hosting;
 using Microsoft.Extensions.DependencyInjection;
 using Xunit;
 
@@ -17,18 +19,17 @@ public class ComponentRenderingFunctionalTests : IClassFixture<MvcTestFixture<Ba
         public ComponentRenderingFunctionalTests(MvcTestFixture<BasicWebSite.StartupWithoutEndpointRouting> fixture)
         {
             Factory = fixture;
-            Client = Client ?? CreateClient(fixture);
         }
 
-        public HttpClient Client { get; }
-
         public MvcTestFixture<StartupWithoutEndpointRouting> Factory { get; }
 
         [Fact]
         public async Task Renders_BasicComponent()
         {
             // Arrange & Act
-            var response = await Client.GetAsync("http://localhost/components");
+            var client = CreateClient(Factory);
+
+            var response = await client.GetAsync("http://localhost/components");
 
             // Assert
             Assert.Equal(HttpStatusCode.OK, response.StatusCode);
@@ -41,9 +42,7 @@ public async Task Renders_BasicComponent()
         public async Task Renders_BasicComponent_UsingRazorComponents_Prerrenderer()
         {
             // Arrange & Act
-            var client = Factory
-                .WithWebHostBuilder(builder => builder.ConfigureServices(services => services.AddRazorComponents()))
-                .CreateClient();
+            var client = CreateClient(Factory, builder => builder.ConfigureServices(services => services.AddRazorComponents()));
 
             var response = await client.GetAsync("http://localhost/components");
 
@@ -58,7 +57,9 @@ public async Task Renders_BasicComponent_UsingRazorComponents_Prerrenderer()
         public async Task Renders_RoutingComponent()
         {
             // Arrange & Act
-            var response = await Client.GetAsync("http://localhost/components/routable");
+            var client = CreateClient(Factory, builder => builder.ConfigureServices(services => services.AddRazorComponents()));
+
+            var response = await client.GetAsync("http://localhost/components/routable");
 
             // Assert
             Assert.Equal(HttpStatusCode.OK, response.StatusCode);
@@ -71,9 +72,7 @@ public async Task Renders_RoutingComponent()
         public async Task Renders_RoutingComponent_UsingRazorComponents_Prerrenderer()
         {
             // Arrange & Act
-            var client = Factory
-                .WithWebHostBuilder(builder => builder.ConfigureServices(services => services.AddRazorComponents()))
-                .CreateClient();
+            var client = CreateClient(Factory, builder => builder.ConfigureServices(services => services.AddRazorComponents()));
 
             var response = await client.GetAsync("http://localhost/components/routable");
 
@@ -84,6 +83,21 @@ public async Task Renders_RoutingComponent_UsingRazorComponents_Prerrenderer()
             AssertComponent("\n    Router component\n<p>Routed successfully</p>\n", "Routing", content);
         }
 
+        [Fact]
+        public async Task Renders_ThrowingComponent_UsingRazorComponents_Prerrenderer()
+        {
+            // Arrange & Act
+            var client = CreateClient(Factory, builder => builder.ConfigureServices(services => services.AddRazorComponents()));
+
+            var response = await client.GetAsync("http://localhost/components/throws");
+
+            // Assert
+            Assert.Equal(HttpStatusCode.InternalServerError, response.StatusCode);
+            var content = await response.Content.ReadAsStringAsync();
+
+            Assert.Contains("InvalidTimeZoneException: test", content);
+        }
+
         [Fact]
         public async Task Renders_AsyncComponent()
         {
@@ -138,8 +152,8 @@ public async Task Renders_AsyncComponent()
     </table>
 
 ";
-
-            var response = await Client.GetAsync("http://localhost/components");
+            var client = CreateClient(Factory);
+            var response = await client.GetAsync("http://localhost/components");
 
             // Assert
             Assert.Equal(HttpStatusCode.OK, response.StatusCode);
@@ -164,12 +178,16 @@ private class LoopHttpHandler : DelegatingHandler
         {
         }
 
-        private HttpClient CreateClient(MvcTestFixture<BasicWebSite.StartupWithoutEndpointRouting> fixture)
+        private HttpClient CreateClient(MvcTestFixture<BasicWebSite.StartupWithoutEndpointRouting> fixture, Action<IWebHostBuilder> configure = null)
         {
             var loopHandler = new LoopHttpHandler();
 
             var client = fixture
-                .WithWebHostBuilder(builder => builder.ConfigureServices(ConfigureTestWeatherForecastService))
+                .WithWebHostBuilder(builder =>
+                {
+                    configure?.Invoke(builder);
+                    builder.ConfigureServices(ConfigureTestWeatherForecastService);
+                })
                 .CreateClient();
 
             // We configure the inner handler with a handler to this TestServer instance so that calls to the

src/Mvc/test/WebSites/BasicWebSite/Controllers/RazorComponentsController.cs

@@ -51,7 +51,7 @@ public class RazorComponentsController : Controller
             };
 
         [HttpGet("/components")]
-        [HttpGet("/components/routable")]
+        [HttpGet("/components/{component}")]
         public IActionResult Index()
         {
             return View();

src/Mvc/test/WebSites/BasicWebSite/RazorComponents/Throws.razor

@@ -0,0 +1,10 @@
+@page "/components/throws"
+    
+@* This is expected to throw and result in a 500 *@
+@functions { 
+    protected override async Task OnInitAsync()
+    {
+        await base.OnInitAsync();
+        throw new InvalidTimeZoneException("test");
+    }
+}