Throw original exception if exception handler is not found

src/Middleware/Diagnostics/src/DiagnosticsLoggerExtensions.cs

@@ -1,4 +1,4 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// 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;
@@ -19,6 +19,9 @@ internal static class DiagnosticsLoggerExtensions
         private static readonly Action<ILogger, Exception> _errorHandlerException =
             LoggerMessage.Define(LogLevel.Error, new EventId(3, "Exception"), "An exception was thrown attempting to execute the error handler.");
 
+        private static readonly Action<ILogger, Exception> _errorHandlerNotFound =
+            LoggerMessage.Define(LogLevel.Warning, new EventId(4, "HandlerNotFound"), "No exception handler was found, rethrowing original exception.");
+
         // DeveloperExceptionPageMiddleware
         private static readonly Action<ILogger, Exception> _responseStartedErrorPageMiddleware =
             LoggerMessage.Define(LogLevel.Warning, new EventId(2, "ResponseStarted"), "The response has already started, the error page middleware will not be executed.");
@@ -41,6 +44,11 @@ public static void ErrorHandlerException(this ILogger logger, Exception exceptio
             _errorHandlerException(logger, exception);
         }
 
+        public static void ErrorHandlerNotFound(this ILogger logger)
+        {
+            _errorHandlerNotFound(logger, null);
+        }
+
         public static void ResponseStartedErrorPageMiddleware(this ILogger logger)
         {
             _responseStartedErrorPageMiddleware(logger, null);

src/Middleware/Diagnostics/src/ExceptionHandler/ExceptionHandlerMiddleware.cs

@@ -113,18 +113,22 @@ private async Task HandleException(HttpContext context, ExceptionDispatchInfo ed
                 };
                 context.Features.Set<IExceptionHandlerFeature>(exceptionHandlerFeature);
                 context.Features.Set<IExceptionHandlerPathFeature>(exceptionHandlerFeature);
-                context.Response.StatusCode = 500;
+                context.Response.StatusCode = StatusCodes.Status500InternalServerError;
                 context.Response.OnStarting(_clearCacheHeadersDelegate, context.Response);
 
                 await _options.ExceptionHandler(context);
 
-                if (_diagnosticListener.IsEnabled() && _diagnosticListener.IsEnabled("Microsoft.AspNetCore.Diagnostics.HandledException"))
+                if (context.Response.StatusCode != StatusCodes.Status404NotFound)
                 {
-                    _diagnosticListener.Write("Microsoft.AspNetCore.Diagnostics.HandledException", new { httpContext = context, exception = edi.SourceException });
+                    if (_diagnosticListener.IsEnabled() && _diagnosticListener.IsEnabled("Microsoft.AspNetCore.Diagnostics.HandledException"))
+                    {
+                        _diagnosticListener.Write("Microsoft.AspNetCore.Diagnostics.HandledException", new { httpContext = context, exception = edi.SourceException });
+                    }
+
+                    return;
                 }
 
-                // TODO: Optional re-throw? We'll re-throw the original exception by default if the error handler throws.
-                return;
+                _logger.ErrorHandlerNotFound();
             }
             catch (Exception ex2)
             {

src/Middleware/Diagnostics/test/UnitTests/ExceptionHandlerTest.cs

@@ -7,17 +7,17 @@
 using System.IO;
 using System.Linq;
 using System.Net;
-using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Http;
-using Microsoft.AspNetCore.Http.Features;
 using Microsoft.AspNetCore.TestHost;
 using Microsoft.AspNetCore.Testing;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Testing;
 using Xunit;
 
 namespace Microsoft.AspNetCore.Diagnostics
@@ -469,5 +469,79 @@ public void UsingExceptionHandler_ThrowsAnException_WhenExceptionHandlingPathNot
                 "Alternatively, set one of the aforementioned properties in 'Startup.ConfigureServices' as follows: 'services.AddExceptionHandler(options => { ... });'.",
                 exception.Message);
         }
+
+        [Fact]
+        public async Task ExceptionHandlerNotFound_RethrowsOriginalError()
+        {
+            var sink = new TestSink(TestSink.EnableWithTypeName<ExceptionHandlerMiddleware>);
+            var loggerFactory = new TestLoggerFactory(sink, enabled: true);
+
+            using var host = new HostBuilder()
+                .ConfigureWebHost(webHostBuilder =>
+                {
+                    webHostBuilder
+                    .UseTestServer()
+                    .ConfigureServices(services =>
+                    {
+                        services.AddSingleton<ILoggerFactory>(loggerFactory);
+                    })
+                    .Configure(app =>
+                    {
+                        app.Use(async (httpContext, next) =>
+                        {
+                            Exception exception = null;
+                            try
+                            {
+                                await next();
+                            }
+                            catch (InvalidOperationException ex)
+                            {
+                                exception = ex;
+
+                                // This mimics what the server would do when an exception occurs
+                                httpContext.Response.StatusCode = StatusCodes.Status500InternalServerError;
+                            }
+
+                            // The original exception is thrown
+                            Assert.NotNull(exception);
+                            Assert.Equal("Something bad happened.", exception.Message);
+
+                        });
+
+                        app.UseExceptionHandler("/non-existent-hander");
+
+                        app.Map("/handle-errors", (innerAppBuilder) =>
+                        {
+                            innerAppBuilder.Run(async (httpContext) =>
+                            {
+                                await httpContext.Response.WriteAsync("Handled error in a custom way.");
+                            });
+                        });
+
+                        app.Map("/throw", (innerAppBuilder) =>
+                        {
+                            innerAppBuilder.Run(httpContext =>
+                            {
+                                throw new InvalidOperationException("Something bad happened.");
+                            });
+                        });
+                    });
+                }).Build();
+
+            await host.StartAsync();
+
+            using (var server = host.GetTestServer())
+            {
+                var client = server.CreateClient();
+                var response = await client.GetAsync("throw");
+                Assert.Equal(HttpStatusCode.InternalServerError, response.StatusCode);
+                Assert.Equal(string.Empty, await response.Content.ReadAsStringAsync());
+            }
+
+            Assert.Contains(sink.Writes, w =>
+                w.LogLevel == LogLevel.Warning
+                && w.EventId == 4
+                && w.Message == "No exception handler was found, rethrowing original exception.");
+        }
     }
 }