Add helpful log message and exception when exception handler returns 404

src/Middleware/Diagnostics/src/DiagnosticsLoggerExtensions.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using Microsoft.AspNetCore.Builder;
 using Microsoft.Extensions.Logging;
 
 namespace Microsoft.AspNetCore.Diagnostics
@@ -20,7 +21,9 @@ internal static class DiagnosticsLoggerExtensions
             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.");
+            LoggerMessage.Define(LogLevel.Warning, new EventId(4, "HandlerNotFound"), "The exception handler configured on ExceptionHandlerOptions produced a 404 status response. " +
+                "An InvalidOperationException containing the original exception will be thrown since this is often due to a misconfigured ExceptionHandlerOptions.ExceptionHandlingPath. " +
+                "If the exception handler is expected to return 404 status responses then set ExceptionHandlerOptions.AllowStatusCode404Response to true.");
 
         // DeveloperExceptionPageMiddleware
         private static readonly Action<ILogger, Exception?> _responseStartedErrorPageMiddleware =

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

@@ -143,6 +143,8 @@ private async Task HandleException(HttpContext context, ExceptionDispatchInfo ed
                 }
 
                 _logger.ErrorHandlerNotFound();
+
+                edi = ExceptionDispatchInfo.Capture(new InvalidOperationException($"No exception handler was found, see inner exception for details of original exception. If an exception should not be thrown for 404 responses, set {nameof(ExceptionHandlerOptions.AllowStatusCode404Response)} to true.", edi.SourceException));
             }
             catch (Exception ex2)
             {
@@ -154,7 +156,7 @@ private async Task HandleException(HttpContext context, ExceptionDispatchInfo ed
                 context.Request.Path = originalPath;
             }
 
-            edi.Throw(); // Re-throw the original if we couldn't handle it
+            edi.Throw(); // Re-throw wrapped exception or the original if we couldn't handle it
         }
 
         private static void ClearHttpContext(HttpContext context)

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

@@ -469,7 +469,7 @@ public void UsingExceptionHandler_ThrowsAnException_WhenExceptionHandlingPathNot
         }
 
         [Fact]
-        public async Task ExceptionHandlerNotFound_RethrowsOriginalError()
+        public async Task ExceptionHandlerNotFound_ThrowsIOEWithOriginalError()
         {
             var sink = new TestSink(TestSink.EnableWithTypeName<ExceptionHandlerMiddleware>);
             var loggerFactory = new TestLoggerFactory(sink, enabled: true);
@@ -500,9 +500,14 @@ public async Task ExceptionHandlerNotFound_RethrowsOriginalError()
                                 httpContext.Response.StatusCode = StatusCodes.Status500InternalServerError;
                             }
 
-                            // The original exception is thrown
+                            // Invalid operation exception
                             Assert.NotNull(exception);
-                            Assert.Equal("Something bad happened.", exception.Message);
+                            Assert.Equal("No exception handler was found, see inner exception for details of original exception. If an exception should not be thrown for 404 responses, set AllowStatusCode404Response to true.", exception.Message);
+
+                            // The original exception is inner exception
+                            Assert.NotNull(exception.InnerException);
+                            Assert.IsType<ApplicationException>(exception.InnerException);
+                            Assert.Equal("Something bad happened.", exception.InnerException.Message);
 
                         });
 
@@ -520,7 +525,7 @@ public async Task ExceptionHandlerNotFound_RethrowsOriginalError()
                         {
                             innerAppBuilder.Run(httpContext =>
                             {
-                                throw new InvalidOperationException("Something bad happened.");
+                                throw new ApplicationException("Something bad happened.");
                             });
                         });
                     });
@@ -539,7 +544,9 @@ public async Task ExceptionHandlerNotFound_RethrowsOriginalError()
             Assert.Contains(sink.Writes, w =>
                 w.LogLevel == LogLevel.Warning
                 && w.EventId == 4
-                && w.Message == "No exception handler was found, rethrowing original exception.");
+                && w.Message == $"The exception handler configured on ExceptionHandlerOptions produced a 404 status response. " +
+                $"An InvalidOperationException containing the original exception will be thrown since this is often due to a misconfigured ExceptionHandlingPath. " +
+                $"If the exception handler should be allowed to return 404 status responses, AllowStatusCode404Response must be set to true.");
         }
 
         [Fact]