Support plain text exception formatting (#9342)

- If there's no Accept: text/html then print the exception.ToString as plaintext

src/Middleware/Diagnostics/src/DeveloperExceptionPage/DeveloperExceptionPageMiddleware.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
+using System.Text;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Diagnostics.Internal;
@@ -17,11 +18,12 @@ using Microsoft.Extensions.FileProviders;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 using Microsoft.Extensions.StackTrace.Sources;
+using Microsoft.Net.Http.Headers;
 
 namespace Microsoft.AspNetCore.Diagnostics
 {
     /// <summary>
-    /// Captures synchronous and asynchronous exceptions from the pipeline and generates HTML error responses.
+    /// Captures synchronous and asynchronous exceptions from the pipeline and generates error responses.
     /// </summary>
     public class DeveloperExceptionPageMiddleware
     {
@@ -32,6 +34,7 @@ namespace Microsoft.AspNetCore.Diagnostics
         private readonly DiagnosticSource _diagnosticSource;
         private readonly ExceptionDetailsProvider _exceptionDetailsProvider;
         private readonly Func<ErrorContext, Task> _exceptionHandler;
+        private static readonly MediaTypeHeaderValue _textPlainMediaType = new MediaTypeHeaderValue("text/html");
 
         /// <summary>
         /// Initializes a new instance of the <see cref="DeveloperExceptionPageMiddleware"/> class
@@ -127,13 +130,34 @@ namespace Microsoft.AspNetCore.Diagnostics
         // Assumes the response headers have not been sent.  If they have, still attempt to write to the body.
         private Task DisplayException(ErrorContext errorContext)
         {
-            var compilationException = errorContext.Exception as ICompilationException;
-            if (compilationException != null)
+            var httpContext = errorContext.HttpContext;
+            var headers = httpContext.Request.GetTypedHeaders();
+            var acceptHeader = headers.Accept;
+
+            // If the client does not ask for HTML just format the exception as plain text
+            if (acceptHeader == null || !acceptHeader.Any(h => h.IsSubsetOf(_textPlainMediaType)))
             {
-                return DisplayCompilationException(errorContext.HttpContext, compilationException);
+                httpContext.Response.ContentType = "text/plain";
+
+                var sb = new StringBuilder();
+                sb.AppendLine(errorContext.Exception.ToString());
+                sb.AppendLine();
+                sb.AppendLine("HEADERS");
+                sb.AppendLine("=======");
+                foreach (var pair in httpContext.Request.Headers)
+                {
+                    sb.AppendLine($"{pair.Key}: {pair.Value}");
+                }
+
+                return httpContext.Response.WriteAsync(sb.ToString());
             }
 
-            return DisplayRuntimeException(errorContext.HttpContext, errorContext.Exception);
+            if (errorContext.Exception is ICompilationException compilationException)
+            {
+                return DisplayCompilationException(httpContext, compilationException);
+            }
+
+            return DisplayRuntimeException(httpContext, errorContext.Exception);
         }
 
         private Task DisplayCompilationException(

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

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Net.Http.Headers;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
@@ -45,6 +46,58 @@ namespace Microsoft.AspNetCore.Diagnostics
             Assert.Null(listener.DiagnosticHandledException?.Exception);
         }
 
+        [Fact]
+        public async Task ErrorPageWithAcceptHeaderForHtmlReturnsHtml()
+        {
+            // Arrange
+            var builder = new WebHostBuilder()
+                .Configure(app =>
+                {
+                    app.UseDeveloperExceptionPage();
+                    app.Run(context =>
+                    {
+                        throw new Exception("Test exception");
+                    });
+                });
+            var server = new TestServer(builder);
+
+            // Act
+            var client = server.CreateClient();
+            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("text/html"));
+            var response = await client.GetAsync("/path");
+
+            // Assert
+            var responseText = await response.Content.ReadAsStringAsync();
+            Assert.Equal("text/html", response.Content.Headers.ContentType.MediaType);
+            Assert.Contains("<html", responseText);
+            Assert.Contains("Test exception", responseText);
+        }
+
+        [Fact]
+        public async Task ErrorPageWithoutAcceptHeaderForHtmlReturnsPlainText()
+        {
+            // Arrange
+            var builder = new WebHostBuilder()
+                .Configure(app =>
+                {
+                    app.UseDeveloperExceptionPage();
+                    app.Run(context =>
+                    {
+                        throw new Exception("Test exception");
+                    });
+                });
+            var server = new TestServer(builder);
+
+            // Act
+            var response = await server.CreateClient().GetAsync("/path");
+
+            // Assert
+            var responseText = await response.Content.ReadAsStringAsync();
+            Assert.Equal("text/plain", response.Content.Headers.ContentType.MediaType);
+            Assert.Contains("Test exception", responseText);
+            Assert.DoesNotContain("<html", responseText);
+        }
+
         [Fact]
         public async Task ExceptionPageFiltersAreApplied()
         {

src/Mvc/test/Mvc.FunctionalTests/ErrorPageTests.cs

@@ -39,6 +39,8 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
             Client = factory
                 .WithWebHostBuilder(builder => builder.ConfigureLogging(l => l.Services.AddSingleton<ILoggerFactory>(loggerProvider)))
                 .CreateDefaultClient();
+            // These tests want to verify runtime compilation and formatting in the HTML of the error page
+            Client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("text/html"));
         }
 
         public HttpClient Client { get; }