Prevent form handling logic to run during exception handling

Author: javiercn

src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs

@@ -7,6 +7,7 @@
 using System.Text.Encodings.Web;
 using Microsoft.AspNetCore.Antiforgery;
 using Microsoft.AspNetCore.Components.Endpoints.Rendering;
+using Microsoft.AspNetCore.Diagnostics;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.WebUtilities;
 using Microsoft.Extensions.DependencyInjection;
@@ -128,7 +129,12 @@ await EndpointHtmlRenderer.InitializeStandardComponentServicesAsync(
 
     private async Task<RequestValidationState> ValidateRequestAsync(HttpContext context, IAntiforgery? antiforgery)
     {
-        var isPost = HttpMethods.IsPost(context.Request.Method);
+        var isPost = HttpMethods.IsPost(context.Request.Method) &&
+            // Disable POST functionality during exception handling.
+            // The exception handler middleware will not update the request method, and we don't
+            // want to run the form handling logic against the error page.
+            context.Features.Get<IExceptionHandlerFeature>() == null;
+
         if (isPost)
         {
             var valid = false;

src/Components/test/E2ETest/ServerRenderingTests/FormHandlingTests/FormWithParentBindingContextTest.cs

@@ -1119,15 +1119,13 @@ private void AssertHasInternalServerError(bool suppressedEnhancedNavigation, boo
         {
             Browser.True(() => Browser.FindElement(By.TagName("html")).Text.Contains("There was an unhandled exception on the current request"));
         }
-        else if (suppressedEnhancedNavigation)
-        {
-            // Chrome's built-in error UI for a 500 response when there's no response content
-            Browser.Exists(By.Id("main-frame-error"));
-        }
         else
         {
-            // The UI generated by enhanced nav when there's no response content
-            Browser.Contains("Error: 500", () => Browser.Exists(By.TagName("html")).Text);
+            // Displays the error page from the exception handler
+            Assert.Collection(
+                Browser.FindElements(By.CssSelector(".text-danger")),
+                item => Assert.Equal("Error.", item.Text),
+                item => Assert.Equal("An error occurred while processing your request.", item.Text));
         }
     }