Fix AspNetCore UseExceptionHandler applied on postbacks

AspNetCore reexecutes the same request with the original POST
method. If a DotVVM page is handling the error, we assumed that
we need to execute a postback in the Error page. The request body
was usually already consumed, and even if wasn't a matching command
wasnt found.

resolves #1700

src/Framework/Framework/Hosting/HostingConstants.cs

@@ -5,6 +5,7 @@ namespace DotVVM.Framework.Hosting
     public class HostingConstants
     {
         public const string DotvvmRequestContextKey = "dotvvm.requestContext";
+        public const string DotvvmIsErrorHandlingKey = "dotvvm.isErrorHandling";
 
         public const string GlobalizeCultureUrlPath = "dotvvmGlobalizeCulture";
         public const string GlobalizeCultureUrlIdParameter = "id";

src/Framework/Hosting.AspNetCore/Hosting/Middlewares/DotvvmMiddleware.cs

@@ -1,11 +1,13 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using DotVVM.Framework.Configuration;
 using DotVVM.Framework.Hosting.ErrorPages;
 using DotVVM.Framework.Hosting.Middlewares;
 using DotVVM.Framework.ResourceManagement;
+using DotVVM.Framework.Utils;
 using DotVVM.Framework.ViewModel.Serialization;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Localization;
@@ -40,8 +42,13 @@ public DotvvmMiddleware(RequestDelegate next, DotvvmConfiguration configuration,
         /// </summary>
         public async Task Invoke(HttpContext context)
         {
-            // create the context
-            var dotvvmContext = CreateDotvvmContext(context);
+            // If we are handling an error, assume that the request is Navigate, otherwise we are attempting to execute postbacks from a different page
+            var assumedRequestType =
+#if NET6_0_OR_GREATER
+                context.Features[typeof(Microsoft.AspNetCore.Diagnostics.IExceptionHandlerFeature)] is {} ? DotvvmRequestType.Navigate :
+#endif
+                (DotvvmRequestType?)null;
+            var dotvvmContext = CreateDotvvmContext(context, assumedRequestType);
             context.RequestServices.GetRequiredService<DotvvmRequestContextStorage>().Context = dotvvmContext;
             context.Items[HostingConstants.DotvvmRequestContextKey] = dotvvmContext;
 
@@ -101,14 +108,13 @@ public static IHttpContext ConvertHttpContext(HttpContext context)
             return httpContext;
         }
 
-        protected DotvvmRequestContext CreateDotvvmContext(HttpContext context)
-        {
-            return new DotvvmRequestContext(
+        protected DotvvmRequestContext CreateDotvvmContext(HttpContext context, DotvvmRequestType? requestType = null) =>
+            new DotvvmRequestContext(
                 ConvertHttpContext(context),
                 Configuration,
-                context.RequestServices
+                context.RequestServices,
+                requestType
             );
-        }
 
         public static bool IsInCurrentVirtualDirectory(IHttpContext context, ref string url)
         {

src/Templates/content/EmptyWeb/Pages/Error/ErrorViewModel.cs

@@ -34,7 +34,7 @@ public override Task Init()
             ExceptionType = "View called without IExceptionHandlerFeature";
             return base.Init();
         }
-        ExceptionType = exceptionInfo.Error.GetType().Name;
+        ExceptionType = exceptionInfo.Error.GetBaseException().GetType().Name;
         RequestId = aspcontext.TraceIdentifier;
         RequestPath = exceptionInfo.Path;
         return base.Init();