Add custom error page for BootFailedException

src/Umbraco.Web/Composing/ModuleInjector.cs

@@ -35,7 +35,16 @@ namespace Umbraco.Web.Composing
                 catch { /* don't make it worse */ }
 
                 if (runtimeState?.BootFailedException != null)
-                    BootFailedException.Rethrow(runtimeState.BootFailedException);
+                {
+                    // if we throw the exception here the HttpApplication.Application_Error method will never be hit
+                    // and our custom error page will not be shown, so we need to wait for the request to begin
+                    // before we throw the exception.
+                    context.BeginRequest += (sender, args) =>
+                    {
+                        BootFailedException.Rethrow(runtimeState.BootFailedException);
+                    };
+                    return;
+                }
 
                 // else... throw what we have
                 throw;

src/Umbraco.Web/UmbracoApplication.cs

@@ -1,7 +1,10 @@
-using System.Configuration;
+using System;
+using System.Configuration;
+using System.IO;
 using System.Threading;
 using System.Web;
 using Umbraco.Core;
+using Umbraco.Core.Exceptions;
 using Umbraco.Core.Logging;
 using Umbraco.Core.Logging.Serilog;
 using Umbraco.Core.Runtime;
@@ -23,6 +26,53 @@ namespace Umbraco.Web
             return runtime;
         }
 
+        protected override void OnApplicationError(object sender, EventArgs evargs)
+        {
+            base.OnApplicationError(sender, evargs);
+
+            // if the exception is a BootFailedException we want to show a custom 500 page
+            if (Server.GetLastError() is BootFailedException)
+            {
+                // if the requested file exists on disk, clear the error and return
+                // this is needed to serve static files
+                if (File.Exists(Request.PhysicalPath))
+                {
+                    Server.ClearError();
+                    return;
+                }
+
+                // if the application is in debug mode we don't want to show the custom 500 page
+                if (Context.IsDebuggingEnabled) return;
+
+                // find the error file to show
+                var fileName = GetBootErrorFileName();
+
+                // if the file doesn't exist we return and a YSOD will be shown
+                if (File.Exists(fileName) == false) return;
+
+                Response.TrySkipIisCustomErrors = true;
+                Server.ClearError();
+                Response.Clear();
+
+                Response.StatusCode = 500;
+                Response.ContentType = "text/html";
+                Response.WriteFile(fileName);
+
+                CompleteRequest();
+            }
+        }
+
+        /// <summary>
+        /// Returns the absolute filename to the BootException html file.
+        /// </summary>
+        protected virtual string GetBootErrorFileName()
+        {
+            var fileName = Server.MapPath("~/config/errors/BootFailed.html");
+            if (File.Exists(fileName)) return fileName;
+
+            return Server.MapPath("~/umbraco/views/errors/BootFailed.html");
+        }
+
         /// <summary>
         /// Returns a new MainDom
         /// </summary>

src/Umbraco.Web/UmbracoApplicationBase.cs

@@ -6,7 +6,6 @@ using System.Web.Hosting;
 using Umbraco.Core;
 using Umbraco.Core.Composing;
 using Umbraco.Core.Logging;
-using Umbraco.Core.Logging.Serilog;
 
 namespace Umbraco.Web
 {

src/Umbraco.Web/UmbracoInjectedModule.cs

@@ -357,15 +357,13 @@ namespace Umbraco.Web
                 // there's nothing we can do really
                 app.BeginRequest += (sender, args) =>
                 {
-                    // would love to avoid throwing, and instead display a customized Umbraco 500
-                    // page - however if we don't throw here, something else might go wrong, and
-                    // it's this later exception that would be reported. could not figure out how
-                    // to prevent it, either with httpContext.Response.End() or .ApplicationInstance
-                    // .CompleteRequest()
+                    // if we don't throw here, something else might go wrong,
+                    // and it's this later exception that would be reported.
 
                     // also, if something goes wrong with our DI setup, the logging subsystem may
                     // not even kick in, so here we try to give as much detail as possible
 
+                    // the exception is handled in UmbracoApplication which shows a custom error page
                     BootFailedException.Rethrow(Core.Composing.Current.RuntimeState.BootFailedException);
                 };
                 return;