Il2Cpp: Redirect stderr on Windows to log CoreCLR crash messages

On Windows, emit BepInEx/ErrorLog.log that will contain the latest stderr
This allows to catch any errors CoreCLR writes before closing on a fatal crash

On Unix, same is already possible via normal piping

BepInEx.IL2CPP/Preloader.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Reflection;
 using System.Runtime.InteropServices;
+using BepInEx.IL2CPP.RuntimeFixes;
 using BepInEx.Logging;
 using BepInEx.Preloader.Core;
 using BepInEx.Preloader.Core.Logging;
@@ -38,6 +39,8 @@ public static class Preloader
                 Logger.Listeners.Add(new ConsoleLogListener());
             }
 
+            RedirectStdErrFix.Apply();
+
             ChainloaderLogHelper.PrintLogInfo(Log);
 
             Logger.Log(LogLevel.Info, $"Running under Unity {UnityInfo.Version}");

BepInEx.IL2CPP/RuntimeFixes/RedirectStdErrFix.cs

@@ -0,0 +1,49 @@
+using System.IO;
+using System.Runtime.InteropServices;
+using BepInEx.Logging;
+using MonoMod.Utils;
+
+namespace BepInEx.IL2CPP.RuntimeFixes;
+
+internal static class RedirectStdErrFix
+{
+    private const int STD_ERROR_HANDLE = -12;
+    private const int INVALID_HANDLE_VALUE = -1;
+    private const int FILE_SHARE_READ = 1;
+    private const int GENERIC_WRITE = 0x40000000;
+    private const int CREATE_ALWAYS = 2;
+    private const int FILE_ATTRIBUTE_NORMAL = 0x00000080;
+
+    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
+    private static extern nint CreateFile(string fileName,
+                                          uint desiredAccess,
+                                          int shareMode,
+                                          nint securityAttributes,
+                                          int creationDisposition,
+                                          int flagsAndAttributes,
+                                          nint templateFile);
+
+    [DllImport("kernel32.dll", SetLastError = true)]
+    private static extern bool SetStdHandle(int nStdHandle, nint hConsoleOutput);
+
+
+    public static void Apply()
+    {
+        if (PlatformHelper.Is(Platform.Windows))
+        {
+            Logger.Log(LogLevel.Info, "Redirecting stderr");
+            var errorFile = CreateFile(Path.Combine(Paths.BepInExRootPath, "ErrorLog.log"), GENERIC_WRITE,
+                                       FILE_SHARE_READ,
+                                       0, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, 0);
+            if (errorFile == INVALID_HANDLE_VALUE)
+            {
+                Logger.Log(LogLevel.Warning, "Failed to open error log file; skipping error redirection");
+                return;
+            }
+
+            if (!SetStdHandle(STD_ERROR_HANDLE, errorFile))
+                Logger.Log(LogLevel.Warning, "Failed to redirect stderr; skipping error redirection");
+        }
+        // On unix, we can generally redirect stderr to a file "normally" via piping
+    }
+}