Bug: Reentracy checks conflict with .Net synchronization mechanisms (e.g. lock), causing random crashes in apps

[benstevens48]

Description

The issue is explained very clearly here - microsoft/microsoft-ui-xaml#11099.

Basically, the problem is that all .Net synchronization primitives, including lock, when used on an STA thread, will eventually call CoWaitForMultipleHandles, which pumps messages. Unfortunately the WindowsAppSdk windowing code (xcpwindow.cpp) doesn't deal with reentrancy very well (just fails fast in a lot of cases), resulting in the app sometimes crashing when any contended lock is encountered (which could even be in CsWinRT code itself). Obviously we need to be able to use locks, and this is quite a critical bug. It seems like there are 2 solutions. The first is for WindowsAppSDK/WinUI to update their code to properly handle reentrancy (at least in the case of messages that can be pumped by CoWaitForMultipleHandles). If this is the preferred option then perhaps you guys can put some pressure on the WindowsAppSDK/WinUI team to fix the issue. The second option is to use a custom synchronization context which overrides Wait to use a non-re-entrant wait (some example code will follow). If this is the case it would involve an update to DispatcherQueueSynchronizationContext (when used on an STA thread). But I'm not sure if this would have any unintended consequnces...

Here is a custom synchronization context that fixes the issue (note it doesn't necessarily handle errors etc correctly at the moment).

    internal partial class CustomSynchronizationContext : DispatcherQueueSynchronizationContext {
        public CustomSynchronizationContext(DispatcherQueue queue) : base(queue) {
            SetWaitNotificationRequired();
        }
        public override int Wait(nint[] waitHandles, bool waitAll, int millisecondsTimeout) {
            return (int)WaitForMultipleObjectsEx((uint)waitHandles.Length, waitHandles, waitAll, (uint)millisecondsTimeout, false); //Note the (uint)millisecondsTimeout cast correctly handles the -1 case
        }
        [LibraryImport("Kernel32.dll")]
        private static partial uint WaitForMultipleObjectsEx(uint nCount, nint[] lpHandles, [MarshalAs(UnmanagedType.Bool)] bool bWaitAll, uint dwMilliseconds, [MarshalAs(UnmanagedType.Bool)] bool bAlertable);
    }

Steps To Reproduce

The minimal code needed to reproduce the crash is as follows (also see the sample solution in microsoft/microsoft-ui-xaml#11099.

 public sealed partial class MainWindow : Window {

     readonly object lockObj = new object();

     public MainWindow() {
         InitializeComponent();
         CompositionTarget.Rendering += CompositionTarget_Rendering;
         Task.Run(() => {
             while (true) {
                 lock (lockObj) { //Ensures contention at some point
                 }
             }
         });
     }

     private void CompositionTarget_Rendering(object? sender, object e) {
         lock (lockObj) {
         }
     }
 }

<?xml version="1.0" encoding="utf-8"?>
<Window x:Class="TestLockReentrancy.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:local="using:TestLockReentrancy" Title="TestLockReentrancy">
    <Grid Background="Gray" ManipulationMode="All">
    </Grid>
</Window>

Note the important ManipulationMode="All". Run the app, randomly drag and release the mouse in the grey area until there is a crash.

Expected Behavior

No crash

Version Info

WindowsAppSdk 2.1, Windows SDK 26100, Windows 25H2.

Additional Context

No response