fix critical deadlock between render and game threads in UE 5.6 when invoking IHeadMountedDisplay::GetHMDMonitorInfo()

Handbook/src/appendix/changelog.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - Replaced Epic Native Toolchain `v24` support with Epic Native Toolchain `v25` due to the fact that now `v25` is the default Linux toolchain for UE `5.6`.
+- Revamped `FSGHMDTracker` to resolve a critical deadlock between the rendering and game threads in Unreal Engine `5.6` that occurrs when `IHeadMountedDisplay::GetHMDMonitorInfo()` is invoked from `FSGXRTracker::GetControllerTransform()`. This is similar to another [critical deadlock issue (UE-212224), occurring during PipelinedFrameState acquisition, addressed in the `v2.5.0` release](#250---2025-05-09).
 - Bumped the SenseGlove libraries to `v2.203.0-f3d3e676`.
 - SenseGlove libraries have migrated to `C++20` from `C++17`.
 - Bumped the SenseGlove Unreal Engine Marketplace Packager `v0.6.0-4108c6f`.
@@ -23,6 +24,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Resolved GNU/Linux build issues for Unreal Engine `5.5` and `5.6` caused by incorrect linkage to GNU/GCC's `libstdc++` instead of LLVM/Clang's `libc++`.
 - Fix `SGLog` build issues on GNU/Linux with UE `5.6`.
 - Fix type conversion safety and consistency issues across all `SGLog` formatters.
+- Resolved a critical deadlock between the rendering and game threads in Unreal Engine `5.6` that occurrs when `IHeadMountedDisplay::GetHMDMonitorInfo()` is invoked from `FSGXRTracker::GetControllerTransform()`. This is similar to another [critical deadlock issue (UE-212224), occurring during PipelinedFrameState acquisition, addressed in the `v2.5.0` release](#250---2025-05-09).
 - Additional minor fixes and improvements that may not be listed here.
 
 ### Removed

Source/SenseGloveTracking/Private/SGTracking/SGHMDTracker.cpp

@@ -35,12 +35,72 @@
 
 #include "SGTracking/SGHMDTracker.h"
 
+#if ENGINE_MAJOR_VERSION >= 5 && ENGINE_MINOR_VERSION >= 5
+#include "Async/Async.h"
+#include "Async/Future.h"
+#include "Async/TaskGraphInterfaces.h"
+#endif  /* ENGINE_MAJOR_VERSION >= 5 && ENGINE_MINOR_VERSION >= 5 */
 #include "Engine/Engine.h"
 #include "IHeadMountedDisplay.h"
 #include "IXRTrackingSystem.h"
+#if ENGINE_MAJOR_VERSION >= 5 && ENGINE_MINOR_VERSION >= 5
+#include "Misc/Optional.h"
+#include "Misc/Timespan.h"
+#endif  /* ENGINE_MAJOR_VERSION >= 5 && ENGINE_MINOR_VERSION >= 5 */
 
 #include "SGSettings/SGSettings.h"
 
+struct FSGHMDTracker::FStaticImpl
+{
+    /************************
+     * Static methods
+     ************************/
+
+    static void GetHMDMonitorInfo(
+        IHeadMountedDisplay* Device, IHeadMountedDisplay::MonitorInfo& OutMonitorInfo);
+
+    /************************
+     * Constructor / Destructor
+     ************************/
+
+    explicit FStaticImpl() = default;
+    virtual ~FStaticImpl() = delete;
+};
+
+void FSGHMDTracker::FStaticImpl::GetHMDMonitorInfo(
+    IHeadMountedDisplay* Device, IHeadMountedDisplay::MonitorInfo& OutMonitorInfo)
+{
+#if ENGINE_MAJOR_VERSION >= 5 && ENGINE_MINOR_VERSION <= 5
+    Device->GetHMDMonitorInfo(OutMonitorInfo);
+    return;
+#endif  /* ENGINE_MAJOR_VERSION >= 5 && ENGINE_MINOR_VERSION <= 5 */
+
+#if ENGINE_MAJOR_VERSION >= 5 && ENGINE_MINOR_VERSION >= 6
+    TPromise<TOptional<IHeadMountedDisplay::MonitorInfo>> Promise;
+    const TFuture<TOptional<IHeadMountedDisplay::MonitorInfo>> Future{Promise.GetFuture()};
+
+    AsyncTask(ENamedThreads::ActualRenderingThread,
+              [
+                  Device,
+                  Promise = MoveTemp(Promise)]() mutable
+              {
+                  IHeadMountedDisplay::MonitorInfo MonitorInfo;
+                  Device->GetHMDMonitorInfo(MonitorInfo);
+
+                  Promise.SetValue(MoveTemp(MonitorInfo));
+              });
+
+    if (Future.WaitFor(FTimespan::FromMilliseconds(100)))
+    {
+        TOptional<IHeadMountedDisplay::MonitorInfo> Result{Future.Get()};
+        if (Result.IsSet())
+        {
+            OutMonitorInfo = Result.GetValue();
+        }
+    }
+#endif  /* ENGINE_MAJOR_VERSION >= 5 && ENGINE_MINOR_VERSION <= 5 */
+}
+
 FName FSGHMDTracker::GetDeviceName()
 {
     if (GEngine && GEngine->XRSystem.IsValid() && GEngine->XRSystem->IsHeadTrackingAllowed())
@@ -145,7 +205,7 @@ bool FSGHMDTracker::IsHtcVivePro()
             if (Device)
             {
                 IHeadMountedDisplay::MonitorInfo HmdMonitorInfo;
-                Device->GetHMDMonitorInfo(HmdMonitorInfo);
+                FStaticImpl::GetHMDMonitorInfo(Device, HmdMonitorInfo);
 
                 /// NOTE
                 // VIVE Pro:
@@ -218,7 +278,7 @@ bool FSGHMDTracker::IsHtcViveFocus3()
             if (Device)
             {
                 IHeadMountedDisplay::MonitorInfo HmdMonitorInfo;
-                Device->GetHMDMonitorInfo(HmdMonitorInfo);
+                FStaticImpl::GetHMDMonitorInfo(Device, HmdMonitorInfo);
 
                 /// NOTE
                 // VIVE Pro:
@@ -292,7 +352,7 @@ bool FSGHMDTracker::IsHtcViveXRElite()
             if (Device)
             {
                 IHeadMountedDisplay::MonitorInfo HmdMonitorInfo;
-                Device->GetHMDMonitorInfo(HmdMonitorInfo);
+                FStaticImpl::GetHMDMonitorInfo(Device, HmdMonitorInfo);
 
                 /// NOTE
                 // VIVE Pro:
@@ -366,7 +426,7 @@ bool FSGHMDTracker::IsHtcViveFocusVision()
             if (Device)
             {
                 IHeadMountedDisplay::MonitorInfo HmdMonitorInfo;
-                Device->GetHMDMonitorInfo(HmdMonitorInfo);
+                FStaticImpl::GetHMDMonitorInfo(Device, HmdMonitorInfo);
 
                 /// NOTE
                 // VIVE Pro:

Source/SenseGloveTracking/Public/SGTracking/SGHMDTracker.h

@@ -51,6 +51,9 @@ struct SENSEGLOVETRACKING_API FSGHMDTracker
 {
     GENERATED_BODY()
 
+private:
+    struct FStaticImpl;
+
 public:
     static FName GetDeviceName();