Fix crash in animations system when assets gets loading/unloaded while async jobs are active

#2974

Source/Engine/Threading/ConcurrentSystemLocker.cpp

@@ -0,0 +1,41 @@
+// Copyright (c) 2012-2024 Wojciech Figat. All rights reserved.
+
+#include "ConcurrentSystemLocker.h"
+#include "Engine/Platform/Platform.h"
+
+ConcurrentSystemLocker::ConcurrentSystemLocker()
+{
+    _counters[0] = _counters[1] = 0;
+}
+
+void ConcurrentSystemLocker::Begin(bool write)
+{
+    volatile int64* thisCounter = &_counters[write];
+    volatile int64* otherCounter = &_counters[!write];
+RETRY:
+    // Check if we can enter (cannot read while someone else is writing and vice versa)
+    if (Platform::AtomicRead(otherCounter) != 0)
+    {
+        // Someone else is doing opposite operation so wait for it's end
+        // TODO: use ConditionVariable+CriticalSection to prevent active-waiting
+        Platform::Sleep(1);
+        goto RETRY;
+    }
+
+    // Mark that we entered this section
+    Platform::InterlockedIncrement(thisCounter);
+
+    // Double-check if we're safe to go
+    if (Platform::InterlockedCompareExchange(otherCounter, 0, 0))
+    {
+        // Someone else is doing opposite operation while this thread was doing counter increment so retry
+        Platform::InterlockedDecrement(thisCounter);
+        goto RETRY;
+    }
+}
+
+void ConcurrentSystemLocker::End(bool write)
+{
+    // Mark that we left this section
+    Platform::InterlockedDecrement(&_counters[write]);
+}