[idd] cancel stale swap chain assignments

Invalidate in-progress swap chain setup when IddCx unassigns the
monitor, and serialize replacement assignments so workers are
never started on abandoned handles.

Also initialize all worker dependencies before starting the processing
thread and treat teardown during SetDevice as normal cancellation.

idd/LGIdd/CIndirectMonitorContext.h

@@ -24,7 +24,9 @@
 #include <wdf.h>
 #include <IddCx.h>
 
+#include <atomic>
 #include <memory>
+#include <mutex>
 #include "CIndirectDeviceContext.h"
 #include "CSwapChainProcessor.h"
 
@@ -39,12 +41,24 @@ private:
   // concurrently (an unassign triggered by the worker's WdfObjectDelete can
   // race the next assign), and shared_ptr copy/reset is not thread safe.
   SRWLOCK m_lock = SRWLOCK_INIT;
+
+  // IddCx can issue a replacement assignment before an earlier assignment
+  // has finished creating its devices. Serialize those expensive setup paths
+  // while still allowing UnassignSwapChain to cancel the active one.
+  std::mutex m_assignMutex;
   std::shared_ptr<CD3D11Device> m_dx11Device;
   std::shared_ptr<CD3D12Device> m_dx12Device;
 
   CIndirectDeviceContext * m_devContext;
   std::unique_ptr<CSwapChainProcessor> m_swapChain;
 
+  // Incremented whenever the current assignment is replaced or unassigned.
+  // Device creation is performed outside m_lock, so this lets an unassign
+  // cancel that work before a processor is started on a stale swap chain.
+  std::atomic<UINT64> m_assignmentGeneration = 0;
+
+  void DetachSwapChain();
+
 public:
   CIndirectMonitorContext(_In_ IDDCX_MONITOR monitor, CIndirectDeviceContext * device);
 
@@ -52,6 +66,10 @@ public:
   
   void AssignSwapChain(IDDCX_SWAPCHAIN swapChain, LUID renderAdapter, HANDLE newFrameEvent);
   void UnassignSwapChain();
+  bool IsAssignmentCurrent(UINT64 generation) const
+  {
+    return m_assignmentGeneration.load(std::memory_order_acquire) == generation;
+  }
 
   CIndirectDeviceContext * GetDeviceContext() { return m_devContext; }
 };
@@ -67,4 +85,4 @@ struct CIndirectMonitorContextWrapper
   }
 };
 
-WDF_DECLARE_CONTEXT_TYPE(CIndirectMonitorContextWrapper);
+WDF_DECLARE_CONTEXT_TYPE(CIndirectMonitorContextWrapper);