Only handle drag-and-drop from this process

When called from another process the data pointer will of course be
invalid, so every process must handle their own drag-and-drop.

src/wine-host/xdnd-proxy.cpp

@@ -46,14 +46,18 @@ void CALLBACK dnd_winevent_callback(HWINEVENTHOOK /*hWinEventHook*/,
                                     LONG /*idChild*/,
                                     DWORD /*idEventThread*/,
                                     DWORD /*dwmsEventTime*/) {
-    // FIXME: Prevent this from being run by multiple processes at the same time
-
-    // XXX: `EVENT_OBJECT_DESTROY` doesn't seem to be implemented by Wine, so we
-    //      can't rely on that.
     if (!(event == EVENT_OBJECT_CREATE && idObject == OBJID_WINDOW)) {
         return;
     }
 
+    // Don't handle windows that weren't created in this process, because
+    // otherwise we obviously cannot access the `IDataObject` object
+    uint32_t process_id = 0;
+    GetWindowThreadProcessId(hwnd, &process_id);
+    if (process_id != GetCurrentProcessId()) {
+        return;
+    }
+
     // Wine's drag-and-drop tracker windows always have the same window
     // class name, so we can easily identify them
     {
@@ -68,7 +72,7 @@ void CALLBACK dnd_winevent_callback(HWINEVENTHOOK /*hWinEventHook*/,
     // data
     auto tracker_info =
         reinterpret_cast<TrackerWindowInfo*>(GetWindowLongPtr(hwnd, 0));
-    if (!tracker_info) {
+    if (!tracker_info || !tracker_info->dataObject) {
         return;
     }
 
@@ -122,9 +126,9 @@ void CALLBACK dnd_winevent_callback(HWINEVENTHOOK /*hWinEventHook*/,
                 } break;
             }
 
-            // We won't release the storage, because we're not actually doing
-            // the Windows drag and drop. We're just snooping in on the data
-            // that's being dragged.
+            if (storage.pUnkForRelease) {
+                storage.pUnkForRelease->Release();
+            }
         }
     }
 }

src/wine-host/xdnd-proxy.h

@@ -35,9 +35,10 @@ class WineXdndProxy {
      * is started, we will initiate the XDND protocol with the same file. This
      * will allow us to drag files from Wine windows to X11 applications,
      * something that's normally not possible. Calling this function more than
-     * once doesn't have any effect, but this will still be called from every
-     * plugin host instance. Because of that we'll use a global Win32 mutex to
-     * ensure that this is done from only one thread at a time.
+     * once doesn't have any effect, but this should still be called at least
+     * once from every plugin host instance. Because the actual data is stored
+     * in a COM object, we can only handle drag-and-drop coming form this
+     * process.
      */
     static WineXdndProxy& init_proxy();