fix close deadlock (#683)

* fix close deadlock

* add comment

Author: Alex2772

aui.core/src/AUI/Common/ASignal.h

@@ -283,10 +283,15 @@ class ASignal final : public AAbstractSignal {
             if (!value) {
                 return;
             }
-            std::unique_lock lock(AObjectBase::SIGNAL_SLOT_GLOBAL_SYNC);
-            // this destructor can be called in ASignal destructor, so it's worth to reset the sender as well.
-            value->sender = nullptr;
-            value->unlinkInReceiverSideOnly(lock);
+            {
+                std::unique_lock lock(AObjectBase::SIGNAL_SLOT_GLOBAL_SYNC);
+                // this destructor can be called in ASignal destructor, so it's worth to reset the sender as well.
+                value->sender = nullptr;
+                value->unlinkInReceiverSideOnly(lock);
+            }
+            // releasing lock before value = nullptr.
+            // destroying `value` object could trigger a cascade of destructions, leading to a re-entrant lock
+            // attempt on a non-recursive mutex.
             value = nullptr;
         }
     };