Fix potential deadlock in state_key::Entry::drop

If before Entry::drop() calls Registry::maybe_remove, the weak ref under
the same key1 and key2 is replaced, Entry::drop can be called
recursively resulting in a deadlock. The crux is we try to see if the
entry has been replaced by trying to upgrading it to a strong ref, and
it possible that the upgrade is successful and the result becomes a only
ref to the new entry.

This tries to fix it by determining the same thing by seeing if
Weak::strong_count() is 0 instead of upgrading the weak ref.

types/src/state_store/state_key/registry.rs

@@ -152,13 +152,13 @@ where
         let mut locked = self.inner.write();
         if let Some(map2) = locked.get_mut(key1) {
             if let Some(entry) = map2.get(key2) {
-                if entry.upgrade().is_none() {
+                if entry.strong_count() == 0 {
                     map2.remove(key2);
+                    if map2.is_empty() {
+                        locked.remove(key1);
+                    }
                 }
             }
-            if map2.is_empty() {
-                locked.remove(key1);
-            }
         }
     }