fix bug in maybe_unlink_parent (#420)

Summary:
Pull Request resolved: #420

It was meant to return the parent only when it was actually unlinked.

Happily, the net effect of this bug was that we printed spurious "instances was dropped while parent was linked".
ghstack-source-id: 294110249

Reviewed By: shayne-fletcher

Differential Revision: D77705185

fbshipit-source-id: 2c03efada909ee4780725c78d98caea92bfb34b9

Author: mariusae

hyperactor/src/proc.rs

@@ -1338,17 +1338,15 @@ impl InstanceState {
     /// Unlink this instance from its parent, if it has one. If it was unlinked,
     /// the parent is returned.
     fn maybe_unlink_parent(&self) -> Option<InstanceCell> {
-        let result = self.parent.upgrade();
-        if let Some(parent) = &result {
-            parent.inner.unlink(self);
-        }
-        result
+        self.parent
+            .upgrade()
+            .filter(|parent| parent.inner.unlink(self))
     }
 
     /// Unlink this instance from a child.
-    fn unlink(&self, child: &InstanceState) {
+    fn unlink(&self, child: &InstanceState) -> bool {
         assert_eq!(self.actor_id.proc_id(), child.actor_id.proc_id());
-        self.children.remove(&child.actor_id.pid());
+        self.children.remove(&child.actor_id.pid()).is_some()
     }
 }