Fix subscriptions for read only signals passed as props (#3173)

* add a fuzzing test for reference counted generational boxes

* Fix subscriptions for readonlysignal passed as props

* Don't subscribe to dropped contexts

---------

Co-authored-by: Jonathan Kelley <jkelleyrtp@gmail.com>

packages/core/src/reactive_context.rs

@@ -5,7 +5,7 @@ use crate::{
     Runtime,
 };
 use futures_channel::mpsc::UnboundedReceiver;
-use generational_box::{GenerationalBox, SyncStorage};
+use generational_box::{BorrowMutError, GenerationalBox, SyncStorage};
 use std::{
     cell::RefCell,
     collections::HashSet,
@@ -243,11 +243,19 @@ impl ReactiveContext {
 
     /// Subscribe to this context. The reactive context will automatically remove itself from the subscriptions when it is reset.
     pub fn subscribe(&self, subscriptions: Arc<Mutex<HashSet<ReactiveContext>>>) {
-        subscriptions.lock().unwrap().insert(*self);
-        self.inner
-            .write()
-            .subscribers
-            .insert(PointerHash(subscriptions));
+        match self.inner.try_write() {
+            Ok(mut inner) => {
+                subscriptions.lock().unwrap().insert(*self);
+                inner.subscribers.insert(PointerHash(subscriptions));
+            }
+            // If the context was dropped, we don't need to subscribe to it anymore
+            Err(BorrowMutError::Dropped(_)) => {}
+            Err(expect) => {
+                panic!(
+                    "Expected to be able to write to reactive context to subscribe, but it failed with: {expect:?}"
+                );
+            }
+        }
     }
 
     /// Get the scope that inner CopyValue is associated with