reflect: avoid deadlock in GenericTypeCell (#8957)

# Objective

- There was a deadlock discovered in the implementation of
`bevy_reflect::utility::GenericTypeCell`, when called on a recursive
type, e.g. `Vec<Vec<VariableCurve>>`

## Solution

- Drop the lock before calling the initialisation function, and then
pick it up again afterwards.

## Additional Context
- [Discussed on
Discord](https://discord.com/channels/691052431525675048/1002362493634629796/1122706835284185108)

crates/bevy_reflect/src/lib.rs

@@ -1848,6 +1848,15 @@ bevy_reflect::tests::should_reflect_debug::Test {
         assert_eq!("123", format!("{:?}", foo));
     }
 
+    #[test]
+    fn recursive_typed_storage_does_not_hang() {
+        #[derive(Reflect)]
+        struct Recurse<T>(T);
+
+        let _ = <Recurse<Recurse<()>> as Typed>::type_info();
+        let _ = <Recurse<Recurse<()>> as TypePath>::type_path();
+    }
+
     #[cfg(feature = "glam")]
     mod glam {
         use super::*;

crates/bevy_reflect/src/utility.rs

@@ -1,7 +1,7 @@
 //! Helpers for working with Bevy reflection.
 
 use crate::TypeInfo;
-use bevy_utils::{FixedState, HashMap};
+use bevy_utils::{FixedState, StableHashMap};
 use once_cell::race::OnceBox;
 use parking_lot::RwLock;
 use std::{
@@ -206,7 +206,7 @@ impl<T: TypedProperty> NonGenericTypeCell<T> {
 /// ```
 /// [`impl_type_path`]: crate::impl_type_path
 /// [`TypePath`]: crate::TypePath
-pub struct GenericTypeCell<T: TypedProperty>(OnceBox<RwLock<HashMap<TypeId, &'static T::Stored>>>);
+pub struct GenericTypeCell<T: TypedProperty>(RwLock<StableHashMap<TypeId, &'static T::Stored>>);
 
 /// See [`GenericTypeCell`].
 pub type GenericTypeInfoCell = GenericTypeCell<TypeInfo>;
@@ -216,7 +216,9 @@ pub type GenericTypePathCell = GenericTypeCell<TypePathComponent>;
 impl<T: TypedProperty> GenericTypeCell<T> {
     /// Initialize a [`GenericTypeCell`] for generic types.
     pub const fn new() -> Self {
-        Self(OnceBox::new())
+        // Use `bevy_utils::StableHashMap` over `bevy_utils::HashMap`
+        // because `BuildHasherDefault` is unfortunately not const.
+        Self(RwLock::new(StableHashMap::with_hasher(FixedState)))
     }
 
     /// Returns a reference to the [`TypedProperty`] stored in the cell.
@@ -229,19 +231,30 @@ impl<T: TypedProperty> GenericTypeCell<T> {
         F: FnOnce() -> T::Stored,
     {
         let type_id = TypeId::of::<G>();
-        // let mapping = self.0.get_or_init(|| Box::new(RwLock::default()));
+
-        let mapping = self.0.get_or_init(Box::default);
+        // Put in a seperate scope, so `mapping` is dropped before `f`,
-        if let Some(info) = mapping.read().get(&type_id) {
+        // since `f` might want to call `get_or_insert` recursively
-            return info;
+        // and we don't want a deadlock!
+        {
+            let mapping = self.0.read();
+            if let Some(info) = mapping.get(&type_id) {
+                return info;
+            }
         }
 
-        mapping.write().entry(type_id).or_insert_with(|| {
+        let value = f();
-            // We leak here in order to obtain a `&'static` reference.
+
-            // Otherwise, we won't be able to return a reference due to the `RwLock`.
+        let mut mapping = self.0.write();
-            // This should be okay, though, since we expect it to remain statically
+        mapping
-            // available over the course of the application.
+            .entry(type_id)
-            Box::leak(Box::new(f()))
+            .insert({
-        })
+                // We leak here in order to obtain a `&'static` reference.
+                // Otherwise, we won't be able to return a reference due to the `RwLock`.
+                // This should be okay, though, since we expect it to remain statically
+                // available over the course of the application.
+                Box::leak(Box::new(value))
+            })
+            .get()
     }
 }