Fix generation race condition with sync storage (#2638)

* Fix generation race condition with sync storage

* Document GenerationalPointer

* check generation before recycling generational box

* fix clippy

* make race_condition_regression test panic if it fails

packages/generational-box/tests/sync.rs

@@ -0,0 +1,42 @@
+// Regression test for https://github.com/DioxusLabs/dioxus/issues/2636
+
+use std::time::Duration;
+
+use generational_box::{AnyStorage, GenerationalBox, SyncStorage};
+
+#[test]
+fn race_condition_regression() {
+    for _ in 0..100 {
+        let handle = {
+            let owner = SyncStorage::owner();
+            let key = owner.insert(1u64);
+            let handle = std::thread::spawn(move || reader(key));
+
+            std::thread::sleep(Duration::from_millis(10));
+            handle
+            // owner is dropped now
+        };
+        // owner is *recycled*
+        let owner = SyncStorage::owner();
+        let _key = owner.insert(2u64);
+        let _ = handle.join();
+    }
+}
+
+fn reader(key: GenerationalBox<u64, SyncStorage>) {
+    for _ in 0..1000000 {
+        match key.try_read() {
+            Ok(value) => {
+                if *value == 2 {
+                    panic!("Read a new value with the old generation");
+                } else {
+                    // fine
+                }
+            }
+            Err(err) => {
+                eprintln!("bailing out - {err:?}");
+                break;
+            }
+        }
+    }
+}