Optimize thread queue storage for spawned and deferred threads

crates/mlua-luau-scheduler/src/queue/deferred.rs

@@ -1,6 +1,6 @@
 use std::ops::{Deref, DerefMut};
 
-use super::generic::ThreadQueue;
+use super::threads::ThreadQueue;
 
 /**
     Alias for [`ThreadQueue`], providing a newtype to store in Lua app data.

crates/mlua-luau-scheduler/src/queue/mod.rs

@@ -1,8 +1,8 @@
 mod deferred;
 mod event;
 mod futures;
-mod generic;
 mod spawned;
+mod threads;
 
 pub(crate) use self::deferred::DeferredThreadQueue;
 pub(crate) use self::futures::FuturesQueue;

crates/mlua-luau-scheduler/src/queue/spawned.rs

@@ -1,6 +1,6 @@
 use std::ops::{Deref, DerefMut};
 
-use super::generic::ThreadQueue;
+use super::threads::ThreadQueue;
 
 /**
     Alias for [`ThreadQueue`], providing a newtype to store in Lua app data.

crates/mlua-luau-scheduler/src/queue/threads.rs

@@ -1,6 +1,7 @@
-use std::rc::Rc;
+#![allow(clippy::inline_always)]
+
+use std::{cell::RefCell, collections::VecDeque, rc::Rc};
 
-use concurrent_queue::ConcurrentQueue;
 use mlua::prelude::*;
 
 use crate::{threads::ThreadId, traits::IntoLuaThread};
@@ -9,15 +10,16 @@ use super::event::QueueEvent;
 
 #[derive(Debug)]
 struct ThreadQueueInner {
-    queue: ConcurrentQueue<(LuaThread, LuaMultiValue)>,
+    queue: RefCell<VecDeque<(LuaThread, LuaMultiValue)>>,
     event: QueueEvent,
 }
 
 impl ThreadQueueInner {
     fn new() -> Self {
-        let queue = ConcurrentQueue::unbounded();
+        Self {
-        let event = QueueEvent::new();
+            queue: RefCell::new(VecDeque::new()),
-        Self { queue, event }
+            event: QueueEvent::new(),
+        }
     }
 }
 
@@ -50,31 +52,48 @@ impl ThreadQueue {
         tracing::trace!("pushing item to queue with {} args", args.len());
         let id = ThreadId::from(&thread);
 
-        let _ = self.inner.queue.push((thread, args));
+        self.inner.queue.borrow_mut().push_back((thread, args));
         self.inner.event.notify();
 
         Ok(id)
     }
 
-    #[inline]
+    #[inline(always)]
-    pub fn drain_items(&self) -> impl Iterator<Item = (LuaThread, LuaMultiValue)> + '_ {
+    pub fn drain_items(&self) -> ThreadQueueDrain<'_> {
-        self.inner.queue.try_iter()
+        ThreadQueueDrain::new(self)
     }
 
-    #[inline]
+    #[inline(always)]
     pub async fn wait_for_item(&self) {
-        if self.inner.queue.is_empty() {
+        if self.inner.queue.borrow().is_empty() {
-            let listener = self.inner.event.listen();
+            self.inner.event.listen().await;
-            // NOTE: Need to check again, we could have gotten
-            // new queued items while creating our listener
-            if self.inner.queue.is_empty() {
-                listener.await;
-            }
         }
     }
 
-    #[inline]
+    #[inline(always)]
     pub fn is_empty(&self) -> bool {
-        self.inner.queue.is_empty()
+        self.inner.queue.borrow().is_empty()
+    }
+}
+
+/**
+    Iterator that drains the thread queue,
+    popping items from the front first.
+*/
+pub(crate) struct ThreadQueueDrain<'a> {
+    queue: &'a ThreadQueue,
+}
+
+impl<'a> ThreadQueueDrain<'a> {
+    pub fn new(queue: &'a ThreadQueue) -> Self {
+        Self { queue }
+    }
+}
+
+impl Iterator for ThreadQueueDrain<'_> {
+    type Item = (LuaThread, LuaMultiValue);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.queue.inner.queue.borrow_mut().pop_front()
     }
 }