Fix asset_debug_server hang. There should be at most one ThreadExecut… (#7825)

…or's ticker for one thread.

# Objective

- Fix debug_asset_server hang.

## Solution

- Reuse the thread_local executor for MainThreadExecutor resource, so there will be only one ThreadExecutor for main thread. 
- If ThreadTickers from same executor, they are conflict with each other. Then only tick one.

crates/bevy_ecs/src/schedule/executor/multi_threaded.rs

@@ -620,11 +620,17 @@ fn evaluate_and_fold_conditions(conditions: &mut [BoxedCondition], world: &World
 }
 
 /// New-typed [`ThreadExecutor`] [`Resource`] that is used to run systems on the main thread
-#[derive(Resource, Default, Clone)]
+#[derive(Resource, Clone)]
 pub struct MainThreadExecutor(pub Arc<ThreadExecutor<'static>>);
 
+impl Default for MainThreadExecutor {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl MainThreadExecutor {
     pub fn new() -> Self {
-        MainThreadExecutor(Arc::new(ThreadExecutor::new()))
+        MainThreadExecutor(TaskPool::get_thread_executor())
     }
 }

crates/bevy_tasks/src/single_threaded_task_pool.rs

@@ -56,6 +56,11 @@ impl TaskPoolBuilder {
 pub struct TaskPool {}
 
 impl TaskPool {
+    /// Just create a new `ThreadExecutor` for wasm
+    pub fn get_thread_executor() -> Arc<ThreadExecutor<'static>> {
+        Arc::new(ThreadExecutor::new())
+    }
+
     /// Create a `TaskPool` with the default configuration.
     pub fn new() -> Self {
         TaskPoolBuilder::new().build()

crates/bevy_tasks/src/task_pool.rs

@@ -112,7 +112,12 @@ pub struct TaskPool {
 impl TaskPool {
     thread_local! {
         static LOCAL_EXECUTOR: async_executor::LocalExecutor<'static> = async_executor::LocalExecutor::new();
-        static THREAD_EXECUTOR: ThreadExecutor<'static> = ThreadExecutor::new();
+        static THREAD_EXECUTOR: Arc<ThreadExecutor<'static>> = Arc::new(ThreadExecutor::new());
+    }
+
+    /// Each thread should only create one `ThreadExecutor`, otherwise, there are good chances they will deadlock
+    pub fn get_thread_executor() -> Arc<ThreadExecutor<'static>> {
+        Self::THREAD_EXECUTOR.with(|executor| executor.clone())
     }
 
     /// Create a `TaskPool` with the default configuration.
@@ -376,9 +381,17 @@ impl TaskPool {
                 let tick_task_pool_executor = tick_task_pool_executor || self.threads.is_empty();
 
                 // we get this from a thread local so we should always be on the scope executors thread.
+                // note: it is possible `scope_executor` and `external_executor` is the same executor,
+                // in that case, we should only tick one of them, otherwise, it may cause deadlock.
                 let scope_ticker = scope_executor.ticker().unwrap();
-                if let Some(external_ticker) = external_executor.ticker() {
-                    if tick_task_pool_executor {
+                let external_ticker = if !external_executor.is_same(scope_executor) {
+                    external_executor.ticker()
+                } else {
+                    None
+                };
+
+                match (external_ticker, tick_task_pool_executor) {
+                    (Some(external_ticker), true) => {
                         Self::execute_global_external_scope(
                             executor,
                             external_ticker,
@@ -386,14 +399,16 @@ impl TaskPool {
                             get_results,
                         )
                         .await
-                    } else {
+                    }
+                    (Some(external_ticker), false) => {
                         Self::execute_external_scope(external_ticker, scope_ticker, get_results)
                             .await
                     }
-                } else if tick_task_pool_executor {
-                    Self::execute_global_scope(executor, scope_ticker, get_results).await
-                } else {
-                    Self::execute_scope(scope_ticker, get_results).await
+                    // either external_executor is none or it is same as scope_executor
+                    (None, true) => {
+                        Self::execute_global_scope(executor, scope_ticker, get_results).await
+                    }
+                    (None, false) => Self::execute_scope(scope_ticker, get_results).await,
                 }
             })
         }

crates/bevy_tasks/src/thread_executor.rs

@@ -77,12 +77,17 @@ impl<'task> ThreadExecutor<'task> {
     pub fn ticker<'ticker>(&'ticker self) -> Option<ThreadExecutorTicker<'task, 'ticker>> {
         if thread::current().id() == self.thread_id {
             return Some(ThreadExecutorTicker {
-                executor: &self.executor,
+                executor: self,
                 _marker: PhantomData::default(),
             });
         }
         None
     }
+
+    /// Returns true if `self` and `other`'s executor is same
+    pub fn is_same(&self, other: &Self) -> bool {
+        std::ptr::eq(self, other)
+    }
 }
 
 /// Used to tick the [`ThreadExecutor`]. The executor does not
@@ -90,20 +95,20 @@ impl<'task> ThreadExecutor<'task> {
 /// created on.
 #[derive(Debug)]
 pub struct ThreadExecutorTicker<'task, 'ticker> {
-    executor: &'ticker Executor<'task>,
+    executor: &'ticker ThreadExecutor<'task>,
     // make type not send or sync
     _marker: PhantomData<*const ()>,
 }
 impl<'task, 'ticker> ThreadExecutorTicker<'task, 'ticker> {
     /// Tick the thread executor.
     pub async fn tick(&self) {
-        self.executor.tick().await;
+        self.executor.executor.tick().await;
     }
 
     /// Synchronously try to tick a task on the executor.
     /// Returns false if if does not find a task to tick.
     pub fn try_tick(&self) -> bool {
-        self.executor.try_tick()
+        self.executor.executor.try_tick()
     }
 }