fix(core): Ensure we don't lose the waker when polling an empty JoinSet (#19655)

This is a reproduction and fix for a very obscure bug where the Deno
runtime locks up we end up polling an empty JoinSet and attempt to
resolve ops after-the-fact. There's a small footgun in the JoinSet API
where polling it while empty returns Ready(None), which means that it
never holds on to the waker. This means that if we aren't testing for
this particular return value and don't stash the waker ourselves for a
future async op to eventually queue, we can end up losing the waker
entirely and the op wakes up, notifies tokio, which notifies the
JoinSet, which then has nobody to notify 😢.

Co-authored-by: Luca Casonato <hello@lcas.dev>
Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>

Cargo.lock

@@ -607,6 +607,12 @@ version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6245d59a3e82a7fc217c5828a6692dbc6dfb63a0c8c90495621f7b9d79704a0e"
 
+[[package]]
+name = "cooked-waker"
+version = "5.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "147be55d677052dabc6b22252d5dd0fd4c29c8c27aa4f2fbef0f94aa003b406f"
+
 [[package]]
 name = "core-foundation"
 version = "0.9.3"
@@ -971,6 +977,7 @@ version = "0.191.0"
 dependencies = [
  "anyhow",
  "bytes",
+ "cooked-waker",
  "deno_ast",
  "deno_ops",
  "futures",

core/Cargo.toml

@@ -46,4 +46,5 @@ path = "examples/http_bench_json_ops/main.rs"
 
 # These dependencies are only used for the 'http_bench_*_ops' examples.
 [dev-dependencies]
+cooked-waker = "5"
 deno_ast.workspace = true

core/joinset.rs

@@ -0,0 +1,92 @@
+// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
+// Some code and comments under MIT license where adapted from Tokio code
+// Copyright (c) 2023 Tokio Contributors
+
+use std::task::Context;
+use std::task::Poll;
+use std::task::Waker;
+
+use futures::Future;
+use tokio::task::AbortHandle;
+use tokio::task::JoinError;
+
+use crate::task::MaskFutureAsSend;
+use crate::task::MaskResultAsSend;
+
+/// Wraps the tokio [`JoinSet`] to make it !Send-friendly and to make it easier and safer for us to
+/// poll while empty.
+pub(crate) struct JoinSet<T> {
+  joinset: tokio::task::JoinSet<MaskResultAsSend<T>>,
+  /// If join_next returns Ready(None), we stash the waker
+  waker: Option<Waker>,
+}
+
+impl<T> Default for JoinSet<T> {
+  fn default() -> Self {
+    Self {
+      joinset: Default::default(),
+      waker: None,
+    }
+  }
+}
+
+impl<T: 'static> JoinSet<T> {
+  /// Spawn the provided task on the `JoinSet`, returning an [`AbortHandle`]
+  /// that can be used to remotely cancel the task.
+  ///
+  /// The provided future will start running in the background immediately
+  /// when this method is called, even if you don't await anything on this
+  /// `JoinSet`.
+  ///
+  /// # Panics
+  ///
+  /// This method panics if called outside of a Tokio runtime.
+  ///
+  /// [`AbortHandle`]: tokio::task::AbortHandle
+  #[track_caller]
+  pub fn spawn<F>(&mut self, task: F) -> AbortHandle
+  where
+    F: Future<Output = T>,
+    F: 'static,
+    T: 'static,
+  {
+    // SAFETY: We only use this with the single-thread executor
+    let handle = self.joinset.spawn(unsafe { MaskFutureAsSend::new(task) });
+
+    // If someone had called poll_join_next while we were empty, ask them to poll again
+    // so we can properly register the waker with the underlying JoinSet.
+    if let Some(waker) = self.waker.take() {
+      waker.wake();
+    }
+    handle
+  }
+
+  /// Returns the number of tasks currently in the `JoinSet`.
+  pub fn len(&self) -> usize {
+    self.joinset.len()
+  }
+
+  /// Waits until one of the tasks in the set completes and returns its output.
+  ///
+  /// # Cancel Safety
+  ///
+  /// This method is cancel safe. If `join_next` is used as the event in a `tokio::select!`
+  /// statement and some other branch completes first, it is guaranteed that no tasks were
+  /// removed from this `JoinSet`.
+  pub fn poll_join_next(
+    &mut self,
+    cx: &mut Context,
+  ) -> Poll<Result<T, JoinError>> {
+    // TODO(mmastrac): Use poll_join_next from Tokio
+    let next = std::pin::pin!(self.joinset.join_next());
+    match next.poll(cx) {
+      Poll::Ready(Some(res)) => Poll::Ready(res.map(|res| res.into_inner())),
+      Poll::Ready(None) => {
+        // Stash waker
+        self.waker = Some(cx.waker().clone());
+        Poll::Pending
+      }
+      Poll::Pending => Poll::Pending,
+    }
+  }
+}

core/lib.rs

@@ -9,6 +9,7 @@ mod flags;
 mod gotham_state;
 mod inspector;
 mod io;
+mod joinset;
 mod module_specifier;
 mod modules;
 mod normalize_path;

core/runtime/jsrealm.rs

@@ -1,10 +1,10 @@
 // Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
 use super::bindings;
 use crate::error::exception_to_err_result;
+use crate::joinset::JoinSet;
 use crate::modules::ModuleCode;
 use crate::ops::OpCtx;
 use crate::runtime::JsRuntimeState;
-use crate::task::MaskResultAsSend;
 use crate::JsRuntime;
 use crate::OpId;
 use crate::OpResult;
@@ -17,7 +17,6 @@ use std::hash::BuildHasherDefault;
 use std::hash::Hasher;
 use std::option::Option;
 use std::rc::Rc;
-use tokio::task::JoinSet;
 use v8::HandleScope;
 use v8::Local;
 
@@ -50,8 +49,7 @@ pub(crate) struct ContextState {
   pub(crate) pending_promise_rejections:
     VecDeque<(v8::Global<v8::Promise>, v8::Global<v8::Value>)>,
   pub(crate) unrefed_ops: HashSet<i32, BuildHasherDefault<IdentityHasher>>,
-  pub(crate) pending_ops:
+  pub(crate) pending_ops: JoinSet<(PromiseId, OpId, OpResult)>,
-    JoinSet<MaskResultAsSend<(PromiseId, OpId, OpResult)>>,
   // We don't explicitly re-read this prop but need the slice to live alongside
   // the context
   pub(crate) op_ctxs: Box<[OpCtx]>,

core/runtime/jsruntime.rs

@@ -37,7 +37,6 @@ use anyhow::Context as AnyhowContext;
 use anyhow::Error;
 use futures::channel::oneshot;
 use futures::future::poll_fn;
-use futures::future::Future;
 use futures::stream::StreamExt;
 use smallvec::SmallVec;
 use std::any::Any;
@@ -2261,14 +2260,11 @@ impl JsRuntime {
         SmallVec::with_capacity(32);
 
       loop {
-        let item = {
+        let Poll::Ready(item) = context_state.pending_ops.poll_join_next(cx) else {
-          let next = std::pin::pin!(context_state.pending_ops.join_next());
-          let Poll::Ready(Some(item)) = next.poll(cx) else {
           break;
         };
-          item
+        // TODO(mmastrac): If this task is really errored, things could be pretty bad
-        };
+        let (promise_id, op_id, mut resp) = item.unwrap();
-        let (promise_id, op_id, mut resp) = item.unwrap().into_inner();
         state
           .borrow()
           .op_state

core/runtime/ops.rs

@@ -25,10 +25,11 @@ pub fn queue_fast_async_op<R: serde::Serialize + 'static>(
     state.get_error_class_fn
   };
   let fut = op.map(|result| crate::_ops::to_op_result(get_class, result));
-  // SAFETY: this is guaranteed to be running on a current-thread executor
+  ctx
-  ctx.context_state.borrow_mut().pending_ops.spawn(unsafe {
+    .context_state
-    crate::task::MaskFutureAsSend::new(OpCall::new(ctx, promise_id, fut))
+    .borrow_mut()
-  });
+    .pending_ops
+    .spawn(OpCall::new(ctx, promise_id, fut));
 }
 
 #[inline]
@@ -123,12 +124,7 @@ pub fn queue_async_op<'s>(
     Poll::Pending => {}
     Poll::Ready(mut res) => {
       if deferred {
-        ctx
+        ctx.context_state.borrow_mut().pending_ops.spawn(ready(res));
-          .context_state
-          .borrow_mut()
-          .pending_ops
-          // SAFETY: this is guaranteed to be running on a current-thread executor
-          .spawn(unsafe { crate::task::MaskFutureAsSend::new(ready(res)) });
         return None;
       } else {
         ctx.state.borrow_mut().tracker.track_async_completed(ctx.id);
@@ -137,12 +133,7 @@ pub fn queue_async_op<'s>(
     }
   }
 
-  ctx
+  ctx.context_state.borrow_mut().pending_ops.spawn(pinned);
-    .context_state
-    .borrow_mut()
-    .pending_ops
-    // SAFETY: this is guaranteed to be running on a current-thread executor
-    .spawn(unsafe { crate::task::MaskFutureAsSend::new(pinned) });
   None
 }
 

core/runtime/tests.rs

@@ -21,6 +21,9 @@ use crate::Extension;
 use crate::JsBuffer;
 use crate::*;
 use anyhow::Error;
+use cooked_waker::IntoWaker;
+use cooked_waker::Wake;
+use cooked_waker::WakeRef;
 use deno_ops::op;
 use futures::future::poll_fn;
 use futures::future::Future;
@@ -28,11 +31,14 @@ use futures::FutureExt;
 use std::cell::RefCell;
 use std::pin::Pin;
 use std::rc::Rc;
+use std::sync::atomic::AtomicBool;
+use std::sync::atomic::AtomicI8;
 use std::sync::atomic::AtomicUsize;
 use std::sync::atomic::Ordering;
 use std::sync::Arc;
 use std::task::Context;
 use std::task::Poll;
+use std::time::Duration;
 
 // deno_ops macros generate code assuming deno_core in scope.
 mod deno_core {
@@ -264,6 +270,90 @@ fn test_execute_script_return_value() {
   }
 }
 
+#[derive(Default)]
+struct LoggingWaker {
+  woken: AtomicBool,
+}
+
+impl Wake for LoggingWaker {
+  fn wake(self) {
+    self.woken.store(true, Ordering::SeqCst);
+  }
+}
+
+impl WakeRef for LoggingWaker {
+  fn wake_by_ref(&self) {
+    self.woken.store(true, Ordering::SeqCst);
+  }
+}
+
+/// This is a reproduction for a very obscure bug where the Deno runtime locks up we end up polling
+/// an empty JoinSet and attempt to resolve ops after-the-fact. There's a small footgun in the JoinSet
+/// API where polling it while empty returns Ready(None), which means that it never holds on to the
+/// waker. This means that if we aren't testing for this particular return value and don't stash the waker
+/// ourselves for a future async op to eventually queue, we can end up losing the waker entirely and the
+/// op wakes up, notifies tokio, which notifies the JoinSet, which then has nobody to notify )`:.
+#[tokio::test]
+async fn test_wakers_for_async_ops() {
+  static STATE: AtomicI8 = AtomicI8::new(0);
+
+  #[op]
+  async fn op_async_sleep() -> Result<(), Error> {
+    STATE.store(1, Ordering::SeqCst);
+    tokio::time::sleep(std::time::Duration::from_millis(1)).await;
+    STATE.store(2, Ordering::SeqCst);
+    Ok(())
+  }
+
+  STATE.store(0, Ordering::SeqCst);
+
+  let logging_waker = Arc::new(LoggingWaker::default());
+  let waker = logging_waker.clone().into_waker();
+
+  deno_core::extension!(test_ext, ops = [op_async_sleep]);
+  let mut runtime = JsRuntime::new(RuntimeOptions {
+    extensions: vec![test_ext::init_ops()],
+    ..Default::default()
+  });
+
+  // Drain events until we get to Ready
+  loop {
+    logging_waker.woken.store(false, Ordering::SeqCst);
+    let res = runtime.poll_event_loop(&mut Context::from_waker(&waker), false);
+    let ready = matches!(res, Poll::Ready(Ok(())));
+    assert!(ready || logging_waker.woken.load(Ordering::SeqCst));
+    if ready {
+      break;
+    }
+  }
+
+  // Start the AIIFE
+  runtime
+    .execute_script(
+      "",
+      FastString::from_static(
+        "(async () => { await Deno.core.opAsync('op_async_sleep'); })()",
+      ),
+    )
+    .unwrap();
+
+  // Wait for future to finish
+  while STATE.load(Ordering::SeqCst) < 2 {
+    tokio::time::sleep(Duration::from_millis(1)).await;
+  }
+
+  // This shouldn't take one minute, but if it does, things are definitely locked up
+  for _ in 0..Duration::from_secs(60).as_millis() {
+    if logging_waker.woken.load(Ordering::SeqCst) {
+      // Success
+      return;
+    }
+    tokio::time::sleep(Duration::from_millis(1)).await;
+  }
+
+  panic!("The waker was never woken after the future completed");
+}
+
 #[tokio::test]
 async fn test_poll_value() {
   let mut runtime = JsRuntime::new(Default::default());