fix(core): poll async ops eagerly (#12385)

Currently all async ops are polled lazily, which means that op
initialization code is postponed until control is yielded to the event
loop. This has some weird consequences, e.g.

```js
let listener = Deno.listen(...);
let conn_promise = listener.accept();
listener.close();
// `BadResource` is thrown. A reasonable error would be `Interrupted`.
let conn = await conn_promise;
```

JavaScript promises are expected to be eagerly evaluated. This patch
makes ops actually do that.

5 files changed, 76 insertions, 14 deletions

diff --git a/core/lib.rs b/core/lib.rs
index ea7b322e2..c0419f8ab 100644
--- a/core/lib.rs
+++ b/core/lib.rs
@@ -68,6 +68,7 @@ pub use crate::normalize_path::normalize_path;
 pub use crate::ops::serialize_op_result;
 pub use crate::ops::Op;
 pub use crate::ops::OpAsyncFuture;
+pub use crate::ops::OpCall;
 pub use crate::ops::OpFn;
 pub use crate::ops::OpId;
 pub use crate::ops::OpPayload;
diff --git a/core/modules.rs b/core/modules.rs
index 2af09057f..31e03196a 100644
--- a/core/modules.rs
+++ b/core/modules.rs
@@ -722,6 +722,7 @@ impl ModuleMap {
 #[cfg(test)]
 mod tests {
   use super::*;
+  use crate::ops::OpCall;
   use crate::serialize_op_result;
   use crate::JsRuntime;
   use crate::Op;
@@ -1009,7 +1010,7 @@ mod tests {
       let (control, _): (u8, ()) = payload.deserialize().unwrap();
       assert_eq!(control, 42);
       let resp = (0, 1, serialize_op_result(Ok(43), state));
-      Op::Async(Box::pin(futures::future::ready(resp)))
+      Op::Async(OpCall::ready(resp))
     };
 
     let mut runtime = JsRuntime::new(RuntimeOptions {
diff --git a/core/ops.rs b/core/ops.rs
index 80bb30eda..05b91f32f 100644
--- a/core/ops.rs
+++ b/core/ops.rs
@@ -6,6 +6,11 @@ use crate::gotham_state::GothamState;
 use crate::ops_metrics::OpsTracker;
 use crate::resources::ResourceTable;
 use crate::runtime::GetErrorClassFn;
+use futures::future::maybe_done;
+use futures::future::FusedFuture;
+use futures::future::MaybeDone;
+use futures::ready;
+use futures::task::noop_waker;
 use futures::Future;
 use indexmap::IndexMap;
 use rusty_v8 as v8;
@@ -17,10 +22,67 @@ use std::ops::Deref;
 use std::ops::DerefMut;
 use std::pin::Pin;
 use std::rc::Rc;
+use std::task::Context;
+use std::task::Poll;
+
+/// Wrapper around a Future, which causes that Future to be polled immediately.
+/// (Background: ops are stored in a `FuturesUnordered` structure which polls
+/// them, but without the `OpCall` wrapper this doesn't happen until the next
+/// turn of the event loop, which is too late for certain ops.)
+pub struct OpCall<T>(MaybeDone<Pin<Box<dyn Future<Output = T>>>>);
+
+impl<T> OpCall<T> {
+  /// Wraps a future, and polls the inner future immediately.
+  /// This should be the default choice for ops.
+  pub fn eager(fut: impl Future<Output = T> + 'static) -> Self {
+    let boxed = Box::pin(fut) as Pin<Box<dyn Future<Output = T>>>;
+    let mut inner = maybe_done(boxed);
+    let waker = noop_waker();
+    let mut cx = Context::from_waker(&waker);
+    let mut pinned = Pin::new(&mut inner);
+    let _ = pinned.as_mut().poll(&mut cx);
+    Self(inner)
+  }
+
+  /// Wraps a future; the inner future is polled the usual way (lazily).
+  pub fn lazy(fut: impl Future<Output = T> + 'static) -> Self {
+    let boxed = Box::pin(fut) as Pin<Box<dyn Future<Output = T>>>;
+    let inner = maybe_done(boxed);
+    Self(inner)
+  }
+
+  /// Create a future by specifying its output. This is basically the same as
+  /// `async { value }` or `futures::future::ready(value)`.
+  pub fn ready(value: T) -> Self {
+    Self(MaybeDone::Done(value))
+  }
+}
+
+impl<T> Future for OpCall<T> {
+  type Output = T;
+
+  fn poll(
+    self: std::pin::Pin<&mut Self>,
+    cx: &mut std::task::Context<'_>,
+  ) -> std::task::Poll<Self::Output> {
+    let inner = unsafe { &mut self.get_unchecked_mut().0 };
+    let mut pinned = Pin::new(inner);
+    ready!(pinned.as_mut().poll(cx));
+    Poll::Ready(pinned.as_mut().take_output().unwrap())
+  }
+}
+
+impl<F> FusedFuture for OpCall<F>
+where
+  F: Future,
+{
+  fn is_terminated(&self) -> bool {
+    self.0.is_terminated()
+  }
+}
 
 pub type PromiseId = u64;
-pub type OpAsyncFuture =
-  Pin<Box<dyn Future<Output = (PromiseId, OpId, OpResult)>>>;
+pub type OpAsyncFuture = OpCall<(PromiseId, OpId, OpResult)>;
 pub type OpFn = dyn Fn(Rc<RefCell<OpState>>, OpPayload) -> Op + 'static;
 pub type OpId = usize;
 
diff --git a/core/ops_json.rs b/core/ops_json.rs
index 0ca7e5ce4..dca9a9a77 100644
--- a/core/ops_json.rs
+++ b/core/ops_json.rs
@@ -1,6 +1,7 @@
 // Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
 
 use crate::error::AnyError;
+use crate::ops::OpCall;
 use crate::serialize_op_result;
 use crate::Op;
 use crate::OpFn;
@@ -35,7 +36,7 @@ pub fn void_op_async() -> Box<OpFn> {
     let op_id = payload.op_id;
     let pid = payload.promise_id;
     let op_result = serialize_op_result(Ok(()), state);
-    Op::Async(Box::pin(futures::future::ready((pid, op_id, op_result))))
+    Op::Async(OpCall::ready((pid, op_id, op_result)))
   })
 }
 
@@ -127,7 +128,7 @@ where
     use crate::futures::FutureExt;
     let fut = op_fn(state.clone(), a, b)
       .map(move |result| (pid, op_id, serialize_op_result(result, state)));
-    Op::Async(Box::pin(fut))
+    Op::Async(OpCall::eager(fut))
   })
 }
 
@@ -159,7 +160,7 @@ where
     use crate::futures::FutureExt;
     let fut = op_fn(state.clone(), a, b)
       .map(move |result| (pid, op_id, serialize_op_result(result, state)));
-    Op::AsyncUnref(Box::pin(fut))
+    Op::AsyncUnref(OpCall::eager(fut))
   })
 }
 
diff --git a/core/runtime.rs b/core/runtime.rs
index 1928ff31c..873dcd3f5 100644
--- a/core/runtime.rs
+++ b/core/runtime.rs
@@ -28,7 +28,6 @@ use futures::future::FutureExt;
 use futures::stream::FuturesUnordered;
 use futures::stream::StreamExt;
 use futures::task::AtomicWaker;
-use futures::Future;
 use std::any::Any;
 use std::cell::RefCell;
 use std::collections::HashMap;
@@ -36,7 +35,6 @@ use std::convert::TryFrom;
 use std::ffi::c_void;
 use std::mem::forget;
 use std::option::Option;
-use std::pin::Pin;
 use std::rc::Rc;
 use std::sync::Arc;
 use std::sync::Mutex;
@@ -44,8 +42,7 @@ use std::sync::Once;
 use std::task::Context;
 use std::task::Poll;
 
-type PendingOpFuture =
-  Pin<Box<dyn Future<Output = (PromiseId, OpId, OpResult)>>>;
+type PendingOpFuture = OpCall<(PromiseId, OpId, OpResult)>;
 
 pub enum Snapshot {
   Static(&'static [u8]),
@@ -1613,6 +1610,7 @@ pub mod tests {
   use crate::ZeroCopyBuf;
   use futures::future::lazy;
   use std::ops::FnOnce;
+  use std::pin::Pin;
   use std::rc::Rc;
   use std::sync::atomic::{AtomicUsize, Ordering};
   use std::sync::Arc;
@@ -1645,16 +1643,15 @@ pub mod tests {
       Mode::Async => {
         assert_eq!(control, 42);
         let resp = (0, 1, serialize_op_result(Ok(43), rc_op_state));
-        Op::Async(Box::pin(futures::future::ready(resp)))
+        Op::Async(OpCall::ready(resp))
       }
       Mode::AsyncZeroCopy(has_buffer) => {
         assert_eq!(buf.is_some(), has_buffer);
         if let Some(buf) = buf {
           assert_eq!(buf.len(), 1);
         }
-
-        let resp = serialize_op_result(Ok(43), rc_op_state);
-        Op::Async(Box::pin(futures::future::ready((0, 1, resp))))
+        let resp = (0, 1, serialize_op_result(Ok(43), rc_op_state));
+        Op::Async(OpCall::ready(resp))
       }
     }
   }