// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.

// Do not add any dependency to modules.rs!
// modules.rs is complex and should remain decoupled from isolate.rs to keep the
// Isolate struct from becoming too bloating for users who do not need
// asynchronous module loading.

use rusty_v8 as v8;

use crate::bindings;
use crate::ops::*;
use crate::shared_queue::SharedQueue;
use crate::shared_queue::RECOMMENDED_SIZE;
use crate::ErrBox;
use crate::JSError;
use crate::ResourceTable;
use crate::ZeroCopyBuf;
use futures::future::FutureExt;
use futures::stream::FuturesUnordered;
use futures::stream::StreamExt;
use futures::task::AtomicWaker;
use futures::Future;
use serde_json::json;
use serde_json::Value;
use std::any::Any;
use std::cell::RefCell;
use std::collections::HashMap;
use std::convert::From;
use std::ffi::c_void;
use std::mem::forget;
use std::ops::Deref;
use std::ops::DerefMut;
use std::option::Option;
use std::pin::Pin;
use std::rc::Rc;
use std::sync::Once;
use std::task::Context;
use std::task::Poll;

type PendingOpFuture = Pin<Box<dyn Future<Output = (OpId, Buf)>>>;

/// Stores a script used to initialize a Isolate
pub struct Script<'a> {
  pub source: &'a str,
  pub filename: &'a str,
}

// TODO(ry) It's ugly that we have both Script and OwnedScript. Ideally we
// wouldn't expose such twiddly complexity.
struct OwnedScript {
  pub source: String,
  pub filename: String,
}

impl From<Script<'_>> for OwnedScript {
  fn from(s: Script) -> OwnedScript {
    OwnedScript {
      source: s.source.to_string(),
      filename: s.filename.to_string(),
    }
  }
}

pub enum Snapshot {
  Static(&'static [u8]),
  JustCreated(v8::StartupData),
  Boxed(Box<[u8]>),
}

/// Represents data used to initialize an isolate at startup, either
/// in the form of a binary snapshot or a JavaScript source file.
pub enum StartupData<'a> {
  Script(Script<'a>),
  Snapshot(Snapshot),
  None,
}

impl StartupData<'_> {
  fn into_options(self) -> (Option<OwnedScript>, Option<Snapshot>) {
    match self {
      Self::Script(script) => (Some(script.into()), None),
      Self::Snapshot(snapshot) => (None, Some(snapshot)),
      Self::None => (None, None),
    }
  }
}

type JSErrorCreateFn = dyn Fn(JSError) -> ErrBox;

pub type GetErrorClassFn = &'static dyn for<'e> Fn(&'e ErrBox) -> &'static str;

/// Objects that need to live as long as the isolate
#[derive(Default)]
struct IsolateAllocations {
  near_heap_limit_callback_data:
    Option<(Box<RefCell<dyn Any>>, v8::NearHeapLimitCallback)>,
}

/// A single execution context of JavaScript. Corresponds roughly to the "Web
/// Worker" concept in the DOM. An CoreIsolate is a Future that can be used with
/// Tokio. The CoreIsolate future completes when there is an error or when all
/// pending ops have completed.
///
/// Ops are created in JavaScript by calling Deno.core.dispatch(), and in Rust
/// by implementing dispatcher function that takes control buffer and optional zero copy buffer
/// as arguments. An async Op corresponds exactly to a Promise in JavaScript.
pub struct CoreIsolate {
  // This is an Option<OwnedIsolate> instead of just OwnedIsolate to workaround
  // an safety issue with SnapshotCreator. See CoreIsolate::drop.
  v8_isolate: Option<v8::OwnedIsolate>,
  snapshot_creator: Option<v8::SnapshotCreator>,
  has_snapshotted: bool,
  needs_init: bool,
  startup_script: Option<OwnedScript>,
  allocations: IsolateAllocations,
}

/// Internal state for CoreIsolate which is stored in one of v8::Isolate's
/// embedder slots.
pub struct CoreIsolateState {
  pub resource_table: Rc<RefCell<ResourceTable>>,
  pub global_context: Option<v8::Global<v8::Context>>,
  pub(crate) shared_ab: Option<v8::Global<v8::SharedArrayBuffer>>,
  pub(crate) js_recv_cb: Option<v8::Global<v8::Function>>,
  pub(crate) js_macrotask_cb: Option<v8::Global<v8::Function>>,
  pub(crate) pending_promise_exceptions: HashMap<i32, v8::Global<v8::Value>>,
  pub(crate) js_error_create_fn: Box<JSErrorCreateFn>,
  pub get_error_class_fn: GetErrorClassFn,
  pub(crate) shared: SharedQueue,
  pending_ops: FuturesUnordered<PendingOpFuture>,
  pending_unref_ops: FuturesUnordered<PendingOpFuture>,
  have_unpolled_ops: bool,
  pub op_registry: OpRegistry,
  waker: AtomicWaker,
}

impl Deref for CoreIsolate {
  type Target = v8::Isolate;
  fn deref(&self) -> &v8::Isolate {
    self.v8_isolate.as_ref().unwrap()
  }
}

impl DerefMut for CoreIsolate {
  fn deref_mut(&mut self) -> &mut v8::Isolate {
    self.v8_isolate.as_mut().unwrap()
  }
}

impl Drop for CoreIsolate {
  fn drop(&mut self) {
    if let Some(creator) = self.snapshot_creator.take() {
      // TODO(ry): in rusty_v8, `SnapShotCreator::get_owned_isolate()` returns
      // a `struct OwnedIsolate` which is not actually owned, hence the need
      // here to leak the `OwnedIsolate` in order to avoid a double free and
      // the segfault that it causes.
      let v8_isolate = self.v8_isolate.take().unwrap();
      forget(v8_isolate);

      // TODO(ry) V8 has a strange assert which prevents a SnapshotCreator from
      // being deallocated if it hasn't created a snapshot yet.
      // https://github.com/v8/v8/blob/73212783fbd534fac76cc4b66aac899c13f71fc8/src/api.cc#L603
      // If that assert is removed, this if guard could be removed.
      // WARNING: There may be false positive LSAN errors here.
      if self.has_snapshotted {
        drop(creator);
      }
    }
  }
}

#[allow(clippy::missing_safety_doc)]
pub unsafe fn v8_init() {
  let platform = v8::new_default_platform().unwrap();
  v8::V8::initialize_platform(platform);
  v8::V8::initialize();
  // TODO(ry) This makes WASM compile synchronously. Eventually we should
  // remove this to make it work asynchronously too. But that requires getting
  // PumpMessageLoop and RunMicrotasks setup correctly.
  // See https://github.com/denoland/deno/issues/2544
  let argv = vec![
    "".to_string(),
    "--wasm-test-streaming".to_string(),
    "--no-wasm-async-compilation".to_string(),
    "--harmony-top-level-await".to_string(),
    "--experimental-wasm-bigint".to_string(),
  ];
  v8::V8::set_flags_from_command_line(argv);
}

/// Minimum and maximum bytes of heap used in an isolate
pub struct HeapLimits {
  /// By default V8 starts with a small heap and dynamically grows it to match
  /// the set of live objects. This may lead to ineffective garbage collections
  /// at startup if the live set is large. Setting the initial heap size avoids
  /// such garbage collections. Note that this does not affect young generation
  /// garbage collections.
  pub initial: usize,
  /// When the heap size approaches `max`, V8 will perform series of
  /// garbage collections and invoke the
  /// [NearHeapLimitCallback](TODO).
  /// If the garbage collections do not help and the callback does not
  /// increase the limit, then V8 will crash with V8::FatalProcessOutOfMemory.
  pub max: usize,
}

pub(crate) struct IsolateOptions {
  will_snapshot: bool,
  startup_script: Option<OwnedScript>,
  startup_snapshot: Option<Snapshot>,
  heap_limits: Option<HeapLimits>,
}

impl CoreIsolate {
  /// startup_data defines the snapshot or script used at startup to initialize
  /// the isolate.
  pub fn new(startup_data: StartupData, will_snapshot: bool) -> Self {
    let (startup_script, startup_snapshot) = startup_data.into_options();
    let options = IsolateOptions {
      will_snapshot,
      startup_script,
      startup_snapshot,
      heap_limits: None,
    };

    Self::from_options(options)
  }

  /// This is useful for controlling memory usage of scripts.
  ///
  /// See [`HeapLimits`](struct.HeapLimits.html) for more details.
  ///
  /// Make sure to use [`add_near_heap_limit_callback`](#method.add_near_heap_limit_callback)
  /// to prevent v8 from crashing when reaching the upper limit.
  pub fn with_heap_limits(
    startup_data: StartupData,
    heap_limits: HeapLimits,
  ) -> Self {
    let (startup_script, startup_snapshot) = startup_data.into_options();
    let options = IsolateOptions {
      will_snapshot: false,
      startup_script,
      startup_snapshot,
      heap_limits: Some(heap_limits),
    };

    Self::from_options(options)
  }

  fn from_options(options: IsolateOptions) -> Self {
    static DENO_INIT: Once = Once::new();
    DENO_INIT.call_once(|| {
      unsafe { v8_init() };
    });

    let global_context;
    let (mut isolate, maybe_snapshot_creator) = if options.will_snapshot {
      // TODO(ry) Support loading snapshots before snapshotting.
      assert!(options.startup_snapshot.is_none());
      let mut creator =
        v8::SnapshotCreator::new(Some(&bindings::EXTERNAL_REFERENCES));
      let isolate = unsafe { creator.get_owned_isolate() };
      let mut isolate = CoreIsolate::setup_isolate(isolate);
      {
        let scope = &mut v8::HandleScope::new(&mut isolate);
        let context = bindings::initialize_context(scope);
        global_context = v8::Global::new(scope, context);
        creator.set_default_context(context);
      }
      (isolate, Some(creator))
    } else {
      let mut params = v8::Isolate::create_params()
        .external_references(&**bindings::EXTERNAL_REFERENCES);
      let snapshot_loaded = if let Some(snapshot) = options.startup_snapshot {
        params = match snapshot {
          Snapshot::Static(data) => params.snapshot_blob(data),
          Snapshot::JustCreated(data) => params.snapshot_blob(data),
          Snapshot::Boxed(data) => params.snapshot_blob(data),
        };
        true
      } else {
        false
      };

      if let Some(heap_limits) = options.heap_limits {
        params = params.heap_limits(heap_limits.initial, heap_limits.max)
      }

      let isolate = v8::Isolate::new(params);
      let mut isolate = CoreIsolate::setup_isolate(isolate);
      {
        let scope = &mut v8::HandleScope::new(&mut isolate);
        let context = if snapshot_loaded {
          v8::Context::new(scope)
        } else {
          // If no snapshot is provided, we initialize the context with empty
          // main source code and source maps.
          bindings::initialize_context(scope)
        };
        global_context = v8::Global::new(scope, context);
      }
      (isolate, None)
    };

    isolate.set_slot(Rc::new(RefCell::new(CoreIsolateState {
      global_context: Some(global_context),
      resource_table: Rc::new(RefCell::new(ResourceTable::default())),
      pending_promise_exceptions: HashMap::new(),
      shared_ab: None,
      js_recv_cb: None,
      js_macrotask_cb: None,
      js_error_create_fn: Box::new(JSError::create),
      get_error_class_fn: &|_| "Error",
      shared: SharedQueue::new(RECOMMENDED_SIZE),
      pending_ops: FuturesUnordered::new(),
      pending_unref_ops: FuturesUnordered::new(),
      have_unpolled_ops: false,
      op_registry: OpRegistry::new(),
      waker: AtomicWaker::new(),
    })));

    Self {
      v8_isolate: Some(isolate),
      snapshot_creator: maybe_snapshot_creator,
      has_snapshotted: false,
      needs_init: true,
      startup_script: options.startup_script,
      allocations: IsolateAllocations::default(),
    }
  }

  fn setup_isolate(mut isolate: v8::OwnedIsolate) -> v8::OwnedIsolate {
    isolate.set_capture_stack_trace_for_uncaught_exceptions(true, 10);
    isolate.set_promise_reject_callback(bindings::promise_reject_callback);
    isolate
  }

  pub fn state(isolate: &v8::Isolate) -> Rc<RefCell<CoreIsolateState>> {
    let s = isolate.get_slot::<Rc<RefCell<CoreIsolateState>>>().unwrap();
    s.clone()
  }

  /// Executes a bit of built-in JavaScript to provide Deno.sharedQueue.
  pub(crate) fn shared_init(&mut self) {
    if self.needs_init {
      self.needs_init = false;
      js_check(self.execute("core.js", include_str!("core.js")));
      // Maybe execute the startup script.
      if let Some(s) = self.startup_script.take() {
        self.execute(&s.filename, &s.source).unwrap()
      }
    }
  }

  /// Executes traditional JavaScript code (traditional = not ES modules)
  ///
  /// ErrBox can be downcast to a type that exposes additional information about
  /// the V8 exception. By default this type is JSError, however it may be a
  /// different type if CoreIsolate::set_js_error_create_fn() has been used.
  pub fn execute(
    &mut self,
    js_filename: &str,
    js_source: &str,
  ) -> Result<(), ErrBox> {
    self.shared_init();

    let state_rc = Self::state(self);
    let state = state_rc.borrow();

    let scope = &mut v8::HandleScope::with_context(
      self.v8_isolate.as_mut().unwrap(),
      state.global_context.as_ref().unwrap(),
    );

    drop(state);

    let source = v8::String::new(scope, js_source).unwrap();
    let name = v8::String::new(scope, js_filename).unwrap();
    let origin = bindings::script_origin(scope, name);

    let tc_scope = &mut v8::TryCatch::new(scope);

    let script = match v8::Script::compile(tc_scope, source, Some(&origin)) {
      Some(script) => script,
      None => {
        let exception = tc_scope.exception().unwrap();
        return exception_to_err_result(tc_scope, exception);
      }
    };

    match script.run(tc_scope) {
      Some(_) => Ok(()),
      None => {
        assert!(tc_scope.has_caught());
        let exception = tc_scope.exception().unwrap();
        exception_to_err_result(tc_scope, exception)
      }
    }
  }

  /// Takes a snapshot. The isolate should have been created with will_snapshot
  /// set to true.
  ///
  /// ErrBox can be downcast to a type that exposes additional information about
  /// the V8 exception. By default this type is JSError, however it may be a
  /// different type if CoreIsolate::set_js_error_create_fn() has been used.
  pub fn snapshot(&mut self) -> v8::StartupData {
    assert!(self.snapshot_creator.is_some());
    let state = Self::state(self);

    // Note: create_blob() method must not be called from within a HandleScope.
    // TODO(piscisaureus): The rusty_v8 type system should enforce this.
    state.borrow_mut().global_context.take();

    let snapshot_creator = self.snapshot_creator.as_mut().unwrap();
    let snapshot = snapshot_creator
      .create_blob(v8::FunctionCodeHandling::Keep)
      .unwrap();
    self.has_snapshotted = true;

    snapshot
  }

  /// Defines the how Deno.core.dispatch() acts.
  /// Called whenever Deno.core.dispatch() is called in JavaScript. zero_copy_buf
  /// corresponds to the second argument of Deno.core.dispatch().
  ///
  /// Requires runtime to explicitly ask for op ids before using any of the ops.
  pub fn register_op<F>(&mut self, name: &str, op: F) -> OpId
  where
    F: Fn(&mut CoreIsolateState, &mut [ZeroCopyBuf]) -> Op + 'static,
  {
    let state_rc = Self::state(self);
    let mut state = state_rc.borrow_mut();
    state.op_registry.register(name, op)
  }

  pub fn register_op_json_sync<F>(&mut self, name: &str, op: F) -> OpId
  where
    F: 'static
      + Fn(
        &mut CoreIsolateState,
        serde_json::Value,
        &mut [ZeroCopyBuf],
      ) -> Result<serde_json::Value, ErrBox>,
  {
    let core_op =
      move |state: &mut CoreIsolateState, bufs: &mut [ZeroCopyBuf]| -> Op {
        let value = serde_json::from_slice(&bufs[0]).unwrap();
        let result = op(state, value, &mut bufs[1..]);
        let buf = serialize_result(None, result, state.get_error_class_fn);
        Op::Sync(buf)
      };

    let state_rc = Self::state(self);
    let mut state = state_rc.borrow_mut();
    state.op_registry.register(name, core_op)
  }

  pub fn register_op_json_async<F, Fut>(&mut self, name: &str, op: F) -> OpId
  where
    Fut: 'static + Future<Output = Result<serde_json::Value, ErrBox>>,
    F: 'static
      + Fn(&mut CoreIsolateState, serde_json::Value, &mut [ZeroCopyBuf]) -> Fut,
  {
    let core_op = move |state: &mut CoreIsolateState,
                        bufs: &mut [ZeroCopyBuf]|
          -> Op {
      let get_error_class_fn = state.get_error_class_fn;
      let value: serde_json::Value = serde_json::from_slice(&bufs[0]).unwrap();
      let promise_id = value.get("promiseId").unwrap().as_u64().unwrap();
      let fut = op(state, value, &mut bufs[1..]);
      let fut2 = fut.map(move |result| {
        serialize_result(Some(promise_id), result, get_error_class_fn)
      });
      Op::Async(Box::pin(fut2))
    };

    let state_rc = Self::state(self);
    let mut state = state_rc.borrow_mut();
    state.op_registry.register(name, core_op)
  }

  /// Registers a callback on the isolate when the memory limits are approached.
  /// Use this to prevent V8 from crashing the process when reaching the limit.
  ///
  /// Calls the closure with the current heap limit and the initial heap limit.
  /// The return value of the closure is set as the new limit.
  pub fn add_near_heap_limit_callback<C>(&mut self, cb: C)
  where
    C: FnMut(usize, usize) -> usize + 'static,
  {
    let boxed_cb = Box::new(RefCell::new(cb));
    let data = boxed_cb.as_ptr() as *mut c_void;

    let prev = self
      .allocations
      .near_heap_limit_callback_data
      .replace((boxed_cb, near_heap_limit_callback::<C>));
    if let Some((_, prev_cb)) = prev {
      self
        .v8_isolate
        .as_mut()
        .unwrap()
        .remove_near_heap_limit_callback(prev_cb, 0);
    }

    self
      .v8_isolate
      .as_mut()
      .unwrap()
      .add_near_heap_limit_callback(near_heap_limit_callback::<C>, data);
  }

  pub fn remove_near_heap_limit_callback(&mut self, heap_limit: usize) {
    if let Some((_, cb)) = self.allocations.near_heap_limit_callback_data.take()
    {
      self
        .v8_isolate
        .as_mut()
        .unwrap()
        .remove_near_heap_limit_callback(cb, heap_limit);
    }
  }
}

extern "C" fn near_heap_limit_callback<F>(
  data: *mut c_void,
  current_heap_limit: usize,
  initial_heap_limit: usize,
) -> usize
where
  F: FnMut(usize, usize) -> usize,
{
  let callback = unsafe { &mut *(data as *mut F) };
  callback(current_heap_limit, initial_heap_limit)
}

fn serialize_result(
  promise_id: Option<u64>,
  result: Result<Value, ErrBox>,
  get_error_class_fn: GetErrorClassFn,
) -> Buf {
  let value = match result {
    Ok(v) => json!({ "ok": v, "promiseId": promise_id }),
    Err(err) => json!({
      "promiseId": promise_id ,
      "err": {
        "className": (get_error_class_fn)(&err),
        "message": err.to_string(),
      }
    }),
  };
  serde_json::to_vec(&value).unwrap().into_boxed_slice()
}

impl Future for CoreIsolate {
  type Output = Result<(), ErrBox>;

  fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
    let core_isolate = self.get_mut();
    core_isolate.shared_init();

    let state_rc = Self::state(core_isolate);
    {
      let state = state_rc.borrow();
      state.waker.register(cx.waker());
    }

    let scope = &mut v8::HandleScope::with_context(
      &mut **core_isolate,
      state_rc.borrow().global_context.as_ref().unwrap(),
    );

    check_promise_exceptions(scope)?;

    let mut overflow_response: Option<(OpId, Buf)> = None;

    loop {
      let mut state = state_rc.borrow_mut();
      // Now handle actual ops.
      state.have_unpolled_ops = false;

      let pending_r = state.pending_ops.poll_next_unpin(cx);
      match pending_r {
        Poll::Ready(None) => break,
        Poll::Pending => break,
        Poll::Ready(Some((op_id, buf))) => {
          let successful_push = state.shared.push(op_id, &buf);
          if !successful_push {
            // If we couldn't push the response to the shared queue, because
            // there wasn't enough size, we will return the buffer via the
            // legacy route, using the argument of deno_respond.
            overflow_response = Some((op_id, buf));
            break;
          }
        }
      };
    }

    loop {
      let mut state = state_rc.borrow_mut();
      let unref_r = state.pending_unref_ops.poll_next_unpin(cx);
      #[allow(clippy::match_wild_err_arm)]
      match unref_r {
        Poll::Ready(None) => break,
        Poll::Pending => break,
        Poll::Ready(Some((op_id, buf))) => {
          let successful_push = state.shared.push(op_id, &buf);
          if !successful_push {
            // If we couldn't push the response to the shared queue, because
            // there wasn't enough size, we will return the buffer via the
            // legacy route, using the argument of deno_respond.
            overflow_response = Some((op_id, buf));
            break;
          }
        }
      };
    }

    {
      let state = state_rc.borrow();
      if state.shared.size() > 0 {
        drop(state);
        async_op_response(scope, None)?;
        // The other side should have shifted off all the messages.
        let state = state_rc.borrow();
        assert_eq!(state.shared.size(), 0);
      }
    }

    {
      if let Some((op_id, buf)) = overflow_response.take() {
        async_op_response(scope, Some((op_id, buf)))?;
      }

      drain_macrotasks(scope)?;

      check_promise_exceptions(scope)?;
    }

    let state = state_rc.borrow();
    // We're idle if pending_ops is empty.
    if state.pending_ops.is_empty() {
      Poll::Ready(Ok(()))
    } else {
      if state.have_unpolled_ops {
        state.waker.wake();
      }
      Poll::Pending
    }
  }
}

impl CoreIsolateState {
  /// Defines the how Deno.core.dispatch() acts.
  /// Called whenever Deno.core.dispatch() is called in JavaScript. zero_copy_buf
  /// corresponds to the second argument of Deno.core.dispatch().
  ///
  /// Requires runtime to explicitly ask for op ids before using any of the ops.
  pub fn register_op<F>(&mut self, name: &str, op: F) -> OpId
  where
    F: Fn(&mut CoreIsolateState, &mut [ZeroCopyBuf]) -> Op + 'static,
  {
    self.op_registry.register(name, op)
  }

  /// Allows a callback to be set whenever a V8 exception is made. This allows
  /// the caller to wrap the JSError into an error. By default this callback
  /// is set to JSError::create.
  pub fn set_js_error_create_fn(
    &mut self,
    f: impl Fn(JSError) -> ErrBox + 'static,
  ) {
    self.js_error_create_fn = Box::new(f);
  }

  pub fn set_get_error_class_fn(&mut self, f: GetErrorClassFn) {
    self.get_error_class_fn = f;
  }

  pub fn dispatch_op<'s>(
    &mut self,
    scope: &mut v8::HandleScope<'s>,
    op_id: OpId,
    zero_copy_bufs: &mut [ZeroCopyBuf],
  ) -> Option<(OpId, Box<[u8]>)> {
    let op = if let Some(dispatcher) = self.op_registry.get(op_id) {
      dispatcher(self, zero_copy_bufs)
    } else {
      let message =
        v8::String::new(scope, &format!("Unknown op id: {}", op_id)).unwrap();
      let exception = v8::Exception::type_error(scope, message);
      scope.throw_exception(exception);
      return None;
    };

    debug_assert_eq!(self.shared.size(), 0);
    match op {
      Op::Sync(buf) => {
        // For sync messages, we always return the response via Deno.core.send's
        // return value. Sync messages ignore the op_id.
        let op_id = 0;
        Some((op_id, buf))
      }
      Op::Async(fut) => {
        let fut2 = fut.map(move |buf| (op_id, buf));
        self.pending_ops.push(fut2.boxed_local());
        self.have_unpolled_ops = true;
        None
      }
      Op::AsyncUnref(fut) => {
        let fut2 = fut.map(move |buf| (op_id, buf));
        self.pending_unref_ops.push(fut2.boxed_local());
        self.have_unpolled_ops = true;
        None
      }
    }
  }
}

fn async_op_response<'s>(
  scope: &mut v8::HandleScope<'s>,
  maybe_buf: Option<(OpId, Box<[u8]>)>,
) -> Result<(), ErrBox> {
  let context = scope.get_current_context();
  let global: v8::Local<v8::Value> = context.global(scope).into();
  let js_recv_cb = CoreIsolate::state(scope)
    .borrow()
    .js_recv_cb
    .as_ref()
    .map(|cb| v8::Local::new(scope, cb))
    .expect("Deno.core.recv has not been called.");

  let tc_scope = &mut v8::TryCatch::new(scope);

  match maybe_buf {
    Some((op_id, buf)) => {
      let op_id: v8::Local<v8::Value> =
        v8::Integer::new(tc_scope, op_id as i32).into();
      let ui8: v8::Local<v8::Value> =
        bindings::boxed_slice_to_uint8array(tc_scope, buf).into();
      js_recv_cb.call(tc_scope, global, &[op_id, ui8])
    }
    None => js_recv_cb.call(tc_scope, global, &[]),
  };

  match tc_scope.exception() {
    None => Ok(()),
    Some(exception) => exception_to_err_result(tc_scope, exception),
  }
}

fn drain_macrotasks<'s>(scope: &mut v8::HandleScope<'s>) -> Result<(), ErrBox> {
  let context = scope.get_current_context();
  let global: v8::Local<v8::Value> = context.global(scope).into();

  let js_macrotask_cb = match CoreIsolate::state(scope)
    .borrow_mut()
    .js_macrotask_cb
    .as_ref()
  {
    Some(cb) => v8::Local::new(scope, cb),
    None => return Ok(()),
  };

  // Repeatedly invoke macrotask callback until it returns true (done),
  // such that ready microtasks would be automatically run before
  // next macrotask is processed.
  let tc_scope = &mut v8::TryCatch::new(scope);

  loop {
    let is_done = js_macrotask_cb.call(tc_scope, global, &[]);

    if let Some(exception) = tc_scope.exception() {
      return exception_to_err_result(tc_scope, exception);
    }

    let is_done = is_done.unwrap();
    if is_done.is_true() {
      break;
    }
  }

  Ok(())
}

pub(crate) fn exception_to_err_result<'s, T>(
  scope: &mut v8::HandleScope<'s>,
  exception: v8::Local<v8::Value>,
) -> Result<T, ErrBox> {
  // TODO(piscisaureus): in rusty_v8, `is_execution_terminating()` should
  // also be implemented on `struct Isolate`.
  let is_terminating_exception =
    scope.thread_safe_handle().is_execution_terminating();
  let mut exception = exception;

  if is_terminating_exception {
    // TerminateExecution was called. Cancel exception termination so that the
    // exception can be created..
    // TODO(piscisaureus): in rusty_v8, `cancel_terminate_execution()` should
    // also be implemented on `struct Isolate`.
    scope.thread_safe_handle().cancel_terminate_execution();

    // Maybe make a new exception object.
    if exception.is_null_or_undefined() {
      let message = v8::String::new(scope, "execution terminated").unwrap();
      exception = v8::Exception::error(scope, message);
    }
  }

  let js_error = JSError::from_v8_exception(scope, exception);

  let state_rc = CoreIsolate::state(scope);
  let state = state_rc.borrow();
  let js_error = (state.js_error_create_fn)(js_error);

  if is_terminating_exception {
    // Re-enable exception termination.
    // TODO(piscisaureus): in rusty_v8, `terminate_execution()` should also
    // be implemented on `struct Isolate`.
    scope.thread_safe_handle().terminate_execution();
  }

  Err(js_error)
}

fn check_promise_exceptions<'s>(
  scope: &mut v8::HandleScope<'s>,
) -> Result<(), ErrBox> {
  let state_rc = CoreIsolate::state(scope);
  let mut state = state_rc.borrow_mut();

  if let Some(&key) = state.pending_promise_exceptions.keys().next() {
    let handle = state.pending_promise_exceptions.remove(&key).unwrap();
    drop(state);
    let exception = v8::Local::new(scope, handle);
    exception_to_err_result(scope, exception)
  } else {
    Ok(())
  }
}

pub fn js_check<T>(r: Result<T, ErrBox>) -> T {
  if let Err(e) = r {
    panic!(e.to_string());
  }
  r.unwrap()
}

#[cfg(test)]
pub mod tests {
  use super::*;
  use futures::future::lazy;
  use std::ops::FnOnce;
  use std::sync::atomic::{AtomicUsize, Ordering};
  use std::sync::Arc;

  pub fn run_in_task<F>(f: F)
  where
    F: FnOnce(&mut Context) + Send + 'static,
  {
    futures::executor::block_on(lazy(move |cx| f(cx)));
  }

  fn poll_until_ready<F>(future: &mut F, max_poll_count: usize) -> F::Output
  where
    F: Future + Unpin,
  {
    let mut cx = Context::from_waker(futures::task::noop_waker_ref());
    for _ in 0..max_poll_count {
      match future.poll_unpin(&mut cx) {
        Poll::Pending => continue,
        Poll::Ready(val) => return val,
      }
    }
    panic!(
      "CoreIsolate still not ready after polling {} times.",
      max_poll_count
    )
  }

  pub enum Mode {
    Async,
    AsyncUnref,
    AsyncZeroCopy(u8),
    OverflowReqSync,
    OverflowResSync,
    OverflowReqAsync,
    OverflowResAsync,
  }

  pub fn setup(mode: Mode) -> (CoreIsolate, Arc<AtomicUsize>) {
    let dispatch_count = Arc::new(AtomicUsize::new(0));
    let dispatch_count_ = dispatch_count.clone();

    let mut isolate = CoreIsolate::new(StartupData::None, false);

    let dispatcher = move |_state: &mut CoreIsolateState,
                           zero_copy: &mut [ZeroCopyBuf]|
          -> Op {
      dispatch_count_.fetch_add(1, Ordering::Relaxed);
      match mode {
        Mode::Async => {
          assert_eq!(zero_copy.len(), 1);
          assert_eq!(zero_copy[0].len(), 1);
          assert_eq!(zero_copy[0][0], 42);
          let buf = vec![43u8].into_boxed_slice();
          Op::Async(futures::future::ready(buf).boxed())
        }
        Mode::AsyncUnref => {
          assert_eq!(zero_copy.len(), 1);
          assert_eq!(zero_copy[0].len(), 1);
          assert_eq!(zero_copy[0][0], 42);
          let fut = async {
            // This future never finish.
            futures::future::pending::<()>().await;
            vec![43u8].into_boxed_slice()
          };
          Op::AsyncUnref(fut.boxed())
        }
        Mode::AsyncZeroCopy(count) => {
          assert_eq!(zero_copy.len(), count as usize);
          zero_copy.iter().enumerate().for_each(|(idx, buf)| {
            assert_eq!(buf.len(), 1);
            assert_eq!(idx, buf[0] as usize);
          });

          let buf = vec![43u8].into_boxed_slice();
          Op::Async(futures::future::ready(buf).boxed())
        }
        Mode::OverflowReqSync => {
          assert_eq!(zero_copy.len(), 1);
          assert_eq!(zero_copy[0].len(), 100 * 1024 * 1024);
          let buf = vec![43u8].into_boxed_slice();
          Op::Sync(buf)
        }
        Mode::OverflowResSync => {
          assert_eq!(zero_copy.len(), 1);
          assert_eq!(zero_copy[0].len(), 1);
          assert_eq!(zero_copy[0][0], 42);
          let mut vec = Vec::<u8>::new();
          vec.resize(100 * 1024 * 1024, 0);
          vec[0] = 99;
          let buf = vec.into_boxed_slice();
          Op::Sync(buf)
        }
        Mode::OverflowReqAsync => {
          assert_eq!(zero_copy.len(), 1);
          assert_eq!(zero_copy[0].len(), 100 * 1024 * 1024);
          let buf = vec![43u8].into_boxed_slice();
          Op::Async(futures::future::ready(buf).boxed())
        }
        Mode::OverflowResAsync => {
          assert_eq!(zero_copy.len(), 1);
          assert_eq!(zero_copy[0].len(), 1);
          assert_eq!(zero_copy[0][0], 42);
          let mut vec = Vec::<u8>::new();
          vec.resize(100 * 1024 * 1024, 0);
          vec[0] = 4;
          let buf = vec.into_boxed_slice();
          Op::Async(futures::future::ready(buf).boxed())
        }
      }
    };

    isolate.register_op("test", dispatcher);

    js_check(isolate.execute(
      "setup.js",
      r#"
        function assert(cond) {
          if (!cond) {
            throw Error("assert");
          }
        }
        "#,
    ));
    assert_eq!(dispatch_count.load(Ordering::Relaxed), 0);
    (isolate, dispatch_count)
  }

  #[test]
  fn test_dispatch() {
    let (mut isolate, dispatch_count) = setup(Mode::Async);
    js_check(isolate.execute(
      "filename.js",
      r#"
        let control = new Uint8Array([42]);
        Deno.core.send(1, control);
        async function main() {
          Deno.core.send(1, control);
        }
        main();
        "#,
    ));
    assert_eq!(dispatch_count.load(Ordering::Relaxed), 2);
  }

  #[test]
  fn test_dispatch_no_zero_copy_buf() {
    let (mut isolate, dispatch_count) = setup(Mode::AsyncZeroCopy(0));
    js_check(isolate.execute(
      "filename.js",
      r#"
        Deno.core.send(1);
        "#,
    ));
    assert_eq!(dispatch_count.load(Ordering::Relaxed), 1);
  }

  #[test]
  fn test_dispatch_stack_zero_copy_bufs() {
    let (mut isolate, dispatch_count) = setup(Mode::AsyncZeroCopy(2));
    js_check(isolate.execute(
      "filename.js",
      r#"
        let zero_copy_a = new Uint8Array([0]);
        let zero_copy_b = new Uint8Array([1]);
        Deno.core.send(1, zero_copy_a, zero_copy_b);
        "#,
    ));
    assert_eq!(dispatch_count.load(Ordering::Relaxed), 1);
  }

  #[test]
  fn test_dispatch_heap_zero_copy_bufs() {
    let (mut isolate, dispatch_count) = setup(Mode::AsyncZeroCopy(5));
    js_check(isolate.execute(
      "filename.js",
      r#"
        let zero_copy_a = new Uint8Array([0]);
        let zero_copy_b = new Uint8Array([1]);
        let zero_copy_c = new Uint8Array([2]);
        let zero_copy_d = new Uint8Array([3]);
        let zero_copy_e = new Uint8Array([4]);
        Deno.core.send(1, zero_copy_a, zero_copy_b, zero_copy_c, zero_copy_d, zero_copy_e);
        "#,
    ));
    assert_eq!(dispatch_count.load(Ordering::Relaxed), 1);
  }

  #[test]
  fn test_poll_async_delayed_ops() {
    run_in_task(|cx| {
      let (mut isolate, dispatch_count) = setup(Mode::Async);

      js_check(isolate.execute(
        "setup2.js",
        r#"
         let nrecv = 0;
         Deno.core.setAsyncHandler(1, (buf) => {
           nrecv++;
         });
         "#,
      ));
      assert_eq!(dispatch_count.load(Ordering::Relaxed), 0);
      js_check(isolate.execute(
        "check1.js",
        r#"
         assert(nrecv == 0);
         let control = new Uint8Array([42]);
         Deno.core.send(1, control);
         assert(nrecv == 0);
         "#,
      ));
      assert_eq!(dispatch_count.load(Ordering::Relaxed), 1);
      assert!(matches!(isolate.poll_unpin(cx), Poll::Ready(Ok(_))));
      assert_eq!(dispatch_count.load(Ordering::Relaxed), 1);
      js_check(isolate.execute(
        "check2.js",
        r#"
         assert(nrecv == 1);
         Deno.core.send(1, control);
         assert(nrecv == 1);
         "#,
      ));
      assert_eq!(dispatch_count.load(Ordering::Relaxed), 2);
      assert!(matches!(isolate.poll_unpin(cx), Poll::Ready(Ok(_))));
      js_check(isolate.execute("check3.js", "assert(nrecv == 2)"));
      assert_eq!(dispatch_count.load(Ordering::Relaxed), 2);
      // We are idle, so the next poll should be the last.
      assert!(matches!(isolate.poll_unpin(cx), Poll::Ready(Ok(_))));
    });
  }

  #[test]
  fn test_poll_async_optional_ops() {
    run_in_task(|cx| {
      let (mut isolate, dispatch_count) = setup(Mode::AsyncUnref);
      js_check(isolate.execute(
        "check1.js",
        r#"
          Deno.core.setAsyncHandler(1, (buf) => {
            // This handler will never be called
            assert(false);
          });
          let control = new Uint8Array([42]);
          Deno.core.send(1, control);
        "#,
      ));
      assert_eq!(dispatch_count.load(Ordering::Relaxed), 1);
      // The above op never finish, but isolate can finish
      // because the op is an unreffed async op.
      assert!(matches!(isolate.poll_unpin(cx), Poll::Ready(Ok(_))));
    })
  }

  #[test]
  fn terminate_execution() {
    let (mut isolate, _dispatch_count) = setup(Mode::Async);
    // TODO(piscisaureus): in rusty_v8, the `thread_safe_handle()` method
    // should not require a mutable reference to `struct rusty_v8::Isolate`.
    let v8_isolate_handle =
      isolate.v8_isolate.as_mut().unwrap().thread_safe_handle();

    let terminator_thread = std::thread::spawn(move || {
      // allow deno to boot and run
      std::thread::sleep(std::time::Duration::from_millis(100));

      // terminate execution
      let ok = v8_isolate_handle.terminate_execution();
      assert!(ok);
    });

    // Rn an infinite loop, which should be terminated.
    match isolate.execute("infinite_loop.js", "for(;;) {}") {
      Ok(_) => panic!("execution should be terminated"),
      Err(e) => {
        assert_eq!(e.to_string(), "Uncaught Error: execution terminated")
      }
    };

    // Cancel the execution-terminating exception in order to allow script
    // execution again.
    // TODO(piscisaureus): in rusty_v8, `cancel_terminate_execution()` should
    // also be implemented on `struct Isolate`.
    let ok = isolate
      .v8_isolate
      .as_mut()
      .unwrap()
      .thread_safe_handle()
      .cancel_terminate_execution();
    assert!(ok);

    // Verify that the isolate usable again.
    isolate
      .execute("simple.js", "1 + 1")
      .expect("execution should be possible again");

    terminator_thread.join().unwrap();
  }

  #[test]
  fn dangling_shared_isolate() {
    let v8_isolate_handle = {
      // isolate is dropped at the end of this block
      let (mut isolate, _dispatch_count) = setup(Mode::Async);
      // TODO(piscisaureus): in rusty_v8, the `thread_safe_handle()` method
      // should not require a mutable reference to `struct rusty_v8::Isolate`.
      isolate.v8_isolate.as_mut().unwrap().thread_safe_handle()
    };

    // this should not SEGFAULT
    v8_isolate_handle.terminate_execution();
  }

  #[test]
  fn overflow_req_sync() {
    let (mut isolate, dispatch_count) = setup(Mode::OverflowReqSync);
    js_check(isolate.execute(
      "overflow_req_sync.js",
      r#"
        let asyncRecv = 0;
        Deno.core.setAsyncHandler(1, (buf) => { asyncRecv++ });
        // Large message that will overflow the shared space.
        let control = new Uint8Array(100 * 1024 * 1024);
        let response = Deno.core.dispatch(1, control);
        assert(response instanceof Uint8Array);
        assert(response.length == 1);
        assert(response[0] == 43);
        assert(asyncRecv == 0);
        "#,
    ));
    assert_eq!(dispatch_count.load(Ordering::Relaxed), 1);
  }

  #[test]
  fn overflow_res_sync() {
    // TODO(ry) This test is quite slow due to memcpy-ing 100MB into JS. We
    // should optimize this.
    let (mut isolate, dispatch_count) = setup(Mode::OverflowResSync);
    js_check(isolate.execute(
      "overflow_res_sync.js",
      r#"
        let asyncRecv = 0;
        Deno.core.setAsyncHandler(1, (buf) => { asyncRecv++ });
        // Large message that will overflow the shared space.
        let control = new Uint8Array([42]);
        let response = Deno.core.dispatch(1, control);
        assert(response instanceof Uint8Array);
        assert(response.length == 100 * 1024 * 1024);
        assert(response[0] == 99);
        assert(asyncRecv == 0);
        "#,
    ));
    assert_eq!(dispatch_count.load(Ordering::Relaxed), 1);
  }

  #[test]
  fn overflow_req_async() {
    run_in_task(|cx| {
      let (mut isolate, dispatch_count) = setup(Mode::OverflowReqAsync);
      js_check(isolate.execute(
        "overflow_req_async.js",
        r#"
         let asyncRecv = 0;
         Deno.core.setAsyncHandler(1, (buf) => {
           assert(buf.byteLength === 1);
           assert(buf[0] === 43);
           asyncRecv++;
         });
         // Large message that will overflow the shared space.
         let control = new Uint8Array(100 * 1024 * 1024);
         let response = Deno.core.dispatch(1, control);
         // Async messages always have null response.
         assert(response == null);
         assert(asyncRecv == 0);
         "#,
      ));
      assert_eq!(dispatch_count.load(Ordering::Relaxed), 1);
      assert!(matches!(isolate.poll_unpin(cx), Poll::Ready(Ok(_))));
      js_check(isolate.execute("check.js", "assert(asyncRecv == 1);"));
    });
  }

  #[test]
  fn overflow_res_async() {
    run_in_task(|_cx| {
      // TODO(ry) This test is quite slow due to memcpy-ing 100MB into JS. We
      // should optimize this.
      let (mut isolate, dispatch_count) = setup(Mode::OverflowResAsync);
      js_check(isolate.execute(
        "overflow_res_async.js",
        r#"
         let asyncRecv = 0;
         Deno.core.setAsyncHandler(1, (buf) => {
           assert(buf.byteLength === 100 * 1024 * 1024);
           assert(buf[0] === 4);
           asyncRecv++;
         });
         // Large message that will overflow the shared space.
         let control = new Uint8Array([42]);
         let response = Deno.core.dispatch(1, control);
         assert(response == null);
         assert(asyncRecv == 0);
         "#,
      ));
      assert_eq!(dispatch_count.load(Ordering::Relaxed), 1);
      poll_until_ready(&mut isolate, 3).unwrap();
      js_check(isolate.execute("check.js", "assert(asyncRecv == 1);"));
    });
  }

  #[test]
  fn overflow_res_multiple_dispatch_async() {
    // TODO(ry) This test is quite slow due to memcpy-ing 100MB into JS. We
    // should optimize this.
    run_in_task(|_cx| {
      let (mut isolate, dispatch_count) = setup(Mode::OverflowResAsync);
      js_check(isolate.execute(
        "overflow_res_multiple_dispatch_async.js",
        r#"
         let asyncRecv = 0;
         Deno.core.setAsyncHandler(1, (buf) => {
           assert(buf.byteLength === 100 * 1024 * 1024);
           assert(buf[0] === 4);
           asyncRecv++;
         });
         // Large message that will overflow the shared space.
         let control = new Uint8Array([42]);
         let response = Deno.core.dispatch(1, control);
         assert(response == null);
         assert(asyncRecv == 0);
         // Dispatch another message to verify that pending ops
         // are done even if shared space overflows
         Deno.core.dispatch(1, control);
         "#,
      ));
      assert_eq!(dispatch_count.load(Ordering::Relaxed), 2);
      poll_until_ready(&mut isolate, 3).unwrap();
      js_check(isolate.execute("check.js", "assert(asyncRecv == 2);"));
    });
  }

  #[test]
  fn test_pre_dispatch() {
    run_in_task(|mut cx| {
      let (mut isolate, _dispatch_count) = setup(Mode::OverflowResAsync);
      js_check(isolate.execute(
        "bad_op_id.js",
        r#"
          let thrown;
          try {
            Deno.core.dispatch(100);
          } catch (e) {
            thrown = e;
          }
          assert(String(thrown) === "TypeError: Unknown op id: 100");
         "#,
      ));
      if let Poll::Ready(Err(_)) = isolate.poll_unpin(&mut cx) {
        unreachable!();
      }
    });
  }

  #[test]
  fn core_test_js() {
    run_in_task(|mut cx| {
      let (mut isolate, _dispatch_count) = setup(Mode::Async);
      js_check(isolate.execute("core_test.js", include_str!("core_test.js")));
      if let Poll::Ready(Err(_)) = isolate.poll_unpin(&mut cx) {
        unreachable!();
      }
    });
  }

  #[test]
  fn syntax_error() {
    let mut isolate = CoreIsolate::new(StartupData::None, false);
    let src = "hocuspocus(";
    let r = isolate.execute("i.js", src);
    let e = r.unwrap_err();
    let js_error = e.downcast::<JSError>().unwrap();
    assert_eq!(js_error.end_column, Some(11));
  }

  #[test]
  fn test_encode_decode() {
    run_in_task(|mut cx| {
      let (mut isolate, _dispatch_count) = setup(Mode::Async);
      js_check(isolate.execute(
        "encode_decode_test.js",
        include_str!("encode_decode_test.js"),
      ));
      if let Poll::Ready(Err(_)) = isolate.poll_unpin(&mut cx) {
        unreachable!();
      }
    });
  }

  #[test]
  fn will_snapshot() {
    let snapshot = {
      let mut isolate = CoreIsolate::new(StartupData::None, true);
      js_check(isolate.execute("a.js", "a = 1 + 2"));
      isolate.snapshot()
    };

    let startup_data = StartupData::Snapshot(Snapshot::JustCreated(snapshot));
    let mut isolate2 = CoreIsolate::new(startup_data, false);
    js_check(isolate2.execute("check.js", "if (a != 3) throw Error('x')"));
  }

  #[test]
  fn test_from_boxed_snapshot() {
    let snapshot = {
      let mut isolate = CoreIsolate::new(StartupData::None, true);
      js_check(isolate.execute("a.js", "a = 1 + 2"));
      let snap: &[u8] = &*isolate.snapshot();
      Vec::from(snap).into_boxed_slice()
    };

    let startup_data = StartupData::Snapshot(Snapshot::Boxed(snapshot));
    let mut isolate2 = CoreIsolate::new(startup_data, false);
    js_check(isolate2.execute("check.js", "if (a != 3) throw Error('x')"));
  }

  #[test]
  fn test_heap_limits() {
    let heap_limits = HeapLimits {
      initial: 0,
      max: 20 * 1024, // 20 kB
    };
    let mut isolate =
      CoreIsolate::with_heap_limits(StartupData::None, heap_limits);
    let cb_handle = isolate.thread_safe_handle();

    let callback_invoke_count = Rc::new(AtomicUsize::default());
    let inner_invoke_count = Rc::clone(&callback_invoke_count);

    isolate.add_near_heap_limit_callback(
      move |current_limit, _initial_limit| {
        inner_invoke_count.fetch_add(1, Ordering::SeqCst);
        cb_handle.terminate_execution();
        current_limit * 2
      },
    );
    let err = isolate
      .execute(
        "script name",
        r#"let s = ""; while(true) { s += "Hello"; }"#,
      )
      .expect_err("script should fail");
    assert_eq!(
      "Uncaught Error: execution terminated",
      err.downcast::<JSError>().unwrap().message
    );
    assert!(callback_invoke_count.load(Ordering::SeqCst) > 0)
  }

  #[test]
  fn test_heap_limit_cb_remove() {
    let mut isolate = CoreIsolate::new(StartupData::None, false);

    isolate.add_near_heap_limit_callback(|current_limit, _initial_limit| {
      current_limit * 2
    });
    isolate.remove_near_heap_limit_callback(20 * 1024);
    assert!(isolate.allocations.near_heap_limit_callback_data.is_none());
  }

  #[test]
  fn test_heap_limit_cb_multiple() {
    let heap_limits = HeapLimits {
      initial: 0,
      max: 20 * 1024, // 20 kB
    };
    let mut isolate =
      CoreIsolate::with_heap_limits(StartupData::None, heap_limits);
    let cb_handle = isolate.thread_safe_handle();

    let callback_invoke_count_first = Rc::new(AtomicUsize::default());
    let inner_invoke_count_first = Rc::clone(&callback_invoke_count_first);
    isolate.add_near_heap_limit_callback(
      move |current_limit, _initial_limit| {
        inner_invoke_count_first.fetch_add(1, Ordering::SeqCst);
        current_limit * 2
      },
    );

    let callback_invoke_count_second = Rc::new(AtomicUsize::default());
    let inner_invoke_count_second = Rc::clone(&callback_invoke_count_second);
    isolate.add_near_heap_limit_callback(
      move |current_limit, _initial_limit| {
        inner_invoke_count_second.fetch_add(1, Ordering::SeqCst);
        cb_handle.terminate_execution();
        current_limit * 2
      },
    );

    let err = isolate
      .execute(
        "script name",
        r#"let s = ""; while(true) { s += "Hello"; }"#,
      )
      .expect_err("script should fail");
    assert_eq!(
      "Uncaught Error: execution terminated",
      err.downcast::<JSError>().unwrap().message
    );
    assert_eq!(0, callback_invoke_count_first.load(Ordering::SeqCst));
    assert!(callback_invoke_count_second.load(Ordering::SeqCst) > 0);
  }
}