gst-plugins-rs/generic/threadshare/src/runtime/executor.rs

// Copyright (C) 2018-2020 Sebastian Dröge <sebastian@centricular.com>
// Copyright (C) 2019-2020 François Laignel <fengalin@free.fr>
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Library General Public
// License as published by the Free Software Foundation; either
// version 2 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Library General Public License for more details.
//
// You should have received a copy of the GNU Library General Public
// License along with this library; if not, write to the
// Free Software Foundation, Inc., 51 Franklin Street, Suite 500,
// Boston, MA 02110-1335, USA.

//! The `Executor` for the `threadshare` GStreamer plugins framework.
//!
//! The [`threadshare`]'s `Executor` consists in a set of [`Context`]s. Each [`Context`] is
//! identified by a `name` and runs a loop in a dedicated `thread`. Users can use the [`Context`]
//! to spawn `Future`s. `Future`s are asynchronous processings which allow waiting for resources
//! in a non-blocking way. Examples of non-blocking operations are:
//!
//! * Waiting for an incoming packet on a Socket.
//! * Waiting for an asynchronous `Mutex` `lock` to succeed.
//! * Waiting for a time related `Future`.
//!
//! `Element` implementations should use [`PadSrc`] & [`PadSink`] which provides high-level features.
//!
//! [`threadshare`]: ../../index.html
//! [`Context`]: struct.Context.html
//! [`PadSrc`]: ../pad/struct.PadSrc.html
//! [`PadSink`]: ../pad/struct.PadSink.html

use futures::channel::oneshot;
use futures::future::BoxFuture;
use futures::prelude::*;

use gst;
use gst::{gst_debug, gst_log, gst_trace, gst_warning};

use lazy_static::lazy_static;

use std::cell::RefCell;
use std::collections::{HashMap, VecDeque};
use std::fmt;
use std::io;
use std::mem;
use std::pin::Pin;
use std::sync::mpsc as sync_mpsc;
use std::sync::{Arc, Mutex, Weak};
use std::task::Poll;
use std::thread;
use std::time::Duration;

use super::RUNTIME_CAT;

// We are bound to using `sync` for the `runtime` `Mutex`es. Attempts to use `async` `Mutex`es
// lead to the following issues:
//
// * `CONTEXTS`: can't `spawn` a `Future` when called from a `Context` thread via `ffi`.
// * `timers`: can't automatically `remove` the timer from `BinaryHeap` because `async drop`
//    is not available.
// * `task_queues`: can't `add` a pending task when called from a `Context` thread via `ffi`.
//
// Also, we want to be able to `acquire` a `Context` outside of an `async` context.
// These `Mutex`es must be `lock`ed for a short period.
lazy_static! {
    static ref CONTEXTS: Mutex<HashMap<String, Weak<ContextInner>>> = Mutex::new(HashMap::new());
}

thread_local!(static CURRENT_THREAD_CONTEXT: RefCell<Option<ContextWeak>> = RefCell::new(None));

tokio::task_local! {
    static CURRENT_TASK_ID: TaskId;
}

/// Blocks on `future` in one way or another if possible.
///
/// IO & time related `Future`s must be handled within their own [`Context`].
/// Wait for the result using a [`JoinHandle`] or a `channel`.
///
/// If there's currently an active `Context` with a task, then the future is only queued up as a
/// pending sub task for that task.
///
/// Otherwise the current thread is blocking and the passed in future is executed.
///
/// Note that you must not pass any futures here that wait for the currently active task in one way
/// or another as this would deadlock!
pub fn block_on_or_add_sub_task<Fut: Future + Send + 'static>(future: Fut) -> Option<Fut::Output> {
    if let Some((cur_context, cur_task_id)) = Context::current_task() {
        gst_debug!(
            RUNTIME_CAT,
            "Adding subtask to task {:?} on context {}",
            cur_task_id,
            cur_context.name()
        );
        let _ = Context::add_sub_task(async move {
            future.await;
            Ok(())
        });
        return None;
    }

    // Not running in a Context thread so we can block
    Some(block_on(future))
}

/// Blocks on `future`.
///
/// IO & time related `Future`s must be handled within their own [`Context`].
/// Wait for the result using a [`JoinHandle`] or a `channel`.
///
/// The current thread is blocking and the passed in future is executed.
///
/// # Panics
///
/// This function panics if called within a [`Context`] thread.
pub fn block_on<Fut: Future>(future: Fut) -> Fut::Output {
    assert!(!Context::is_context_thread());

    // Not running in a Context thread so we can block
    gst_debug!(RUNTIME_CAT, "Blocking on new dummy context");

    let context = Context(Arc::new(ContextInner {
        real: None,
        task_queues: Mutex::new((0, HashMap::new())),
    }));

    CURRENT_THREAD_CONTEXT.with(move |cur_ctx| {
        *cur_ctx.borrow_mut() = Some(context.downgrade());

        let res = futures::executor::block_on(async move {
            CURRENT_TASK_ID
                .scope(TaskId(0), async move {
                    let task_id = CURRENT_TASK_ID.try_with(|task_id| *task_id).ok();
                    assert_eq!(task_id, Some(TaskId(0)));

                    let res = future.await;

                    while Context::current_has_sub_tasks() {
                        if Context::drain_sub_tasks().await.is_err() {
                            break;
                        }
                    }

                    res
                })
                .await
        });

        *cur_ctx.borrow_mut() = None;

        res
    })
}

/// Yields execution back to the runtime
#[inline]
pub async fn yield_now() {
    tokio::task::yield_now().await;
}

struct ContextThread {
    name: String,
}

impl ContextThread {
    fn start(name: &str, wait: u32) -> Context {
        let context_thread = ContextThread { name: name.into() };
        let (context_sender, context_receiver) = sync_mpsc::channel();
        let join = thread::spawn(move || {
            context_thread.spawn(wait, context_sender);
        });

        let context = context_receiver.recv().expect("Context thread init failed");
        *context
            .0
            .real
            .as_ref()
            .unwrap()
            .shutdown
            .join
            .lock()
            .unwrap() = Some(join);

        context
    }

    fn spawn(&self, wait: u32, context_sender: sync_mpsc::Sender<Context>) {
        gst_debug!(RUNTIME_CAT, "Started context thread '{}'", self.name);

        let mut runtime = tokio::runtime::Builder::new()
            .basic_scheduler()
            .thread_name(self.name.clone())
            .enable_all()
            .max_throttling(Duration::from_millis(wait as u64))
            .build()
            .expect("Couldn't build the runtime");

        let (shutdown_sender, shutdown_receiver) = oneshot::channel();

        let shutdown = ContextShutdown {
            name: self.name.clone(),
            shutdown: Some(shutdown_sender),
            join: Mutex::new(None),
        };

        let context = Context(Arc::new(ContextInner {
            real: Some(ContextRealInner {
                name: self.name.clone(),
                handle: Mutex::new(runtime.handle().clone()),
                shutdown,
            }),
            task_queues: Mutex::new((0, HashMap::new())),
        }));

        CURRENT_THREAD_CONTEXT.with(|cur_ctx| {
            *cur_ctx.borrow_mut() = Some(context.downgrade());
        });

        context_sender.send(context).unwrap();

        let _ = runtime.block_on(shutdown_receiver);
    }
}

impl Drop for ContextThread {
    fn drop(&mut self) {
        gst_debug!(RUNTIME_CAT, "Terminated: context thread '{}'", self.name);
    }
}

#[derive(Debug)]
struct ContextShutdown {
    name: String,
    shutdown: Option<oneshot::Sender<()>>,
    join: Mutex<Option<thread::JoinHandle<()>>>,
}

impl Drop for ContextShutdown {
    fn drop(&mut self) {
        gst_debug!(
            RUNTIME_CAT,
            "Shutting down context thread thread '{}'",
            self.name
        );
        self.shutdown.take().unwrap();

        gst_trace!(
            RUNTIME_CAT,
            "Waiting for context thread '{}' to shutdown",
            self.name
        );
        let join_handle = self.join.lock().unwrap().take().unwrap();
        let _ = join_handle.join();
    }
}

#[derive(Clone, Copy, Eq, PartialEq, Hash, Debug)]
pub struct TaskId(u64);

pub type SubTaskOutput = Result<(), gst::FlowError>;
pub struct SubTaskQueue(VecDeque<BoxFuture<'static, SubTaskOutput>>);

impl fmt::Debug for SubTaskQueue {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.debug_tuple("SubTaskQueue").finish()
    }
}

pub struct JoinError(tokio::task::JoinError);

impl fmt::Display for JoinError {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Display::fmt(&self.0, fmt)
    }
}

impl fmt::Debug for JoinError {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&self.0, fmt)
    }
}

impl std::error::Error for JoinError {}

impl From<tokio::task::JoinError> for JoinError {
    fn from(src: tokio::task::JoinError) -> Self {
        JoinError(src)
    }
}

/// Wrapper for the underlying runtime JoinHandle implementation.
pub struct JoinHandle<T> {
    join_handle: tokio::task::JoinHandle<T>,
    context: ContextWeak,
    task_id: TaskId,
}

unsafe impl<T: Send> Send for JoinHandle<T> {}
unsafe impl<T: Send> Sync for JoinHandle<T> {}

impl<T> JoinHandle<T> {
    pub fn is_current(&self) -> bool {
        if let Some((context, task_id)) = Context::current_task() {
            let self_context = self.context.upgrade();
            self_context.map(|c| c == context).unwrap_or(false) && task_id == self.task_id
        } else {
            false
        }
    }

    pub fn context(&self) -> Option<Context> {
        self.context.upgrade()
    }

    pub fn task_id(&self) -> TaskId {
        self.task_id
    }
}

impl<T> Unpin for JoinHandle<T> {}

impl<T> Future for JoinHandle<T> {
    type Output = Result<T, JoinError>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
        if self.as_ref().is_current() {
            panic!("Trying to join task {:?} from itself", self.as_ref());
        }

        self.as_mut()
            .join_handle
            .poll_unpin(cx)
            .map_err(JoinError::from)
    }
}

impl<T> fmt::Debug for JoinHandle<T> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        let context_name = self.context.upgrade().map(|c| String::from(c.name()));

        fmt.debug_struct("JoinHandle")
            .field("context", &context_name)
            .field("task_id", &self.task_id)
            .finish()
    }
}

#[derive(Debug)]
struct ContextRealInner {
    name: String,
    handle: Mutex<tokio::runtime::Handle>,
    // Only used for dropping
    shutdown: ContextShutdown,
}

#[derive(Debug)]
struct ContextInner {
    // Otherwise a dummy context
    real: Option<ContextRealInner>,
    task_queues: Mutex<(u64, HashMap<u64, SubTaskQueue>)>,
}

impl Drop for ContextInner {
    fn drop(&mut self) {
        if let Some(ref real) = self.real {
            let mut contexts = CONTEXTS.lock().unwrap();
            gst_debug!(RUNTIME_CAT, "Finalizing context '{}'", real.name);
            contexts.remove(&real.name);
        }
    }
}

#[derive(Clone, Debug)]
pub struct ContextWeak(Weak<ContextInner>);

impl ContextWeak {
    pub fn upgrade(&self) -> Option<Context> {
        self.0.upgrade().map(Context)
    }
}

/// A `threadshare` `runtime` `Context`.
///
/// The `Context` provides low-level asynchronous processing features to
/// multiplex task execution on a single thread.
///
/// `Element` implementations should use [`PadSrc`] and [`PadSink`] which
///  provide high-level features.
///
/// See the [module-level documentation](index.html) for more.
///
/// [`PadSrc`]: ../pad/struct.PadSrc.html
/// [`PadSink`]: ../pad/struct.PadSink.html
#[derive(Clone, Debug)]
pub struct Context(Arc<ContextInner>);

impl PartialEq for Context {
    fn eq(&self, other: &Self) -> bool {
        Arc::ptr_eq(&self.0, &other.0)
    }
}

impl Eq for Context {}

impl Context {
    pub fn acquire(context_name: &str, wait: u32) -> Result<Self, io::Error> {
        assert_ne!(context_name, "DUMMY");

        let mut contexts = CONTEXTS.lock().unwrap();

        if let Some(inner_weak) = contexts.get(context_name) {
            if let Some(inner_strong) = inner_weak.upgrade() {
                gst_debug!(
                    RUNTIME_CAT,
                    "Joining Context '{}'",
                    inner_strong.real.as_ref().unwrap().name
                );
                return Ok(Context(inner_strong));
            }
        }

        let context = ContextThread::start(context_name, wait);
        contexts.insert(context_name.into(), Arc::downgrade(&context.0));

        gst_debug!(
            RUNTIME_CAT,
            "New Context '{}'",
            context.0.real.as_ref().unwrap().name
        );
        Ok(context)
    }

    pub fn downgrade(&self) -> ContextWeak {
        ContextWeak(Arc::downgrade(&self.0))
    }

    pub fn name(&self) -> &str {
        match self.0.real {
            Some(ref real) => real.name.as_str(),
            None => "DUMMY",
        }
    }

    /// Returns `true` if a `Context` is running on current thread.
    pub fn is_context_thread() -> bool {
        CURRENT_THREAD_CONTEXT.with(|cur_ctx| cur_ctx.borrow().is_some())
    }

    /// Returns the `Context` running on current thread, if any.
    pub fn current() -> Option<Context> {
        CURRENT_THREAD_CONTEXT.with(|cur_ctx| {
            cur_ctx
                .borrow()
                .as_ref()
                .and_then(|ctx_weak| ctx_weak.upgrade())
        })
    }

    /// Returns the `TaskId` running on current thread, if any.
    pub fn current_task() -> Option<(Context, TaskId)> {
        CURRENT_THREAD_CONTEXT.with(|cur_ctx| {
            cur_ctx
                .borrow()
                .as_ref()
                .and_then(|ctx_weak| ctx_weak.upgrade())
                .and_then(|ctx| {
                    let task_id = CURRENT_TASK_ID.try_with(|task_id| *task_id).ok();

                    task_id.map(move |task_id| (ctx, task_id))
                })
        })
    }

    pub fn enter<F, R>(&self, f: F) -> R
    where
        F: FnOnce() -> R,
    {
        let real = match self.0.real {
            Some(ref real) => real,
            None => panic!("Can't enter on dummy context"),
        };

        real.handle.lock().unwrap().enter(f)
    }

    pub fn spawn<Fut>(&self, future: Fut) -> JoinHandle<Fut::Output>
    where
        Fut: Future + Send + 'static,
        Fut::Output: Send + 'static,
    {
        let real = match self.0.real {
            Some(ref real) => real,
            None => panic!("Can't spawn new tasks on dummy context"),
        };

        let mut task_queues = self.0.task_queues.lock().unwrap();
        let id = task_queues.0;
        task_queues.0 += 1;
        task_queues.1.insert(id, SubTaskQueue(VecDeque::new()));

        let id = TaskId(id);
        gst_trace!(
            RUNTIME_CAT,
            "Spawning new task {:?} on context {}",
            id,
            real.name
        );

        let join_handle = real.handle.lock().unwrap().spawn(async move {
            let ctx = Context::current().unwrap();
            let real = ctx.0.real.as_ref().unwrap();

            gst_trace!(
                RUNTIME_CAT,
                "Running task {:?} on context {}",
                id,
                real.name
            );
            let res = CURRENT_TASK_ID.scope(id, future).await;

            // Remove task from the list
            {
                let mut task_queues = ctx.0.task_queues.lock().unwrap();
                if let Some(task_queue) = task_queues.1.remove(&id.0) {
                    let l = task_queue.0.len();
                    if l > 0 {
                        gst_warning!(
                            RUNTIME_CAT,
                            "Task {:?} on context {} has {} pending sub tasks",
                            id,
                            real.name,
                            l
                        );
                    }
                }
            }

            gst_trace!(RUNTIME_CAT, "Task {:?} on context {} done", id, real.name);

            res
        });

        JoinHandle {
            join_handle,
            context: self.downgrade(),
            task_id: id,
        }
    }

    pub fn current_has_sub_tasks() -> bool {
        let (ctx, task_id) = match Context::current_task() {
            Some(task) => task,
            None => {
                gst_trace!(RUNTIME_CAT, "No current task");
                return false;
            }
        };

        let task_queues = ctx.0.task_queues.lock().unwrap();
        task_queues
            .1
            .get(&task_id.0)
            .map(|t| !t.0.is_empty())
            .unwrap_or(false)
    }

    pub fn add_sub_task<T>(sub_task: T) -> Result<(), T>
    where
        T: Future<Output = SubTaskOutput> + Send + 'static,
    {
        let (ctx, task_id) = match Context::current_task() {
            Some(task) => task,
            None => {
                gst_trace!(RUNTIME_CAT, "No current task");
                return Err(sub_task);
            }
        };

        let mut task_queues = ctx.0.task_queues.lock().unwrap();
        match task_queues.1.get_mut(&task_id.0) {
            Some(task_queue) => {
                if let Some(ref real) = ctx.0.real {
                    gst_trace!(
                        RUNTIME_CAT,
                        "Adding subtask to {:?} on context {}",
                        task_id,
                        real.name
                    );
                } else {
                    gst_trace!(
                        RUNTIME_CAT,
                        "Adding subtask to {:?} on dummy context",
                        task_id,
                    );
                }
                task_queue.0.push_back(sub_task.boxed());
                Ok(())
            }
            None => {
                gst_trace!(RUNTIME_CAT, "Task was removed in the meantime");
                Err(sub_task)
            }
        }
    }

    pub fn drain_sub_tasks() -> impl Future<Output = SubTaskOutput> + Send + 'static {
        async {
            let (ctx, task_id) = match Context::current_task() {
                Some(task) => task,
                None => return Ok(()),
            };

            ctx.drain_sub_tasks_internal(task_id).await
        }
    }

    fn drain_sub_tasks_internal(
        &self,
        id: TaskId,
    ) -> impl Future<Output = SubTaskOutput> + Send + 'static {
        let mut task_queue = {
            let mut task_queues = self.0.task_queues.lock().unwrap();
            if let Some(task_queue) = task_queues.1.get_mut(&id.0) {
                mem::replace(task_queue, SubTaskQueue(VecDeque::new()))
            } else {
                SubTaskQueue(VecDeque::new())
            }
        };

        let name = self
            .0
            .real
            .as_ref()
            .map(|r| r.name.clone())
            .unwrap_or_else(|| String::from("DUMMY"));
        async move {
            if !task_queue.0.is_empty() {
                gst_log!(
                    RUNTIME_CAT,
                    "Scheduling draining {} sub tasks from {:?} on '{}'",
                    task_queue.0.len(),
                    id,
                    &name,
                );

                for task in task_queue.0.drain(..) {
                    task.await?;
                }
            }

            Ok(())
        }
    }
}

#[cfg(test)]
mod tests {
    use futures;
    use futures::channel::mpsc;
    use futures::lock::Mutex;
    use futures::prelude::*;

    use gst;

    use std::net::{IpAddr, Ipv4Addr, SocketAddr, UdpSocket};
    use std::sync::Arc;
    use std::time::{Duration, Instant};

    use super::Context;

    type Item = i32;

    const SLEEP_DURATION_MS: u32 = 2;
    const SLEEP_DURATION: Duration = Duration::from_millis(SLEEP_DURATION_MS as u64);
    const DELAY: Duration = Duration::from_millis(SLEEP_DURATION_MS as u64 * 10);

    #[tokio::test]
    async fn drain_sub_tasks() {
        // Setup
        gst::init().unwrap();

        let context = Context::acquire("drain_sub_tasks", SLEEP_DURATION_MS).unwrap();

        let join_handle = context.spawn(async move {
            let (sender, mut receiver) = mpsc::channel(1);
            let sender: Arc<Mutex<mpsc::Sender<Item>>> = Arc::new(Mutex::new(sender));

            let add_sub_task = move |item| {
                let sender = sender.clone();
                Context::add_sub_task(async move {
                    sender
                        .lock()
                        .await
                        .send(item)
                        .await
                        .map_err(|_| gst::FlowError::Error)
                })
            };

            // Tests

            // Drain empty queue
            let drain_fut = Context::drain_sub_tasks();
            drain_fut.await.unwrap();

            // Add a subtask
            add_sub_task(0).map_err(drop).unwrap();

            // Check that it was not executed yet
            receiver.try_next().unwrap_err();

            // Drain it now and check that it was executed
            let drain_fut = Context::drain_sub_tasks();
            drain_fut.await.unwrap();
            assert_eq!(receiver.try_next().unwrap(), Some(0));

            // Add another task and check that it's not executed yet
            add_sub_task(1).map_err(drop).unwrap();
            receiver.try_next().unwrap_err();

            // Return the receiver
            receiver
        });

        let mut receiver = join_handle.await.unwrap();

        // The last sub task should be simply dropped at this point
        assert_eq!(receiver.try_next().unwrap(), None);
    }

    #[tokio::test]
    async fn block_on_within_tokio() {
        let context = Context::acquire("block_on_within_tokio", SLEEP_DURATION_MS).unwrap();

        let bytes_sent = crate::runtime::executor::block_on(context.spawn(async {
            let saddr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 5000);
            let socket = UdpSocket::bind(saddr).unwrap();
            let mut socket = tokio::net::UdpSocket::from_std(socket).unwrap();
            let saddr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 4000);
            socket.send_to(&[0; 10], saddr).await.unwrap()
        }))
        .unwrap();
        assert_eq!(bytes_sent, 10);

        let elapsed = crate::runtime::executor::block_on(context.spawn(async {
            let now = Instant::now();
            crate::runtime::time::delay_for(DELAY).await;
            now.elapsed()
        }))
        .unwrap();
        // Due to throttling, `Delay` may be fired earlier
        assert!(elapsed + SLEEP_DURATION / 2 >= DELAY);
    }

    #[test]
    fn block_on_from_sync() {
        let context = Context::acquire("block_on_from_sync", SLEEP_DURATION_MS).unwrap();

        let bytes_sent = crate::runtime::executor::block_on(context.spawn(async {
            let saddr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 5001);
            let socket = UdpSocket::bind(saddr).unwrap();
            let mut socket = tokio::net::UdpSocket::from_std(socket).unwrap();
            let saddr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 4000);
            socket.send_to(&[0; 10], saddr).await.unwrap()
        }))
        .unwrap();
        assert_eq!(bytes_sent, 10);

        let elapsed = crate::runtime::executor::block_on(context.spawn(async {
            let now = Instant::now();
            crate::runtime::time::delay_for(DELAY).await;
            now.elapsed()
        }))
        .unwrap();
        // Due to throttling, `Delay` may be fired earlier
        assert!(elapsed + SLEEP_DURATION / 2 >= DELAY);
    }

    #[test]
    fn block_on_from_context() {
        gst::init().unwrap();

        let context = Context::acquire("block_on_from_context", SLEEP_DURATION_MS).unwrap();
        let join_handle = context.spawn(async {
            crate::runtime::executor::block_on(async {
                crate::runtime::time::delay_for(DELAY).await;
            });
        });
        // Panic: attempt to `runtime::executor::block_on` within a `Context` thread
        futures::executor::block_on(join_handle).unwrap_err();
    }

    #[tokio::test]
    async fn enter_context_from_tokio() {
        gst::init().unwrap();

        let context = Context::acquire("enter_context_from_tokio", SLEEP_DURATION_MS).unwrap();
        let mut socket = context
            .enter(|| {
                let saddr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 5002);
                let socket = UdpSocket::bind(saddr).unwrap();
                tokio::net::UdpSocket::from_std(socket)
            })
            .unwrap();

        let saddr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 4000);
        let bytes_sent = socket.send_to(&[0; 10], saddr).await.unwrap();
        assert_eq!(bytes_sent, 10);

        let elapsed = context.enter(|| {
            futures::executor::block_on(async {
                let now = Instant::now();
                crate::runtime::time::delay_for(DELAY).await;
                now.elapsed()
            })
        });
        // Due to throttling, `Delay` may be fired earlier
        assert!(elapsed + SLEEP_DURATION / 2 >= DELAY);
    }

    #[test]
    fn enter_context_from_sync() {
        gst::init().unwrap();

        let context = Context::acquire("enter_context_from_sync", SLEEP_DURATION_MS).unwrap();
        let mut socket = context
            .enter(|| {
                let saddr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 5003);
                let socket = UdpSocket::bind(saddr).unwrap();
                tokio::net::UdpSocket::from_std(socket)
            })
            .unwrap();

        let saddr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 4000);
        let bytes_sent = futures::executor::block_on(socket.send_to(&[0; 10], saddr)).unwrap();
        assert_eq!(bytes_sent, 10);

        let elapsed = context.enter(|| {
            futures::executor::block_on(async {
                let now = Instant::now();
                crate::runtime::time::delay_for(DELAY).await;
                now.elapsed()
            })
        });
        // Due to throttling, `Delay` may be fired earlier
        assert!(elapsed + SLEEP_DURATION / 2 >= DELAY);
    }
}