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// Copyright 2019 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
//! Values of this type represent "execution scopes" used by the library to give fine grained
//! control of the lifetimes of the tasks associated with particular connections. When a new
//! connection is attached to a pseudo directory tree, an execution scope is provided. This scope
//! is then used to start any tasks related to this connection. All connections opened as a result
//! of operations on this first connection will also use the same scope, as well as any tasks
//! related to those connections.
//!
//! This way, it is possible to control the lifetime of a group of connections. All connections
//! and their tasks can be shutdown by calling `shutdown` method on the scope that is hosting them.
//! Scope will also shutdown all the tasks when it goes out of scope.
//!
//! Implementation wise, execution scope is just a proxy, that forwards all the tasks to an actual
//! executor, provided as an instance of a [`futures::task::Spawn`] trait.
use crate::token_registry::TokenRegistry;
use fuchsia_async::{JoinHandle, Scope, Task};
use futures::task::{self, Poll};
use futures::Future;
use std::future::{pending, poll_fn};
use std::pin::pin;
use std::sync::{Arc, Mutex, OnceLock};
use std::task::ready;
#[cfg(target_os = "fuchsia")]
use fuchsia_async::EHandle;
pub type SpawnError = task::SpawnError;
/// An execution scope that is hosting tasks for a group of connections. See the module level
/// documentation for details.
///
/// Actual execution will be delegated to an "upstream" executor - something that implements
/// [`futures::task::Spawn`]. In a sense, this is somewhat of an analog of a multithreaded capable
/// [`futures::stream::FuturesUnordered`], but this some additional functionality specific to the
/// vfs library.
///
/// Use [`ExecutionScope::new()`] or [`ExecutionScope::build()`] to construct new
/// `ExecutionScope`es.
#[derive(Clone)]
pub struct ExecutionScope {
executor: Arc<Executor>,
}
struct Executor {
inner: Mutex<Inner>,
token_registry: TokenRegistry,
scope: OnceLock<Scope>,
}
struct Inner {
/// Records the kind of shutdown that has been called on the executor.
shutdown_state: ShutdownState,
/// The number of active tasks preventing shutdown.
active_count: usize,
/// A fake active task that we use when there are no other tasks yet there's still an an active
/// count.
fake_active_task: Option<Task<()>>,
}
#[derive(Copy, Clone, PartialEq)]
enum ShutdownState {
Active,
Shutdown,
ForceShutdown,
}
impl ExecutionScope {
/// Constructs an execution scope. Use [`ExecutionScope::build()`] if you want to specify
/// parameters.
pub fn new() -> Self {
Self::build().new()
}
/// Constructs a new execution scope builder, wrapping the specified executor and optionally
/// accepting additional parameters. Run [`ExecutionScopeParams::new()`] to get an actual
/// [`ExecutionScope`] object.
pub fn build() -> ExecutionScopeParams {
ExecutionScopeParams::default()
}
/// Returns the active count: the number of tasks that are active and will prevent shutdown.
pub fn active_count(&self) -> usize {
self.executor.inner.lock().unwrap().active_count
}
/// Sends a `task` to be executed in this execution scope. This is very similar to
/// [`futures::task::Spawn::spawn_obj()`] with a minor difference that `self` reference is not
/// exclusive.
///
/// If the task needs to prevent itself from being shutdown, then it should use the
/// `try_active_guard` function below.
///
/// For the "vfs" library it is more convenient that this method allows non-exclusive
/// access. And as the implementation is employing internal mutability there are no downsides.
/// This way `ExecutionScope` can actually also implement [`futures::task::Spawn`] - it just was
/// not necessary for now.
pub fn spawn(&self, task: impl Future<Output = ()> + Send + 'static) -> JoinHandle<()> {
let executor = self.executor.clone();
self.executor.scope().spawn(async move {
let mut task = std::pin::pin!(task);
poll_fn(|cx| {
let shutdown_state = executor.inner.lock().unwrap().shutdown_state;
match task.as_mut().poll(cx) {
Poll::Ready(()) => Poll::Ready(()),
Poll::Pending => match shutdown_state {
ShutdownState::Active => Poll::Pending,
ShutdownState::Shutdown
if executor.inner.lock().unwrap().active_count > 0 =>
{
Poll::Pending
}
_ => Poll::Ready(()),
},
}
})
.await;
})
}
pub fn token_registry(&self) -> &TokenRegistry {
&self.executor.token_registry
}
pub fn shutdown(&self) {
self.executor.shutdown();
}
/// Forcibly shut down the executor without respecting the active guards.
pub fn force_shutdown(&self) {
let mut inner = self.executor.inner.lock().unwrap();
inner.shutdown_state = ShutdownState::ForceShutdown;
self.executor.scope().wake_all();
}
/// Restores the executor so that it is no longer in the shut-down state. Any tasks
/// that are still running will continue to run after calling this.
pub fn resurrect(&self) {
self.executor.inner.lock().unwrap().shutdown_state = ShutdownState::Active;
}
/// Wait for all tasks to complete.
pub async fn wait(&self) {
let mut on_no_tasks = pin!(self.executor.scope().on_no_tasks());
poll_fn(|cx| {
// Hold the lock whilst we poll the scope so that the active count can't change.
let mut inner = self.executor.inner.lock().unwrap();
ready!(on_no_tasks.as_mut().poll(cx));
if inner.active_count == 0 {
Poll::Ready(())
} else {
// There are no tasks but there's an active count and we must only finish when there
// are no tasks *and* the active count is zero. To address this, we spawn a fake
// task so that we can just use `on_no_tasks`, and then we'll cancel the task when
// the active count drops to zero.
let scope = self.executor.scope();
inner.fake_active_task = Some(scope.compute(pending::<()>()));
on_no_tasks.set(scope.on_no_tasks());
assert!(on_no_tasks.as_mut().poll(cx).is_pending());
Poll::Pending
}
})
.await;
}
/// Prevents the executor from shutting down whilst the guard is held. Returns None if the
/// executor is shutting down.
pub fn try_active_guard(&self) -> Option<ActiveGuard> {
let mut inner = self.executor.inner.lock().unwrap();
if inner.shutdown_state != ShutdownState::Active {
return None;
}
inner.active_count += 1;
Some(ActiveGuard(self.executor.clone()))
}
/// As above, but succeeds even if the executor is shutting down. This can be used in drop
/// implementations to spawn tasks that *must* run before the executor shuts down.
pub fn active_guard(&self) -> ActiveGuard {
self.executor.inner.lock().unwrap().active_count += 1;
ActiveGuard(self.executor.clone())
}
}
impl PartialEq for ExecutionScope {
fn eq(&self, other: &Self) -> bool {
Arc::as_ptr(&self.executor) == Arc::as_ptr(&other.executor)
}
}
impl Eq for ExecutionScope {}
impl std::fmt::Debug for ExecutionScope {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
f.write_fmt(format_args!("ExecutionScope {:?}", Arc::as_ptr(&self.executor)))
}
}
#[derive(Default)]
pub struct ExecutionScopeParams {
#[cfg(target_os = "fuchsia")]
async_executor: Option<EHandle>,
}
impl ExecutionScopeParams {
#[cfg(target_os = "fuchsia")]
pub fn executor(mut self, value: EHandle) -> Self {
assert!(self.async_executor.is_none(), "`executor` is already set");
self.async_executor = Some(value);
self
}
pub fn new(self) -> ExecutionScope {
ExecutionScope {
executor: Arc::new(Executor {
token_registry: TokenRegistry::new(),
inner: Mutex::new(Inner {
shutdown_state: ShutdownState::Active,
active_count: 0,
fake_active_task: None,
}),
#[cfg(target_os = "fuchsia")]
scope: self
.async_executor
.map_or_else(|| OnceLock::new(), |e| e.root_scope().new_child().into()),
#[cfg(not(target_os = "fuchsia"))]
scope: OnceLock::new(),
}),
}
}
}
impl Executor {
fn scope(&self) -> &Scope {
// We lazily initialize the executor rather than at construction time as there are currently
// a few tests that create the ExecutionScope before the async executor has been initialized
// (which means we cannot call EHandle::local()).
self.scope.get_or_init(|| Scope::new())
}
fn shutdown(&self) {
let wake_all = {
let mut inner = self.inner.lock().unwrap();
inner.shutdown_state = ShutdownState::Shutdown;
inner.active_count == 0
};
if wake_all {
if let Some(scope) = self.scope.get() {
scope.wake_all();
}
}
}
}
impl Drop for Executor {
fn drop(&mut self) {
self.shutdown();
}
}
// ActiveGuard prevents the executor from shutting down until the guard is dropped.
pub struct ActiveGuard(Arc<Executor>);
impl Drop for ActiveGuard {
fn drop(&mut self) {
let wake_all = {
let mut inner = self.0.inner.lock().unwrap();
inner.active_count -= 1;
if inner.active_count == 0 {
if let Some(task) = inner.fake_active_task.take() {
let _ = task.cancel();
}
}
inner.active_count == 0 && inner.shutdown_state == ShutdownState::Shutdown
};
if wake_all {
self.0.scope().wake_all();
}
}
}
/// Yields to the executor, providing an opportunity for other futures to run.
pub async fn yield_to_executor() {
let mut done = false;
poll_fn(|cx| {
if done {
Poll::Ready(())
} else {
done = true;
cx.waker().wake_by_ref();
Poll::Pending
}
})
.await;
}
#[cfg(test)]
mod tests {
use super::{yield_to_executor, ExecutionScope};
use fuchsia_async::{Task, TestExecutor, Timer};
use futures::channel::oneshot;
use futures::stream::FuturesUnordered;
use futures::task::Poll;
use futures::{Future, StreamExt};
use std::pin::pin;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;
use std::time::Duration;
#[cfg(target_os = "fuchsia")]
fn run_test<GetTest, GetTestRes>(get_test: GetTest)
where
GetTest: FnOnce(ExecutionScope) -> GetTestRes,
GetTestRes: Future<Output = ()>,
{
let mut exec = TestExecutor::new();
let scope = ExecutionScope::new();
let test = get_test(scope);
assert_eq!(
exec.run_until_stalled(&mut pin!(test)),
Poll::Ready(()),
"Test did not complete"
);
}
#[cfg(not(target_os = "fuchsia"))]
fn run_test<GetTest, GetTestRes>(get_test: GetTest)
where
GetTest: FnOnce(ExecutionScope) -> GetTestRes,
GetTestRes: Future<Output = ()>,
{
use fuchsia_async::TimeoutExt;
let mut exec = TestExecutor::new();
let scope = ExecutionScope::new();
// This isn't a perfect equivalent to the target version, but Tokio
// doesn't have run_until_stalled and it sounds like it's
// architecturally impossible.
let test =
get_test(scope).on_stalled(Duration::from_secs(30), || panic!("Test did not complete"));
exec.run_singlethreaded(&mut pin!(test));
}
#[test]
fn simple() {
run_test(|scope| {
async move {
let (sender, receiver) = oneshot::channel();
let (counters, task) = mocks::ImmediateTask::new(sender);
scope.spawn(task);
// Make sure our task had a chance to execute.
receiver.await.unwrap();
assert_eq!(counters.drop_call(), 1);
assert_eq!(counters.poll_call(), 1);
}
});
}
#[test]
fn simple_drop() {
run_test(|scope| {
async move {
let (poll_sender, poll_receiver) = oneshot::channel();
let (processing_done_sender, processing_done_receiver) = oneshot::channel();
let (drop_sender, drop_receiver) = oneshot::channel();
let (counters, task) =
mocks::ControlledTask::new(poll_sender, processing_done_receiver, drop_sender);
scope.spawn(task);
poll_receiver.await.unwrap();
processing_done_sender.send(()).unwrap();
scope.shutdown();
drop_receiver.await.unwrap();
// poll might be called one or two times depending on the order in which the
// executor decides to poll the two tasks (this one and the one we spawned).
let poll_count = counters.poll_call();
assert!(poll_count >= 1, "poll was not called");
assert_eq!(counters.drop_call(), 1);
}
});
}
#[test]
fn test_wait_waits_for_tasks_to_finish() {
let mut executor = TestExecutor::new();
let scope = ExecutionScope::new();
executor.run_singlethreaded(async {
let (poll_sender, poll_receiver) = oneshot::channel();
let (processing_done_sender, processing_done_receiver) = oneshot::channel();
let (drop_sender, _drop_receiver) = oneshot::channel();
let (_, task) =
mocks::ControlledTask::new(poll_sender, processing_done_receiver, drop_sender);
scope.spawn(task);
poll_receiver.await.unwrap();
// We test that wait is working correctly by concurrently waiting and telling the
// task to complete, and making sure that the order is correct.
let done = std::sync::Mutex::new(false);
futures::join!(
async {
scope.wait().await;
assert_eq!(*done.lock().unwrap(), true);
},
async {
// This is a Turing halting problem so the sleep is justified.
Timer::new(Duration::from_millis(100)).await;
*done.lock().unwrap() = true;
processing_done_sender.send(()).unwrap();
}
);
});
}
#[fuchsia::test]
async fn test_active_guard() {
let scope = ExecutionScope::new();
let (guard_taken_tx, guard_taken_rx) = oneshot::channel();
let (shutdown_triggered_tx, shutdown_triggered_rx) = oneshot::channel();
let (drop_task_tx, drop_task_rx) = oneshot::channel();
let scope_clone = scope.clone();
let done = Arc::new(AtomicBool::new(false));
let done_clone = done.clone();
scope.spawn(async move {
{
struct OnDrop((ExecutionScope, Option<oneshot::Receiver<()>>));
impl Drop for OnDrop {
fn drop(&mut self) {
let guard = self.0 .0.active_guard();
let rx = self.0 .1.take().unwrap();
Task::spawn(async move {
rx.await.unwrap();
std::mem::drop(guard);
})
.detach();
}
}
let _guard = scope_clone.try_active_guard().unwrap();
let _on_drop = OnDrop((scope_clone, Some(drop_task_rx)));
guard_taken_tx.send(()).unwrap();
shutdown_triggered_rx.await.unwrap();
// Stick a timer here and record whether we're done to make sure we get to run to
// completion.
Timer::new(std::time::Duration::from_millis(100)).await;
done_clone.store(true, Ordering::SeqCst);
}
});
guard_taken_rx.await.unwrap();
scope.shutdown();
// The task should keep running whilst it has an active guard. Introduce a timer here to
// make failing more likely if it's broken.
Timer::new(std::time::Duration::from_millis(100)).await;
let mut shutdown_wait = std::pin::pin!(scope.wait());
assert_eq!(futures::poll!(shutdown_wait.as_mut()), Poll::Pending);
shutdown_triggered_tx.send(()).unwrap();
// The drop task should now start running and the executor still shouldn't have finished.
Timer::new(std::time::Duration::from_millis(100)).await;
assert_eq!(futures::poll!(shutdown_wait.as_mut()), Poll::Pending);
drop_task_tx.send(()).unwrap();
shutdown_wait.await;
assert!(done.load(Ordering::SeqCst));
}
#[cfg(target_os = "fuchsia")]
#[fuchsia::test]
async fn test_shutdown_waits_for_channels() {
use fuchsia_async as fasync;
let scope = ExecutionScope::new();
let (rx, tx) = zx::Channel::create();
let received_msg = Arc::new(AtomicBool::new(false));
let (sender, receiver) = futures::channel::oneshot::channel();
{
let received_msg = received_msg.clone();
scope.spawn(async move {
let mut msg_buf = zx::MessageBuf::new();
msg_buf.ensure_capacity_bytes(64);
let _ = sender.send(());
let _ = fasync::Channel::from_channel(rx).recv_msg(&mut msg_buf).await;
received_msg.store(true, Ordering::Relaxed);
});
}
// Wait until the spawned future has been polled once.
let _ = receiver.await;
tx.write(b"hello", &mut []).expect("write failed");
scope.shutdown();
scope.wait().await;
assert!(received_msg.load(Ordering::Relaxed));
}
#[fuchsia::test]
async fn test_force_shutdown() {
let scope = ExecutionScope::new();
let scope_clone = scope.clone();
let ref_count = Arc::new(());
let ref_count_clone = ref_count.clone();
// Spawn a task that holds a reference. When the task is dropped the reference will get
// dropped with it.
scope.spawn(async move {
let _ref_count_clone = ref_count_clone;
// Hold an active guard so that only a forced shutdown will work.
let _guard = scope_clone.active_guard();
let _: () = std::future::pending().await;
});
scope.force_shutdown();
scope.wait().await;
// The task should have been dropped leaving us with the only reference.
assert_eq!(Arc::strong_count(&ref_count), 1);
// Test resurrection...
scope.resurrect();
let ref_count_clone = ref_count.clone();
scope.spawn(async move {
// Yield so that if the executor is in the shutdown state, it will kill this task.
yield_to_executor().await;
// Take another reference count so that we can check we got here below.
let _ref_count = ref_count_clone.clone();
let _: () = std::future::pending().await;
});
while Arc::strong_count(&ref_count) != 3 {
yield_to_executor().await;
}
// Yield some more just to be sure the task isn't killed.
for _ in 0..5 {
yield_to_executor().await;
assert_eq!(Arc::strong_count(&ref_count), 3);
}
}
#[fuchsia::test]
async fn test_task_runs_once() {
let scope = ExecutionScope::new();
// Spawn a task.
scope.spawn(async {});
scope.shutdown();
let polled = Arc::new(AtomicBool::new(false));
let polled_clone = polled.clone();
let scope_clone = scope.clone();
// Use FuturesUnordered so that it uses its own waker.
let mut futures = FuturesUnordered::new();
futures.push(async move { scope_clone.wait().await });
// Poll it now to set up a waker.
assert_eq!(futures::poll!(futures.next()), Poll::Pending);
// Spawn another task. When this task runs, wait still shouldn't be resolved because at
// this point the first task hasn't finished.
scope.spawn(async move {
assert_eq!(futures::poll!(futures.next()), Poll::Pending);
polled_clone.store(true, Ordering::Relaxed);
});
scope.wait().await;
// Make sure the second spawned task actually ran.
assert!(polled.load(Ordering::Relaxed));
}
mod mocks {
use futures::channel::oneshot;
use futures::task::{Context, Poll};
use futures::Future;
use std::pin::Pin;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::Arc;
pub(super) struct TaskCounters {
poll_call_count: Arc<AtomicUsize>,
drop_call_count: Arc<AtomicUsize>,
}
impl TaskCounters {
fn new() -> (Arc<AtomicUsize>, Arc<AtomicUsize>, Self) {
let poll_call_count = Arc::new(AtomicUsize::new(0));
let drop_call_count = Arc::new(AtomicUsize::new(0));
(
poll_call_count.clone(),
drop_call_count.clone(),
Self { poll_call_count, drop_call_count },
)
}
pub(super) fn poll_call(&self) -> usize {
self.poll_call_count.load(Ordering::Relaxed)
}
pub(super) fn drop_call(&self) -> usize {
self.drop_call_count.load(Ordering::Relaxed)
}
}
pub(super) struct ImmediateTask {
poll_call_count: Arc<AtomicUsize>,
drop_call_count: Arc<AtomicUsize>,
done_sender: Option<oneshot::Sender<()>>,
}
impl ImmediateTask {
pub(super) fn new(done_sender: oneshot::Sender<()>) -> (TaskCounters, Self) {
let (poll_call_count, drop_call_count, counters) = TaskCounters::new();
(
counters,
Self { poll_call_count, drop_call_count, done_sender: Some(done_sender) },
)
}
}
impl Future for ImmediateTask {
type Output = ();
fn poll(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
self.poll_call_count.fetch_add(1, Ordering::Relaxed);
if let Some(sender) = self.done_sender.take() {
sender.send(()).unwrap();
}
Poll::Ready(())
}
}
impl Drop for ImmediateTask {
fn drop(&mut self) {
self.drop_call_count.fetch_add(1, Ordering::Relaxed);
}
}
impl Unpin for ImmediateTask {}
pub(super) struct ControlledTask {
poll_call_count: Arc<AtomicUsize>,
drop_call_count: Arc<AtomicUsize>,
drop_sender: Option<oneshot::Sender<()>>,
future: Pin<Box<dyn Future<Output = ()> + Send>>,
}
impl ControlledTask {
pub(super) fn new(
poll_sender: oneshot::Sender<()>,
processing_complete: oneshot::Receiver<()>,
drop_sender: oneshot::Sender<()>,
) -> (TaskCounters, Self) {
let (poll_call_count, drop_call_count, counters) = TaskCounters::new();
(
counters,
Self {
poll_call_count,
drop_call_count,
drop_sender: Some(drop_sender),
future: Box::pin(async move {
poll_sender.send(()).unwrap();
processing_complete.await.unwrap();
}),
},
)
}
}
impl Future for ControlledTask {
type Output = ();
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
self.poll_call_count.fetch_add(1, Ordering::Relaxed);
self.future.as_mut().poll(cx)
}
}
impl Drop for ControlledTask {
fn drop(&mut self) {
self.drop_call_count.fetch_add(1, Ordering::Relaxed);
self.drop_sender.take().unwrap().send(()).unwrap();
}
}
}
}