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// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
//! A global dispatcher queue.
//!
//! # Example - Global Dispatch queue
//!
//! The global dispatch queue is pre-configured with a maximum queue size of 100 tasks.
//!
//! ```rust,ignore
//! // Ensure the dispatcher queue is being worked on.
//! dispatcher::flush_init();
//!
//! dispatcher::launch(|| {
//! println!("Executing expensive task");
//! // Run your expensive task in a separate thread.
//! });
//!
//! dispatcher::launch(|| {
//! println!("A second task that's executed sequentially, but off the main thread.");
//! });
//! ```
use std::{
mem,
sync::{
atomic::{AtomicBool, AtomicUsize, Ordering},
Arc,
},
thread::{self, JoinHandle},
time::Duration,
};
use crossbeam_channel::{bounded, unbounded, RecvTimeoutError, SendError, Sender};
use thiserror::Error;
pub use global::*;
pub(crate) mod global;
/// Command received while blocked from further work.
enum Blocked {
/// Shutdown immediately without processing the queue.
Shutdown,
/// Unblock and continue with work as normal.
Continue,
}
/// The command a worker should execute.
enum Command {
/// A task is a user-defined function to run.
Task(Box<dyn FnOnce() + Send>),
/// Swap the channel
Swap(Sender<()>),
/// Signal the worker to finish work and shut down.
Shutdown,
}
/// The error returned from operations on the dispatcher
#[derive(Error, Debug, PartialEq, Eq)]
pub enum DispatchError {
/// The worker panicked while running a task
#[error("The worker panicked while running a task")]
WorkerPanic,
/// Maximum queue size reached
#[error("Maximum queue size reached")]
QueueFull,
/// Pre-init buffer was already flushed
#[error("Pre-init buffer was already flushed")]
AlreadyFlushed,
/// Failed to send command to worker thread
#[error("Failed to send command to worker thread")]
SendError,
/// Failed to receive from channel
#[error("Failed to receive from channel")]
RecvError(#[from] crossbeam_channel::RecvError),
}
impl<T> From<SendError<T>> for DispatchError {
fn from(_: SendError<T>) -> Self {
DispatchError::SendError
}
}
/// A clonable guard for a dispatch queue.
#[derive(Clone)]
struct DispatchGuard {
/// Whether to queue on the preinit buffer or on the unbounded queue
queue_preinit: Arc<AtomicBool>,
/// The number of items that were added to the queue after it filled up.
overflow_count: Arc<AtomicUsize>,
/// The maximum pre-init queue size
max_queue_size: usize,
/// Used to unblock the worker thread initially.
block_sender: Sender<Blocked>,
/// Sender for the preinit queue.
preinit_sender: Sender<Command>,
/// Sender for the unbounded queue.
sender: Sender<Command>,
}
impl DispatchGuard {
pub fn launch(&self, task: impl FnOnce() + Send + 'static) -> Result<(), DispatchError> {
let task = Command::Task(Box::new(task));
self.send(task)
}
pub fn shutdown(&mut self) -> Result<(), DispatchError> {
// Need to flush in order for the thread to actually process anything,
// including the shutdown command.
self.flush_init().ok();
self.send(Command::Shutdown)
}
fn send(&self, task: Command) -> Result<(), DispatchError> {
if self.queue_preinit.load(Ordering::SeqCst) {
if self.preinit_sender.len() < self.max_queue_size {
self.preinit_sender.send(task)?;
Ok(())
} else {
self.overflow_count.fetch_add(1, Ordering::SeqCst);
// Instead of using a bounded queue, we are handling the bounds
// checking ourselves. If a bounded queue were full, we would return
// a QueueFull DispatchError, so we do the same here.
Err(DispatchError::QueueFull)
}
} else {
self.sender.send(task)?;
Ok(())
}
}
fn block_on_queue(&self) {
let (tx, rx) = crossbeam_channel::bounded(0);
// We explicitly don't use `self.launch` here.
// We always put this task on the unbounded queue.
// The pre-init queue might be full before its flushed, in which case this would panic.
// Blocking on the queue can only work if it is eventually flushed anyway.
let task = Command::Task(Box::new(move || {
tx.send(())
.expect("(worker) Can't send message on single-use channel");
}));
self.sender
.send(task)
.expect("Failed to launch the blocking task");
rx.recv()
.expect("Failed to receive message on single-use channel");
}
/// Block on the task queue emptying, with a timeout.
fn block_on_queue_timeout(&self, timeout: Duration) -> Result<(), RecvTimeoutError> {
let (tx, rx) = crossbeam_channel::bounded(0);
// We explicitly don't use `self.launch` here.
// We always put this task on the unbounded queue.
// The pre-init queue might be full before its flushed, in which case this would panic.
// Blocking on the queue can only work if it is eventually flushed anyway.
let task = Command::Task(Box::new(move || {
// In case the calling thread times out waiting for this
// the channel will be dropped.
// But in case the work continues we don't want to panic.
_ = tx.send(());
}));
self.sender
.send(task)
.expect("Failed to launch the blocking task");
rx.recv_timeout(timeout)
}
fn kill(&mut self) -> Result<(), DispatchError> {
// We immediately stop queueing in the pre-init buffer.
let old_val = self.queue_preinit.swap(false, Ordering::SeqCst);
if !old_val {
return Err(DispatchError::AlreadyFlushed);
}
// Unblock the worker thread exactly once.
self.block_sender.send(Blocked::Shutdown)?;
Ok(())
}
/// Flushes the pre-init buffer.
///
/// This function blocks until tasks queued prior to this call are finished.
/// Once the initial queue is empty the dispatcher will wait for new tasks to be launched.
///
/// Returns an error if called multiple times.
fn flush_init(&mut self) -> Result<usize, DispatchError> {
// We immediately stop queueing in the pre-init buffer.
let old_val = self.queue_preinit.swap(false, Ordering::SeqCst);
if !old_val {
return Err(DispatchError::AlreadyFlushed);
}
// Unblock the worker thread exactly once.
self.block_sender.send(Blocked::Continue)?;
// Single-use channel to communicate with the worker thread.
let (swap_sender, swap_receiver) = bounded(0);
// Send final command and block until it is sent.
self.preinit_sender
.send(Command::Swap(swap_sender))
.map_err(|_| DispatchError::SendError)?;
// Now wait for the worker thread to do the swap and inform us.
// This blocks until all tasks in the preinit buffer have been processed.
swap_receiver.recv()?;
// We're not queueing anymore.
global::QUEUE_TASKS.store(false, Ordering::SeqCst);
let overflow_count = self.overflow_count.load(Ordering::SeqCst);
if overflow_count > 0 {
Ok(overflow_count)
} else {
Ok(0)
}
}
}
/// A dispatcher.
///
/// Run expensive processing tasks sequentially off the main thread.
/// Tasks are processed in a single separate thread in the order they are submitted.
/// The dispatch queue will enqueue tasks while not flushed, up to the maximum queue size.
/// Processing will start after flushing once, processing already enqueued tasks first, then
/// waiting for further tasks to be enqueued.
pub struct Dispatcher {
/// Guard used for communication with the worker thread.
guard: DispatchGuard,
/// Handle to the worker thread, allows to wait for it to finish.
worker: Option<JoinHandle<()>>,
}
impl Dispatcher {
/// Creates a new dispatcher with a maximum queue size.
///
/// Launched tasks won't run until [`flush_init`] is called.
///
/// [`flush_init`]: #method.flush_init
pub fn new(max_queue_size: usize) -> Self {
let (block_sender, block_receiver) = bounded(1);
let (preinit_sender, preinit_receiver) = unbounded();
let (sender, mut unbounded_receiver) = unbounded();
let queue_preinit = Arc::new(AtomicBool::new(true));
let overflow_count = Arc::new(AtomicUsize::new(0));
let worker = thread::Builder::new()
.name("glean.dispatcher".into())
.spawn(move || {
match block_receiver.recv() {
Err(_) => {
// The other side was disconnected.
// There's nothing the worker thread can do.
log::error!("The task producer was disconnected. Worker thread will exit.");
return;
}
Ok(Blocked::Shutdown) => {
// The other side wants us to stop immediately
return;
}
Ok(Blocked::Continue) => {
// Queue is unblocked, processing continues as normal.
}
}
let mut receiver = preinit_receiver;
loop {
use Command::*;
match receiver.recv() {
Ok(Shutdown) => {
break;
}
Ok(Task(f)) => {
(f)();
}
Ok(Swap(swap_done)) => {
// A swap should only occur exactly once.
// This is upheld by `flush_init`, which errors out if the preinit buffer
// was already flushed.
// We swap the channels we listen on for new tasks.
// The next iteration will continue with the unbounded queue.
mem::swap(&mut receiver, &mut unbounded_receiver);
// The swap command MUST be the last one received on the preinit buffer,
// so by the time we run this we know all preinit tasks were processed.
// We can notify the other side.
swap_done
.send(())
.expect("The caller of `flush_init` has gone missing");
}
// Other side was disconnected.
Err(_) => {
log::error!(
"The task producer was disconnected. Worker thread will exit."
);
return;
}
}
}
})
.expect("Failed to spawn Glean's dispatcher thread");
let guard = DispatchGuard {
queue_preinit,
overflow_count,
max_queue_size,
block_sender,
preinit_sender,
sender,
};
Dispatcher {
guard,
worker: Some(worker),
}
}
fn guard(&self) -> DispatchGuard {
self.guard.clone()
}
/// Waits for the worker thread to finish and finishes the dispatch queue.
///
/// You need to call `shutdown` to initiate a shutdown of the queue.
#[cfg(test)]
fn join(mut self) -> Result<(), DispatchError> {
if let Some(worker) = self.worker.take() {
worker.join().map_err(|_| DispatchError::WorkerPanic)?;
}
Ok(())
}
}
#[cfg(test)]
mod test {
use super::*;
use std::sync::atomic::AtomicU8;
use std::sync::Mutex;
fn enable_test_logging() {
// When testing we want all logs to go to stdout/stderr by default,
// without requiring each individual test to activate it.
let _ = env_logger::builder().is_test(true).try_init();
}
#[test]
fn tasks_run_off_the_main_thread() {
enable_test_logging();
let main_thread_id = thread::current().id();
let thread_canary = Arc::new(AtomicBool::new(false));
let dispatcher = Dispatcher::new(100);
// Force the Dispatcher out of the pre-init queue mode.
dispatcher
.guard()
.flush_init()
.expect("Failed to get out of preinit queue mode");
let canary_clone = thread_canary.clone();
dispatcher
.guard()
.launch(move || {
assert!(thread::current().id() != main_thread_id);
// Use the canary bool to make sure this is getting called before
// the test completes.
assert!(!canary_clone.load(Ordering::SeqCst));
canary_clone.store(true, Ordering::SeqCst);
})
.expect("Failed to dispatch the test task");
dispatcher.guard().block_on_queue();
assert!(thread_canary.load(Ordering::SeqCst));
assert_eq!(main_thread_id, thread::current().id());
}
#[test]
fn launch_correctly_adds_tasks_to_preinit_queue() {
enable_test_logging();
let main_thread_id = thread::current().id();
let thread_canary = Arc::new(AtomicU8::new(0));
let dispatcher = Dispatcher::new(100);
// Add 3 tasks to queue each one increasing thread_canary by 1 to
// signal that the tasks ran.
for _ in 0..3 {
let canary_clone = thread_canary.clone();
dispatcher
.guard()
.launch(move || {
// Make sure the task is flushed off-the-main thread.
assert!(thread::current().id() != main_thread_id);
canary_clone.fetch_add(1, Ordering::SeqCst);
})
.expect("Failed to dispatch the test task");
}
// Ensure that no task ran.
assert_eq!(0, thread_canary.load(Ordering::SeqCst));
// Flush the queue and wait for the tasks to complete.
dispatcher
.guard()
.flush_init()
.expect("Failed to get out of preinit queue mode");
// Validate that we have the expected canary value.
assert_eq!(3, thread_canary.load(Ordering::SeqCst));
}
#[test]
fn preinit_tasks_are_processed_after_flush() {
enable_test_logging();
let dispatcher = Dispatcher::new(10);
let result = Arc::new(Mutex::new(vec![]));
for i in 1..=5 {
let result = Arc::clone(&result);
dispatcher
.guard()
.launch(move || {
result.lock().unwrap().push(i);
})
.unwrap();
}
result.lock().unwrap().push(0);
dispatcher.guard().flush_init().unwrap();
for i in 6..=10 {
let result = Arc::clone(&result);
dispatcher
.guard()
.launch(move || {
result.lock().unwrap().push(i);
})
.unwrap();
}
dispatcher.guard().block_on_queue();
// This additionally checks that tasks were executed in order.
assert_eq!(
&*result.lock().unwrap(),
&[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
);
}
#[test]
fn tasks_after_shutdown_are_not_processed() {
enable_test_logging();
let dispatcher = Dispatcher::new(10);
let result = Arc::new(Mutex::new(vec![]));
dispatcher.guard().flush_init().unwrap();
dispatcher.guard().shutdown().unwrap();
{
let result = Arc::clone(&result);
// This might fail because the shutdown is quick enough,
// or it might succeed and still send the task.
// In any case that task should not be executed.
let _ = dispatcher.guard().launch(move || {
result.lock().unwrap().push(0);
});
}
dispatcher.join().unwrap();
assert!(result.lock().unwrap().is_empty());
}
#[test]
fn preinit_buffer_fills_up() {
enable_test_logging();
let dispatcher = Dispatcher::new(5);
let result = Arc::new(Mutex::new(vec![]));
for i in 1..=5 {
let result = Arc::clone(&result);
dispatcher
.guard()
.launch(move || {
result.lock().unwrap().push(i);
})
.unwrap();
}
{
let result = Arc::clone(&result);
let err = dispatcher.guard().launch(move || {
result.lock().unwrap().push(10);
});
assert_eq!(Err(DispatchError::QueueFull), err);
}
dispatcher.guard().flush_init().unwrap();
{
let result = Arc::clone(&result);
dispatcher
.guard()
.launch(move || {
result.lock().unwrap().push(20);
})
.unwrap();
}
dispatcher.guard().block_on_queue();
assert_eq!(&*result.lock().unwrap(), &[1, 2, 3, 4, 5, 20]);
}
#[test]
fn normal_queue_is_unbounded() {
enable_test_logging();
// Note: We can't actually test that it's fully unbounded,
// but we can quickly queue more slow tasks than the pre-init buffer holds
// and then guarantuee they all run.
let dispatcher = Dispatcher::new(5);
let result = Arc::new(Mutex::new(vec![]));
for i in 1..=5 {
let result = Arc::clone(&result);
dispatcher
.guard()
.launch(move || {
result.lock().unwrap().push(i);
})
.unwrap();
}
dispatcher.guard().flush_init().unwrap();
// Queue more than 5 tasks,
// Each one is slow to process, so we should be faster in queueing
// them up than they are processed.
for i in 6..=20 {
let result = Arc::clone(&result);
dispatcher
.guard()
.launch(move || {
thread::sleep(Duration::from_millis(50));
result.lock().unwrap().push(i);
})
.unwrap();
}
dispatcher.guard().shutdown().unwrap();
dispatcher.join().unwrap();
let expected = (1..=20).collect::<Vec<_>>();
assert_eq!(&*result.lock().unwrap(), &expected);
}
}