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//! [![Documentation](https://docs.rs/ste/badge.svg)](https://docs.rs/ste) //! [![Crates](https://img.shields.io/crates/v/ste.svg)](https://crates.io/crates/ste) //! [![Actions Status](https://github.com/udoprog/rotary/workflows/Rust/badge.svg)](https://github.com/udoprog/rotary/actions) //! //! A single-threaded executor with some tricks up its sleeve. //! //! This was primarily written for use in [rotary] as a low-latency way of //! interacting with a single background thread for audio-related purposes, but //! is otherwise a general purpose library that can be used by anyone. //! //! **Warning:** Some of the tricks used in this crate needs to be sanity //! checked for safety before you can rely on this for production uses. //! //! # Examples //! //! ```rust //! # fn main() -> anyhow::Result<()> { //! let thread = ste::Thread::new()?; //! //! let mut n = 10; //! thread.submit(|| n += 10)?; //! assert_eq!(20, n); //! //! thread.join()?; //! # Ok(()) } //! ``` //! //! [rotary]: https://github.com/udoprog/rotary use parking_lot::{Condvar, Mutex}; use std::io; use std::mem; use std::ptr; use std::sync::atomic::{AtomicUsize, Ordering}; use std::thread; use thiserror::Error; #[cfg(test)] mod tests; mod parker; use self::parker::Unparker; mod tagged; pub use self::tagged::Tagged; use self::tagged::{with_tag, Tag}; /// Error raised when we try to interact with a background thread that has /// panicked. #[derive(Debug, Error)] #[error("background thread panicked")] pub struct Panicked(()); /// The handle for a background thread. /// /// The background thread can be interacted with in a couple of ways: /// * [submit][Thread::submit] - for submitted tasks, the call will block until /// it has been executed on the thread (or the thread has panicked). /// * [drop][Thread::drop] - for dropping value *on* the background thread. This /// is necessary for [Tagged] values that requires drop. /// /// # Examples /// /// ```rust /// use std::sync::Arc; /// /// # fn main() -> anyhow::Result<()> { /// let thread = Arc::new(ste::Thread::new()?); /// let mut threads = Vec::new(); /// /// for n in 0..10 { /// let thread = thread.clone(); /// /// threads.push(std::thread::spawn(move || { /// thread.submit(move || n) /// })); /// } /// /// let mut result = 0; /// /// for t in threads { /// result += t.join().unwrap()?; /// } /// /// assert_eq!(result, (0..10).sum()); /// /// // Unwrap the thread. /// let thread = Arc::try_unwrap(thread).map_err(|_| "unwrap failed").unwrap(); /// /// let value = thread.submit(|| { /// panic!("Background thread: {:?}", std::thread::current().id()); /// }); /// /// println!("Main thread: {:?}", std::thread::current().id()); /// assert!(value.is_err()); /// /// assert!(thread.join().is_err()); /// # Ok(()) } /// ``` #[must_use = "The thread should be joined with Thread::join once no longer used, \ otherwise it will block while being dropped."] pub struct Thread { /// Things that have been submitted for execution on the background thread. shared: ptr::NonNull<Shared>, /// The handle associated with the background thread. handle: Option<thread::JoinHandle<()>>, } /// Safety: The handle is both send and sync because it joins the background /// thread which keeps track of the state of `shared` and cleans it up once it's /// no longer needed. unsafe impl Send for Thread {} unsafe impl Sync for Thread {} impl Thread { /// Construct a default background thread executor. /// /// These both do the same thing: /// /// ```rust /// # fn main() -> anyhow::Result<()> { /// let thread1 = ste::Thread::new()?; /// let thread2 = ste::Builder::new().build()?; /// # Ok(()) } /// ``` pub fn new() -> io::Result<Self> { Builder::new().build() } /// Submit a task to run on the background thread. /// /// The call will block until it has been executed on the thread (or the /// thread has panicked). /// /// Because this function blocks until completion, it can safely access /// values which are outside of the scope of the provided closure. /// /// If you however need to store and access things which are `!Sync`, you /// can use [Tagged]. /// /// # Examples /// /// ```rust /// # fn main() -> anyhow::Result<()> { /// let thread = ste::Thread::new()?; /// /// let mut n = 10; /// thread.submit(|| n += 10)?; /// assert_eq!(20, n); /// /// thread.join()?; /// # Ok(()) } /// ``` pub fn submit<F, T>(&self, task: F) -> Result<T, Panicked> where F: Send + FnOnce() -> T, T: Send, { let flag = AtomicUsize::new(0); let mut storage = None; { let storage = ptr::NonNull::from(&mut storage); let (parker, unparker) = parker::new(storage.as_ptr()); let mut task = into_task(task, RawSend(storage)); // Safety: We're constructing a pointer to a local stack location. It // will never be null. // // The transmute is necessary because we're constructing a trait object // with a `'static` lifetime. let task = unsafe { ptr::NonNull::new_unchecked(mem::transmute::<&mut (dyn FnMut(Tag) + Send), _>( &mut task, )) }; self.set_state(State::Schedule(Schedule { task, unparker, flag: ptr::NonNull::from(&flag), }))?; match self.handle.as_ref() { Some(handle) => handle.thread().unpark(), None => panic!("missing thread handle"), } // If 0, we know we got here first and have to park until the thread // is ready. if flag.fetch_add(1, Ordering::AcqRel) == NONE_READY { // Safety: we're the only ones controlling these, so we know that // they are correctly allocated and who owns what with // synchronization. parker.park(|| flag.load(Ordering::Relaxed) == BOTH_READY); } } return match storage { Some(result) => Ok(result), None => Err(Panicked(())), }; fn into_task<F, T>(task: F, storage: RawSend<Option<T>>) -> impl FnMut(Tag) + Send where F: FnOnce() -> T + Send, T: Send, { let mut task = Some(task); move |tag| { let RawSend(mut storage) = storage; if let Some(task) = task.take() { let output = with_tag(tag, task); // Safety: we're the only one with access to this pointer, // and we know it hasn't been de-allocated yet. unsafe { *storage.as_mut() = Some(output); } } } } } /// Move the provided `value` onto the background thread and drop it. /// /// This is necessary for [Tagged] values that needs to be dropped which /// would otherwise panic. /// /// # Examples /// /// ```rust /// // Ensure that `Foo` is `!Send` and `!Sync`. /// struct Foo(*mut ()); /// /// impl Foo { /// fn test(&self) -> u32 { /// 42 /// } /// } /// /// impl Drop for Foo { /// fn drop(&mut self) { /// println!("Foo was dropped"); /// } /// } /// /// # fn main() -> anyhow::Result<()> { /// let thread = ste::Thread::new()?; /// /// let value = thread.submit(|| ste::Tagged::new(Foo(0 as *mut ())))?; /// let out = thread.submit(|| value.test())?; /// assert_eq!(42, out); /// /// thread.drop(value)?; /// thread.join()?; /// # Ok(()) } /// ``` /// /// If we omit the drop the above will panic. /// /// ```rust,should_panic /// # struct Foo(*mut ()); /// # impl Drop for Foo { fn drop(&mut self) {} } /// /// # fn main() -> anyhow::Result<()> { /// let thread = ste::Thread::new()?; /// /// let value = thread.submit(|| ste::Tagged::new(Foo(0 as *mut ())))?; /// /// thread.join()?; /// # Ok(()) } /// ``` pub fn drop<T>(&self, value: T) -> Result<(), Panicked> where T: Send, { self.submit(move || drop(value))?; Ok(()) } /// Join the background thread. /// /// Will block until the background thread is joined. /// /// This is the clean way to join a background thread, the alternative is to /// let [Thread] drop and this will be performed in the drop handler /// instead. /// /// # Examples /// /// ```rust /// # fn main() -> anyhow::Result<()> { /// let thread = ste::Thread::new()?; /// /// let mut n = 10; /// thread.submit(|| n += 10)?; /// assert_eq!(20, n); /// /// thread.join()?; /// # Ok(()) } /// ``` pub fn join(mut self) -> Result<(), Panicked> { self.inner_join() } /// Update the shared state. /// /// This will *not* notify the background thread to allow it to be used in /// contexts where the handle has been removed. fn set_state(&self, state: State) -> Result<(), Panicked> { let mut guard = loop { unsafe { let mut guard = self.shared.as_ref().state.lock(); match &*guard { State::Busy | State::Schedule(..) => { self.shared.as_ref().cond.wait(&mut guard); continue; } State::Panicked => { return Err(Panicked(())); } State::Waiting => break guard, State::Join => unreachable!(), } } }; *guard = state; Ok(()) } fn inner_join(&mut self) -> Result<(), Panicked> { if let Some(handle) = self.handle.take() { self.set_state(State::Join)?; handle.thread().unpark(); return handle.join().map_err(|_| Panicked(())); } Ok(()) } /// Worker thread. fn worker(prelude: Option<Box<Prelude>>, RawSend(shared): RawSend<Shared>) { let poison_guard = PoisonGuard { shared }; if let Some(prelude) = prelude { prelude(); } unsafe { *shared.as_ref().state.lock() = State::Waiting; } 'outer: loop { let mut schedule = loop { unsafe { if !shared.as_ref().cond.notify_one() { thread::park(); } let mut state = shared.as_ref().state.lock(); match mem::replace(&mut *state, State::Waiting) { State::Waiting => continue, State::Schedule(submit) => break submit, _ => break 'outer, } } }; let tag = Tag(shared.as_ptr() as usize); unsafe { schedule.task.as_mut()(tag) }; } // Forget the guard to disarm the panic. mem::forget(poison_guard); /// Guard used to mark the state of the executed as "panicked". This is /// accomplished by asserting that the only reason this destructor would /// be called would be due to an unwinding panic. struct PoisonGuard { shared: ptr::NonNull<Shared>, } impl Drop for PoisonGuard { fn drop(&mut self) { unsafe { let old = mem::replace(&mut *self.shared.as_ref().state.lock(), State::Panicked); // It's not possible for the state to be anything but empty // here, because the worker thread takes the state before // executing user code which might panic. debug_assert!(!matches!(old, State::Panicked)); self.shared.as_ref().cond.notify_all(); } } } } } impl Drop for Thread { fn drop(&mut self) { // Note: we can safely ignore the result, because it will only error in // case the background thread has panicked. At which point we're still // free to assume it's no longer using the shared state. let _ = self.inner_join(); // Safety: at this point it's guaranteed that we've synchronized with // the thread enough that the shared state can be safely deallocated. // // The background thread is in one of two states: // * It has panicked, which means the shared state will not be used any // longer. // * It has successfully been joined in. Which has the same // implications. unsafe { let _ = Box::from_raw(self.shared.as_ptr()); } } } /// The builder for a [Thread] which can be configured a bit more. pub struct Builder { prelude: Option<Box<Prelude>>, } impl Builder { /// Construct a new builder. pub fn new() -> Self { Self { prelude: None } } /// Configure a prelude to the [Thread]. This is code that will run just as /// the thread is spinning up. /// /// # Examples /// /// ```rust /// fn say_hello(main_thread: std::thread::ThreadId) { /// println!("Hello from the prelude!"); /// assert_ne!(main_thread, std::thread::current().id()); /// } /// /// # fn main() -> anyhow::Result<()> { /// let main_thread = std::thread::current().id(); /// /// let thread = ste::Builder::new().prelude(move || say_hello(main_thread)).build(); /// # Ok(()) } /// ``` pub fn prelude<P>(self, prelude: P) -> Self where P: Fn() + Send + 'static, { Self { prelude: Some(Box::new(prelude)), } } /// Construct the background thread. /// /// # Examples /// /// ```rust /// # fn main() -> anyhow::Result<()> { /// let thread = ste::Builder::new().build()?; /// thread.join()?; /// # Ok(()) } /// ``` pub fn build(self) -> io::Result<Thread> { let shared = ptr::NonNull::from(Box::leak(Box::new(Shared { state: Mutex::new(State::Busy), cond: Condvar::new(), }))); let prelude = self.prelude; let shared2 = RawSend(shared); let handle = thread::Builder::new() .name(String::from("ste-thread")) .spawn(move || Thread::worker(prelude, shared2))?; Ok(Thread { shared, handle: Some(handle), }) } } /// Small helper for sending things which are not Send. struct RawSend<T>(ptr::NonNull<T>); unsafe impl<T> Send for RawSend<T> {} /// The state of the executor. enum State { /// The background thread is busy and cannot process tasks yet. The /// scheduler starts out in this state, before the prelude has been /// executed. Busy, /// Scheduler is waiting for tasks. Waiting, /// A task that is expected to be scheduled. Schedule(Schedule), /// Scheduler has panicked. Panicked, /// Scheduler is being joined. Join, } /// A task submitted to the executor. struct Schedule { task: ptr::NonNull<dyn FnMut(Tag) + Send + 'static>, unparker: Unparker, flag: ptr::NonNull<AtomicUsize>, } // The implementation of [Schedule] is safe because it's privately constructed // inside of this module. unsafe impl Send for Schedule {} impl Drop for Schedule { fn drop(&mut self) { // Safety: We know that the task holding the flag owns the // reference. if unsafe { self.flag.as_ref().fetch_add(1, Ordering::AcqRel) == NONE_READY } { // We got here first, so we know the calling thread won't // park since they will see our update. If the calling // thread got here before us, the value would be 1. return; } while !self.unparker.unpark_one() { thread::yield_now(); } } } // Shared state between the worker thread and [Thread]. struct Shared { state: Mutex<State>, cond: Condvar, } /// The type of the prelude function. type Prelude = dyn Fn() + Send + 'static; const NONE_READY: usize = 0; const BOTH_READY: usize = 2;