New zero alloc, no_std compatible implementation

This implementation relies on the fact that the Thread is cloneable and layout-compatible with a pointer, thus one may just build a waker out of the thread without unnecessary wrapping behind Arcs.

Author: h33p

Cargo.toml

@@ -15,7 +15,9 @@ license = "Apache-2.0/MIT"
 readme = "README.md"
 
 [features]
+default = ["std"]
 macro = ["pollster-macro"]
+std = []
 
 [dependencies]
 pollster-macro = { version = "0.1", path = "macro", optional = true }

src/lib.rs

@@ -1,12 +1,17 @@
 #![doc = include_str!("../README.md")]
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 
-use std::{
+#![cfg_attr(not(feature = "std"), no_std)]
+
+use core::{
     future::Future,
-    sync::{Arc, Condvar, Mutex},
-    task::{Context, Poll, Wake, Waker},
+    mem,
+    task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
 };
 
+#[cfg(feature = "std")]
+use std::thread::{self, Thread};
+
 #[cfg(feature = "macro")]
 pub use pollster_macro::{main, test};
 
@@ -24,108 +29,199 @@ pub trait FutureExt: Future {
     /// let result = my_fut.block_on();
     /// ```
     fn block_on(self) -> Self::Output where Self: Sized { block_on(self) }
+
+    /// Block the thread until the future is ready with custom thread parking implementation.
+    ///
+    /// This allows one to use custom thread parking mechanisms in `no_std` environments.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use pollster::FutureExt as _;
+    /// use std::thread::Thread;
+    ///
+    /// let my_fut = async {};
+    ///
+    /// let result = my_fut.block_on_t::<Thread>();
+    /// ```
+    fn block_on_t<T: Parkable>(self) -> Self::Output where Self: Sized { block_on_t::<T, Self>(self) }
 }
 
 impl<F: Future> FutureExt for F {}
 
-enum SignalState {
-    Empty,
-    Waiting,
-    Notified,
+/// Parkable handle.
+///
+/// This handle allows a thread to potentially be efficiently blocked. This is used in the polling
+/// implementation to wait for wakeups.
+///
+/// The interface models that of `std::thread`, and many functions, such as
+/// [`current`](Parkable::current), [`park`](Parkable::park), and [`unpark`](Parkable::unpark)
+/// map to `std::thread` equivalents.
+pub trait Parkable: Sized + Clone {
+    /// Get handle to current thread.
+    fn current() -> Self;
+
+    /// Park the current thread.
+    fn park();
+
+    /// Unpark specified thread.
+    fn unpark(&self);
+
+    /// Convert self into opaque pointer.
+    ///
+    /// This requires `Self` to either be layout compatible with `*const ()` or heap allocated upon
+    /// switch.
+    fn into_opaque(self) -> *const ();
+
+    /// Convert opaque pointer into `Self`.
+    ///
+    /// # Safety
+    ///
+    /// This function is safe if the `data` argument is a valid park handle created by
+    /// `Self::into_opaque`.
+    unsafe fn from_opaque(data: *const ()) -> Self;
+
+    /// Create a waker out of `self`
+    ///
+    /// This function will clone self and build a `Waker` object.
+    fn waker(&self) -> Waker {
+        let data = self.clone().into_opaque();
+        // SAFETY: `RawWaker` created by `raw_waker` builds a waker object out of the raw data and
+        // vtable methods of this type which we assume are correct.
+        unsafe {
+            Waker::from_raw(raw_waker::<Self>(data))
+        }
+    }
 }
 
-struct Signal {
-    state: Mutex<SignalState>,
-    cond: Condvar,
+#[cfg(feature = "std")]
+pub type DefaultHandle = Thread;
+#[cfg(not(feature = "std"))]
+pub type DefaultHandle = *const ();
+
+fn raw_waker<T: Parkable>(data: *const ()) -> RawWaker {
+    RawWaker::new(
+        data,
+        &RawWakerVTable::new(
+            clone_waker::<T>,
+            wake::<T>,
+            wake_by_ref::<T>,
+            drop_waker::<T>,
+        ),
+    )
 }
 
-impl Signal {
-    fn new() -> Self {
-        Self {
-            state: Mutex::new(SignalState::Empty),
-            cond: Condvar::new(),
-        }
+unsafe fn clone_waker<T: Parkable>(data: *const ()) -> RawWaker {
+    let waker = T::from_opaque(data);
+    mem::forget(waker.clone());
+    mem::forget(waker);
+    raw_waker::<T>(data)
+}
+
+unsafe fn wake<T: Parkable>(data: *const ()) {
+    let waker = T::from_opaque(data);
+    waker.unpark();
+}
+
+unsafe fn wake_by_ref<T: Parkable>(data: *const ()) {
+    let waker = T::from_opaque(data);
+    waker.unpark();
+    mem::forget(waker);
+}
+
+unsafe fn drop_waker<T: Parkable>(data: *const ()) {
+    let _ = T::from_opaque(data);
+}
+
+#[cfg(feature = "std")]
+impl Parkable for Thread {
+    fn current() -> Self {
+        thread::current()
     }
 
-    fn wait(&self) {
-        let mut state = self.state.lock().unwrap();
-        match *state {
-            // Notify() was called before we got here, consume it here without waiting and return immediately.
-            SignalState::Notified => *state = SignalState::Empty,
-            // This should not be possible because our signal is created within a function and never handed out to any
-            // other threads. If this is the case, we have a serious problem so we panic immediately to avoid anything
-            // more problematic happening.
-            SignalState::Waiting => {
-                unreachable!("Multiple threads waiting on the same signal: Open a bug report!");
-            }
-            SignalState::Empty => {
-                // Nothing has happened yet, and we're the only thread waiting (as should be the case!). Set the state
-                // accordingly and begin polling the condvar in a loop until it's no longer telling us to wait. The
-                // loop prevents incorrect spurious wakeups.
-                *state = SignalState::Waiting;
-                while let SignalState::Waiting = *state {
-                    state = self.cond.wait(state).unwrap();
-                }
-            }
-        }
+    fn park() {
+        thread::park();
     }
 
-    fn notify(&self) {
-        let mut state = self.state.lock().unwrap();
-        match *state {
-            // The signal was already notified, no need to do anything because the thread will be waking up anyway
-            SignalState::Notified => {}
-            // The signal wasn't notified but a thread isn't waiting on it, so we can avoid doing unnecessary work by
-            // skipping the condvar and leaving behind a message telling the thread that a notification has already
-            // occurred should it come along in the future.
-            SignalState::Empty => *state = SignalState::Notified,
-            // The signal wasn't notified and there's a waiting thread. Reset the signal so it can be wait()'ed on again
-            // and wake up the thread. Because there should only be a single thread waiting, `notify_all` would also be
-            // valid.
-            SignalState::Waiting => {
-                *state = SignalState::Empty;
-                self.cond.notify_one();
-            }
-        }
+    fn unpark(&self) {
+        Thread::unpark(self);
+    }
+
+    fn into_opaque(self) -> *const () {
+        // SAFETY: `Thread` internal layout is an Arc to inner type, which is represented as a
+        // single pointer. The only thing we do with the pointer is transmute it back to
+        // ThreadWaker in the waker functions. If for whatever reason Thread layout will change to
+        // contain multiple fields, this will still be safe, because the compiler will simply
+        // refuse to compile the program.
+        unsafe { mem::transmute::<_, *const ()>(self) }
+    }
+
+    unsafe fn from_opaque(data: *const ()) -> Self {
+        mem::transmute(data)
     }
 }
 
-impl Wake for Signal {
-    fn wake(self: Arc<Self>) {
-        self.notify();
+impl Parkable for *const () {
+    fn current() -> Self {
+        core::ptr::null()
+    }
+
+    fn park() {
+        core::hint::spin_loop()
+    }
+
+    fn unpark(&self) {}
+
+    fn into_opaque(self) -> *const () {
+        self
+    }
+
+    unsafe fn from_opaque(data: *const ()) -> Self {
+        data
     }
 }
 
-/// Block the thread until the future is ready.
+/// Block the thread until the future is ready with custom parking implementation.
+///
+/// This allows one to use custom thread parking mechanisms in `no_std` environments.
 ///
 /// # Example
 ///
 /// ```
+/// use std::thread::Thread;
+///
 /// let my_fut = async {};
-/// let result = pollster::block_on(my_fut);
+/// let result = pollster::block_on_t::<Thread, _>(my_fut);
 /// ```
-pub fn block_on<F: Future>(mut fut: F) -> F::Output {
+pub fn block_on_t<T: Parkable, F: Future>(mut fut: F) -> F::Output {
     // Pin the future so that it can be polled.
     // SAFETY: We shadow `fut` so that it cannot be used again. The future is now pinned to the stack and will not be
     // moved until the end of this scope. This is, incidentally, exactly what the `pin_mut!` macro from `pin_utils`
     // does.
     let mut fut = unsafe { std::pin::Pin::new_unchecked(&mut fut) };
 
-    // Signal used to wake up the thread for polling as the future moves to completion. We need to use an `Arc`
-    // because, although the lifetime of `fut` is limited to this function, the underlying IO abstraction might keep
-    // the signal alive for far longer. `Arc` is a thread-safe way to allow this to happen.
-    // TODO: Investigate ways to reuse this `Arc<Signal>`... perhaps via a `static`?
-    let signal = Arc::new(Signal::new());
+    let handle = T::current();
 
-    // Create a context that will be passed to the future.
-    let waker = Waker::from(Arc::clone(&signal));
+    let waker: Waker = handle.waker();
     let mut context = Context::from_waker(&waker);
 
     // Poll the future to completion
     loop {
         match fut.as_mut().poll(&mut context) {
-            Poll::Pending => signal.wait(),
+            Poll::Pending => T::park(),
             Poll::Ready(item) => break item,
         }
     }
 }
+
+/// Block the thread until the future is ready.
+///
+/// # Example
+///
+/// ```
+/// let my_fut = async {};
+/// let result = pollster::block_on(my_fut);
+/// ```
+pub fn block_on<F: Future>(fut: F) -> F::Output {
+    return block_on_t::<DefaultHandle, _>(fut);
+}