sync: move into `tokio` crate (#1705)

A step towards collapsing Tokio sub crates into a single `tokio`
crate (#1318).

The sync implementation is now provided by the main `tokio` crate.
Functionality can be opted out of by using the various net related
feature flags.

1 files changed, 135 insertions, 0 deletions

diff --git a/tokio/src/sync/barrier.rs b/tokio/src/sync/barrier.rs
new file mode 100644
index 00000000..1582120e
--- /dev/null
+++ b/tokio/src/sync/barrier.rs
@@ -0,0 +1,135 @@
+use crate::sync::watch;
+
+use std::sync::Mutex;
+
+/// A barrier enables multiple threads to synchronize the beginning of some computation.
+///
+/// ```
+/// # #[tokio::main]
+/// # async fn main() {
+/// use tokio::sync::Barrier;
+/// use std::sync::Arc;
+/// use futures_util::future::join_all;
+///
+/// let mut handles = Vec::with_capacity(10);
+/// let barrier = Arc::new(Barrier::new(10));
+/// for _ in 0..10 {
+///     let c = barrier.clone();
+///     // The same messages will be printed together.
+///     // You will NOT see any interleaving.
+///     handles.push(async move {
+///         println!("before wait");
+///         let wr = c.wait().await;
+///         println!("after wait");
+///         wr
+///     });
+/// }
+/// // Will not resolve until all "before wait" messages have been printed
+/// let wrs = join_all(handles).await;
+/// // Exactly one barrier will resolve as the "leader"
+/// assert_eq!(wrs.into_iter().filter(|wr| wr.is_leader()).count(), 1);
+/// # }
+/// ```
+#[derive(Debug)]
+pub struct Barrier {
+    state: Mutex<BarrierState>,
+    wait: watch::Receiver<usize>,
+    n: usize,
+}
+
+#[derive(Debug)]
+struct BarrierState {
+    waker: watch::Sender<usize>,
+    arrived: usize,
+    generation: usize,
+}
+
+impl Barrier {
+    /// Creates a new barrier that can block a given number of threads.
+    ///
+    /// A barrier will block `n`-1 threads which call [`Barrier::wait`] and then wake up all
+    /// threads at once when the `n`th thread calls `wait`.
+    pub fn new(mut n: usize) -> Barrier {
+        let (waker, wait) = crate::sync::watch::channel(0);
+
+        if n == 0 {
+            // if n is 0, it's not clear what behavior the user wants.
+            // in std::sync::Barrier, an n of 0 exhibits the same behavior as n == 1, where every
+            // .wait() immediately unblocks, so we adopt that here as well.
+            n = 1;
+        }
+
+        Barrier {
+            state: Mutex::new(BarrierState {
+                waker,
+                arrived: 0,
+                generation: 1,
+            }),
+            n,
+            wait,
+        }
+    }
+
+    /// Does not resolve until all tasks have rendezvoused here.
+    ///
+    /// Barriers are re-usable after all threads have rendezvoused once, and can
+    /// be used continuously.
+    ///
+    /// A single (arbitrary) future will receive a [`BarrierWaitResult`] that returns `true` from
+    /// [`BarrierWaitResult::is_leader`] when returning from this function, and all other threads
+    /// will receive a result that will return `false` from `is_leader`.
+    pub async fn wait(&self) -> BarrierWaitResult {
+        // NOTE: we are taking a _synchronous_ lock here.
+        // It is okay to do so because the critical section is fast and never yields, so it cannot
+        // deadlock even if another future is concurrently holding the lock.
+        // It is _desireable_ to do so as synchronous Mutexes are, at least in theory, faster than
+        // the asynchronous counter-parts, so we should use them where possible [citation needed].
+        // NOTE: the extra scope here is so that the compiler doesn't think `state` is held across
+        // a yield point, and thus marks the returned future as !Send.
+        let generation = {
+            let mut state = self.state.lock().unwrap();
+            let generation = state.generation;
+            state.arrived += 1;
+            if state.arrived == self.n {
+                // we are the leader for this generation
+                // wake everyone, increment the generation, and return
+                state
+                    .waker
+                    .broadcast(state.generation)
+                    .expect("there is at least one receiver");
+                state.arrived = 0;
+                state.generation += 1;
+                return BarrierWaitResult(true);
+            }
+
+            generation
+        };
+
+        // we're going to have to wait for the last of the generation to arrive
+        let mut wait = self.wait.clone();
+
+        loop {
+            // note that the first time through the loop, this _will_ yield a generation
+            // immediately, since we cloned a receiver that has never seen any values.
+            if wait.recv().await.expect("sender hasn't been closed") >= generation {
+                break;
+            }
+        }
+
+        BarrierWaitResult(false)
+    }
+}
+
+/// A `BarrierWaitResult` is returned by `wait` when all threads in the `Barrier` have rendezvoused.
+#[derive(Debug, Clone)]
+pub struct BarrierWaitResult(bool);
+
+impl BarrierWaitResult {
+    /// Returns true if this thread from wait is the "leader thread".
+    ///
+    /// Only one thread will have `true` returned from their result, all other threads will have
+    /// `false` returned.
+    pub fn is_leader(&self) -> bool {
+        self.0
+    }
+}