sync: add broadcast::Sender::new (#5824)

tokio/src/sync/broadcast.rs

@@ -444,42 +444,13 @@ const MAX_RECEIVERS: usize = usize::MAX >> 2;
 /// This will panic if `capacity` is equal to `0` or larger
 /// than `usize::MAX / 2`.
 #[track_caller]
-pub fn channel<T: Clone>(mut capacity: usize) -> (Sender<T>, Receiver<T>) {
-    assert!(capacity > 0, "capacity is empty");
-    assert!(capacity <= usize::MAX >> 1, "requested capacity too large");
-
-    // Round to a power of two
-    capacity = capacity.next_power_of_two();
-
-    let mut buffer = Vec::with_capacity(capacity);
-
-    for i in 0..capacity {
-        buffer.push(RwLock::new(Slot {
-            rem: AtomicUsize::new(0),
-            pos: (i as u64).wrapping_sub(capacity as u64),
-            val: UnsafeCell::new(None),
-        }));
-    }
-
-    let shared = Arc::new(Shared {
-        buffer: buffer.into_boxed_slice(),
-        mask: capacity - 1,
-        tail: Mutex::new(Tail {
-            pos: 0,
-            rx_cnt: 1,
-            closed: false,
-            waiters: LinkedList::new(),
-        }),
-        num_tx: AtomicUsize::new(1),
-    });
-
+pub fn channel<T: Clone>(capacity: usize) -> (Sender<T>, Receiver<T>) {
+    // SAFETY: In the line below we are creating one extra receiver, so there will be 1 in total.
+    let tx = unsafe { Sender::new_with_receiver_count(1, capacity) };
     let rx = Receiver {
-        shared: shared.clone(),
+        shared: tx.shared.clone(),
         next: 0,
     };
-
-    let tx = Sender { shared };
-
     (tx, rx)
 }
 
@@ -490,6 +461,65 @@ unsafe impl<T: Send> Send for Receiver<T> {}
 unsafe impl<T: Send> Sync for Receiver<T> {}
 
 impl<T> Sender<T> {
+    /// Creates the sending-half of the [`broadcast`] channel.
+    ///
+    /// See documentation of [`broadcast::channel`] for errors when calling this function.
+    ///
+    /// [`broadcast`]: crate::sync::broadcast
+    /// [`broadcast::channel`]: crate::sync::broadcast
+    #[track_caller]
+    pub fn new(capacity: usize) -> Self {
+        // SAFETY: We don't create extra receivers, so there are 0.
+        unsafe { Self::new_with_receiver_count(0, capacity) }
+    }
+
+    /// Creates the sending-half of the [`broadcast`](self) channel, and provide the receiver
+    /// count.
+    ///
+    /// See the documentation of [`broadcast::channel`](self::channel) for more errors when
+    /// calling this function.
+    ///
+    /// # Safety:
+    ///
+    /// The caller must ensure that the amount of receivers for this Sender is correct before
+    /// the channel functionalities are used, the count is zero by default, as this function
+    /// does not create any receivers by itself.
+    #[track_caller]
+    unsafe fn new_with_receiver_count(receiver_count: usize, mut capacity: usize) -> Self {
+        assert!(capacity > 0, "broadcast channel capacity cannot be zero");
+        assert!(
+            capacity <= usize::MAX >> 1,
+            "broadcast channel capacity exceeded `usize::MAX / 2`"
+        );
+
+        // Round to a power of two
+        capacity = capacity.next_power_of_two();
+
+        let mut buffer = Vec::with_capacity(capacity);
+
+        for i in 0..capacity {
+            buffer.push(RwLock::new(Slot {
+                rem: AtomicUsize::new(0),
+                pos: (i as u64).wrapping_sub(capacity as u64),
+                val: UnsafeCell::new(None),
+            }));
+        }
+
+        let shared = Arc::new(Shared {
+            buffer: buffer.into_boxed_slice(),
+            mask: capacity - 1,
+            tail: Mutex::new(Tail {
+                pos: 0,
+                rx_cnt: receiver_count,
+                closed: false,
+                waiters: LinkedList::new(),
+            }),
+            num_tx: AtomicUsize::new(1),
+        });
+
+        Sender { shared }
+    }
+
     /// Attempts to send a value to all active [`Receiver`] handles, returning
     /// it back if it could not be sent.
     ///
@@ -1370,3 +1400,41 @@ impl<'a, T> Drop for RecvGuard<'a, T> {
 }
 
 fn is_unpin<T: Unpin>() {}
+
+#[cfg(not(loom))]
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn receiver_count_on_sender_constructor() {
+        let sender = Sender::<i32>::new(16);
+        assert_eq!(sender.receiver_count(), 0);
+
+        let rx_1 = sender.subscribe();
+        assert_eq!(sender.receiver_count(), 1);
+
+        let rx_2 = rx_1.resubscribe();
+        assert_eq!(sender.receiver_count(), 2);
+
+        let rx_3 = sender.subscribe();
+        assert_eq!(sender.receiver_count(), 3);
+
+        drop(rx_3);
+        drop(rx_1);
+        assert_eq!(sender.receiver_count(), 1);
+
+        drop(rx_2);
+        assert_eq!(sender.receiver_count(), 0);
+    }
+
+    #[cfg(not(loom))]
+    #[test]
+    fn receiver_count_on_channel_constructor() {
+        let (sender, rx) = channel::<i32>(16);
+        assert_eq!(sender.receiver_count(), 1);
+
+        let _rx_2 = rx.resubscribe();
+        assert_eq!(sender.receiver_count(), 2);
+    }
+}