sync::broadcast: don't lock in channel()

tokio/src/sync/broadcast.rs

@@ -445,8 +445,12 @@ const MAX_RECEIVERS: usize = usize::MAX >> 2;
 /// than `usize::MAX / 2`.
 #[track_caller]
 pub fn channel<T: Clone>(capacity: usize) -> (Sender<T>, Receiver<T>) {
-    let tx = Sender::new(capacity);
-    let rx = tx.subscribe();
+    // SAFETY: In the line below we are creating one extra receiver, so will be 1 in total.
+    let tx = unsafe { Sender::new_with_receiver_count(1, capacity) };
+    let rx = Receiver {
+        shared: tx.shared.clone(),
+        next: 0,
+    };
     (tx, rx)
 }
 
@@ -464,9 +468,28 @@ impl<T> Sender<T> {
     /// [`broadcast`]: crate::sync::broadcast
     /// [`broadcast::channel`]: crate::sync::broadcast
     #[track_caller]
-    pub fn new(mut capacity: usize) -> Self {
-        assert!(capacity > 0, "capacity is empty");
-        assert!(capacity <= usize::MAX >> 1, "requested capacity too large");
+    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`] channel, and provide the receiver count.
+    ///
+    /// See the documentation of [`broadcast::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();
@@ -486,7 +509,7 @@ impl<T> Sender<T> {
             mask: capacity - 1,
             tail: Mutex::new(Tail {
                 pos: 0,
-                rx_cnt: 0,
+                rx_cnt: receiver_count,
                 closed: false,
                 waiters: LinkedList::new(),
             }),
@@ -1383,37 +1406,33 @@ mod tests {
 
     #[test]
     fn receiver_count_on_sender_constructor() {
-        let count_of = |sender: &Sender<i32>| sender.shared.tail.lock().rx_cnt;
-
         let sender = Sender::<i32>::new(16);
-        assert_eq!(count_of(&sender), 0);
+        assert_eq!(sender.receiver_count(), 0);
 
         let rx_1 = sender.subscribe();
-        assert_eq!(count_of(&sender), 1);
+        assert_eq!(sender.receiver_count(), 1);
 
         let rx_2 = rx_1.resubscribe();
-        assert_eq!(count_of(&sender), 2);
+        assert_eq!(sender.receiver_count(), 2);
 
         let rx_3 = sender.subscribe();
-        assert_eq!(count_of(&sender), 3);
+        assert_eq!(sender.receiver_count(), 3);
 
         drop(rx_3);
         drop(rx_1);
-        assert_eq!(count_of(&sender), 1);
+        assert_eq!(sender.receiver_count(), 1);
 
         drop(rx_2);
-        assert_eq!(count_of(&sender), 0);
+        assert_eq!(sender.receiver_count(), 0);
     }
 
     #[cfg(not(loom))]
     #[test]
     fn receiver_count_on_channel_constructor() {
-        let count_of = |sender: &Sender<i32>| sender.shared.tail.lock().rx_cnt;
-
         let (sender, rx) = channel::<i32>(16);
-        assert_eq!(count_of(&sender), 1);
+        assert_eq!(sender.receiver_count(), 1);
 
         let _rx_2 = rx.resubscribe();
-        assert_eq!(count_of(&sender), 2);
+        assert_eq!(sender.receiver_count(), 2);
     }
 }