Make timer method available on both foreground and background executors

Also, make it return a static future.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/gpui/src/executor.rs

@@ -306,6 +306,32 @@ impl Deterministic {
         None
     }
 
+    pub fn timer(&self, duration: Duration) -> impl Future<Output = ()> {
+        let (tx, mut rx) = postage::barrier::channel();
+        let timer_id;
+        {
+            let mut state = self.state.lock();
+            let wakeup_at = state.now + duration;
+            timer_id = util::post_inc(&mut state.next_timer_id);
+            state.pending_timers.push((timer_id, wakeup_at, tx));
+        }
+
+        let remove_timer = util::defer({
+            let state = self.state.clone();
+            move || {
+                state
+                    .lock()
+                    .pending_timers
+                    .retain(|(id, _, _)| *id != timer_id);
+            }
+        });
+
+        async move {
+            postage::prelude::Stream::recv(&mut rx).await;
+            drop(remove_timer);
+        }
+    }
+
     pub fn advance_clock(&self, duration: Duration) {
         let mut state = self.state.lock();
         state.now += duration;
@@ -438,41 +464,18 @@ impl Foreground {
         }
     }
 
-    pub async fn timer(&self, duration: Duration) {
-        match self {
-            #[cfg(any(test, feature = "test-support"))]
-            Self::Deterministic { executor, .. } => {
-                use postage::prelude::Stream as _;
+    pub fn timer(&self, duration: Duration) -> impl Future<Output = ()> {
+        let mut timer = None;
 
-                let (tx, mut rx) = postage::barrier::channel();
-                let timer_id;
-                {
-                    let mut state = executor.state.lock();
-                    let wakeup_at = state.now + duration;
-                    timer_id = util::post_inc(&mut state.next_timer_id);
-                    state.pending_timers.push((timer_id, wakeup_at, tx));
-                }
+        #[cfg(any(test, feature = "test-support"))]
+        if let Self::Deterministic { executor, .. } = self {
+            timer = Some(executor.timer(duration));
+        }
 
-                struct DropTimer<'a>(usize, &'a Foreground);
-                impl<'a> Drop for DropTimer<'a> {
-                    fn drop(&mut self) {
-                        match self.1 {
-                            Foreground::Deterministic { executor, .. } => {
-                                executor
-                                    .state
-                                    .lock()
-                                    .pending_timers
-                                    .retain(|(timer_id, _, _)| *timer_id != self.0);
-                            }
-                            _ => unreachable!(),
-                        }
-                    }
-                }
-
-                let _guard = DropTimer(timer_id, self);
-                rx.recv().await;
-            }
-            _ => {
+        async move {
+            if let Some(timer) = timer {
+                timer.await;
+            } else {
                 Timer::after(duration).await;
             }
         }
@@ -600,6 +603,23 @@ impl Background {
         }
     }
 
+    pub fn timer(&self, duration: Duration) -> impl Future<Output = ()> {
+        let mut timer = None;
+
+        #[cfg(any(test, feature = "test-support"))]
+        if let Self::Deterministic { executor, .. } = self {
+            timer = Some(executor.timer(duration));
+        }
+
+        async move {
+            if let Some(timer) = timer {
+                timer.await;
+            } else {
+                Timer::after(duration).await;
+            }
+        }
+    }
+
     #[cfg(any(test, feature = "test-support"))]
     pub async fn simulate_random_delay(&self) {
         use rand::prelude::*;

crates/gpui/src/util.rs

@@ -1,5 +1,6 @@
 use smol::future::FutureExt;
 use std::{future::Future, time::Duration};
+pub use util::*;
 
 pub fn post_inc(value: &mut usize) -> usize {
     let prev = *value;

crates/util/src/lib.rs

@@ -123,6 +123,18 @@ where
     }
 }
 
+struct Defer<F: FnOnce()>(Option<F>);
+
+impl<F: FnOnce()> Drop for Defer<F> {
+    fn drop(&mut self) {
+        self.0.take().map(|f| f());
+    }
+}
+
+pub fn defer<F: FnOnce()>(f: F) -> impl Drop {
+    Defer(Some(f))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;