util: make WakeList::wake_all use FIFO ordering

tokio/src/util/mod.rs

@@ -31,6 +31,26 @@ mod wake_list;
 ))]
 pub(crate) use wake_list::WakeList;
 
+// Used by `wake_list`
+#[cfg(any(
+    feature = "net",
+    feature = "process",
+    feature = "sync",
+    feature = "fs",
+    feature = "rt",
+    feature = "signal",
+))]
+mod noop_waker;
+#[cfg(any(
+    feature = "net",
+    feature = "process",
+    feature = "sync",
+    feature = "fs",
+    feature = "rt",
+    feature = "signal",
+))]
+pub(crate) use noop_waker::noop_waker;
+
 #[cfg(any(
     feature = "fs",
     feature = "net",

tokio/src/util/noop_waker.rs

@@ -0,0 +1,19 @@
+use core::ptr::null;
+use core::task::{RawWaker, RawWakerVTable, Waker};
+
+unsafe fn noop_clone(_data: *const ()) -> RawWaker {
+    noop_raw_waker()
+}
+
+unsafe fn noop(_data: *const ()) {}
+
+const NOOP_WAKER_VTABLE: RawWakerVTable = RawWakerVTable::new(noop_clone, noop, noop, noop);
+
+fn noop_raw_waker() -> RawWaker {
+    RawWaker::new(null(), &NOOP_WAKER_VTABLE)
+}
+
+/// A [`Waker`] which does nothing.
+pub(crate) fn noop_waker() -> Waker {
+    unsafe { Waker::from_raw(noop_raw_waker()) }
+}

tokio/src/util/wake_list.rs

@@ -2,6 +2,8 @@ use core::mem::MaybeUninit;
 use core::ptr;
 use std::task::Waker;
 
+use crate::util::noop_waker;
+
 const NUM_WAKERS: usize = 32;
 
 /// A list of wakers to be woken.
@@ -38,11 +40,11 @@ impl WakeList {
 
     pub(crate) fn wake_all(&mut self) {
         assert!(self.curr <= NUM_WAKERS);
-        while self.curr > 0 {
-            self.curr -= 1;
-            // SAFETY: The first `curr` elements of `WakeList` are initialized, so by decrementing
-            // `curr`, we can take ownership of the last item.
-            let waker = unsafe { ptr::read(self.inner[self.curr].as_mut_ptr()) };
+        for i in 0..self.curr {
+            // SAFETY: The first `curr` elements are initialized
+            let waker = unsafe {
+                ptr::replace(self.inner.as_mut_ptr().add(i).cast::<Waker>(), noop_waker())
+            };
             waker.wake();
         }
     }