Add sleep based on spin to bevy_platform_support (#18633)

# Objective

- Fixes #18617

## Solution

- Added `thread::sleep` to `bevy_platform_support` using a spin-based
fallback.
- Fixed bug in `bevy_platform_support::time::Instant::elapsed`
(comparison was backwards)
- Switched `ScheduleRunnerPlugin` to use
`bevy_platform_support:🧵:sleep` on `std` and `no_std` platforms
(WASM + Browser excluded)

## Testing

- Ran reproduction code from @mockersf in linked issue and confirmed a
consistent 60 counts per `println!`.

---

## Notes

- I chose to add `bevy_platform_support:🧵:sleep` instead of
putting the fix in-line within `ScheduleRunnerPlugin` to keep the
separation of concerns clean. `sleep` is only used in one other location
in Bevy, `bevy_asset`, but I have decided to leave that as-is since
`bevy_asset` isn't `no_std` compatible anyway.
- The bug in `bevy_platform_support::time::Instant::elapsed` wasn't the
cause of this issue, but it did prevent this fix from working so I have
included the it in this PR.

crates/bevy_app/src/schedule_runner.rs

@@ -159,9 +159,8 @@ impl Plugin for ScheduleRunnerPlugin {
                         } else {
                             loop {
                                 match tick(&mut app, wait) {
-                                    Ok(Some(_delay)) => {
-                                        #[cfg(feature = "std")]
-                                        std::thread::sleep(_delay);
+                                    Ok(Some(delay)) => {
+                                        bevy_platform_support::thread::sleep(delay);
                                     }
                                     Ok(None) => continue,
                                     Err(exit) => return exit,

crates/bevy_platform_support/src/lib.rs

@@ -17,6 +17,7 @@ extern crate alloc;
 
 pub mod hash;
 pub mod sync;
+pub mod thread;
 pub mod time;
 
 #[cfg(feature = "alloc")]

crates/bevy_platform_support/src/thread.rs

@@ -0,0 +1,29 @@
+//! Provides `sleep` for all platforms.
+
+pub use thread::sleep;
+
+cfg_if::cfg_if! {
+    // TODO: use browser timeouts based on ScheduleRunnerPlugin::build
+    if #[cfg(feature = "std")] {
+        use std::thread;
+    } else {
+        mod fallback {
+            use core::{hint::spin_loop, time::Duration};
+
+            use crate::time::Instant;
+
+            /// Puts the current thread to sleep for at least the specified amount of time.
+            ///
+            /// As this is a `no_std` fallback implementation, this will spin the current thread.
+            pub fn sleep(dur: Duration) {
+                let start = Instant::now();
+
+                while start.elapsed() < dur {
+                    spin_loop()
+                }
+            }
+        }
+
+        use fallback as thread;
+    }
+}

crates/bevy_platform_support/src/time/fallback.rs

@@ -80,7 +80,7 @@ impl Instant {
     /// Returns the amount of time elapsed since this instant.
     #[must_use]
     pub fn elapsed(&self) -> Duration {
-        self.saturating_duration_since(Instant::now())
+        Instant::now().saturating_duration_since(*self)
     }
 
     /// Returns `Some(t)` where `t` is the time `self + duration` if `t` can be represented as