Improve waiting for messages on Windows

Previous version used [`SetTimer`] with `GetMessageW` for waiting.
The downside of UI timers like ones created by `SetTimer`,
is that they may be late by up to 15-16 ms.

To fix this behaviour, I added use of high resolution timers created by [`CreateWaitableTimerExW`] with the flag `CREATE_WAITABLE_TIMER_HIGH_RESOLUTION`.
In my previous experience, waiting on such timers have precision of roundly 0.5 ms which is the best available on Windows at the moment.
I use [`MsgWaitForMultipleObjectsEx`] to wait simultaneously for both timer and newly arriving events.

Unfortunately, high resolution timers are available only since Windows 10 1803. However:

1. Win 10 is already getting to the end of support, like all previous versions, so it is OK to rely on APIs introduced in it;
2. I use `dwMilliseconds` parameter of `MsgWaitForMultipleObjectsEx` as a fallback. It should perform not worse compared to waiting for events from `SetTimer`.

I also refactored code to remove event dispatching from function responsible for waiting for events. This provides more clear separations of concern and avoids unnecessary duplication of dispatching logic.

I have tested behaviour using a egui app with Vulkan rendering with `VK_PRESENT_MODE_IMMEDIATE_KHR`, and older version consistently have twice less FPS than requested (e.g. 30 FPS when limit is 60 and 60 FPS when limit is 120) while newer version works more correctly (almost always 60 FPS when limit is 60, and only 5-10 frames missing when FPS is set to 120 or more).

Fixes #1610

[`CreateWaitableTimerExW`]: https://learn.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-createwaitabletimerexw
[`MsgWaitForMultipleObjectsEx`]: https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-msgwaitformultipleobjectsex
[`SetTimer`]: https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-settimer

Cargo.toml

@@ -214,6 +214,7 @@ windows-sys = { version = "0.52.0", features = [
     "Win32_System_Com",
     "Win32_System_LibraryLoader",
     "Win32_System_Ole",
+    "Win32_Security",
     "Win32_System_SystemInformation",
     "Win32_System_SystemServices",
     "Win32_System_Threading",

src/changelog/unreleased.md

@@ -189,3 +189,4 @@ changelog entry.
 - On MacOS, package manifest definitions of `LSUIElement` will no longer be overridden with the
   default activation policy, unless explicitly provided during initialization.
 - On X11, key events forward to IME anyway, even when it's disabled.
+- On Windows, make `ControlFlow::WaitUntil` work more precisely using `CREATE_WAITABLE_TIMER_HIGH_RESOLUTION`.

src/platform_impl/windows/event_loop.rs

@@ -4,6 +4,7 @@ mod runner;
 
 use std::cell::Cell;
 use std::ffi::c_void;
+use std::os::windows::io::{AsRawHandle as _, FromRawHandle as _, OwnedHandle, RawHandle};
 use std::rc::Rc;
 use std::sync::atomic::{AtomicU32, Ordering};
 use std::sync::{Arc, Mutex, MutexGuard};
@@ -12,13 +13,18 @@ use std::{mem, panic, ptr};
 
 use runner::EventLoopRunner;
 use windows_sys::Win32::Devices::HumanInterfaceDevice::MOUSE_MOVE_RELATIVE;
-use windows_sys::Win32::Foundation::{HWND, LPARAM, LRESULT, POINT, RECT, WPARAM};
+use windows_sys::Win32::Foundation::{
+    GetLastError, FALSE, HANDLE, HWND, LPARAM, LRESULT, POINT, RECT, WAIT_FAILED, WPARAM,
+};
 use windows_sys::Win32::Graphics::Gdi::{
     GetMonitorInfoW, MonitorFromRect, MonitorFromWindow, RedrawWindow, ScreenToClient,
     ValidateRect, MONITORINFO, MONITOR_DEFAULTTONULL, RDW_INTERNALPAINT, SC_SCREENSAVE,
 };
 use windows_sys::Win32::System::Ole::RevokeDragDrop;
-use windows_sys::Win32::System::Threading::{GetCurrentThreadId, INFINITE};
+use windows_sys::Win32::System::Threading::{
+    CreateWaitableTimerExW, GetCurrentThreadId, SetWaitableTimer,
+    CREATE_WAITABLE_TIMER_HIGH_RESOLUTION, INFINITE, TIMER_ALL_ACCESS,
+};
 use windows_sys::Win32::UI::Controls::{HOVER_DEFAULT, WM_MOUSELEAVE};
 use windows_sys::Win32::UI::Input::Ime::{GCS_COMPSTR, GCS_RESULTSTR, ISC_SHOWUICOMPOSITIONWINDOW};
 use windows_sys::Win32::UI::Input::KeyboardAndMouse::{
@@ -34,14 +40,15 @@ use windows_sys::Win32::UI::Input::Touch::{
 use windows_sys::Win32::UI::Input::{RAWINPUT, RIM_TYPEKEYBOARD, RIM_TYPEMOUSE};
 use windows_sys::Win32::UI::WindowsAndMessaging::{
     CreateWindowExW, DefWindowProcW, DestroyWindow, DispatchMessageW, GetClientRect, GetCursorPos,
-    GetMenu, GetMessageW, KillTimer, LoadCursorW, PeekMessageW, PostMessageW, RegisterClassExW,
-    RegisterWindowMessageA, SetCursor, SetTimer, SetWindowPos, TranslateMessage, CREATESTRUCTW,
-    GWL_STYLE, GWL_USERDATA, HTCAPTION, HTCLIENT, MINMAXINFO, MNC_CLOSE, MSG, NCCALCSIZE_PARAMS,
-    PM_REMOVE, PT_TOUCH, RI_MOUSE_HWHEEL, RI_MOUSE_WHEEL, SC_MINIMIZE, SC_RESTORE, SIZE_MAXIMIZED,
-    SWP_NOACTIVATE, SWP_NOMOVE, SWP_NOSIZE, SWP_NOZORDER, WHEEL_DELTA, WINDOWPOS, WMSZ_BOTTOM,
-    WMSZ_BOTTOMLEFT, WMSZ_BOTTOMRIGHT, WMSZ_LEFT, WMSZ_RIGHT, WMSZ_TOP, WMSZ_TOPLEFT,
-    WMSZ_TOPRIGHT, WM_CAPTURECHANGED, WM_CLOSE, WM_CREATE, WM_DESTROY, WM_DPICHANGED,
-    WM_ENTERSIZEMOVE, WM_EXITSIZEMOVE, WM_GETMINMAXINFO, WM_IME_COMPOSITION, WM_IME_ENDCOMPOSITION,
+    GetMenu, LoadCursorW, MsgWaitForMultipleObjectsEx, PeekMessageW, PostMessageW,
+    RegisterClassExW, RegisterWindowMessageA, SetCursor, SetWindowPos, TranslateMessage,
+    CREATESTRUCTW, GWL_STYLE, GWL_USERDATA, HTCAPTION, HTCLIENT, MINMAXINFO, MNC_CLOSE, MSG,
+    MWMO_INPUTAVAILABLE, NCCALCSIZE_PARAMS, PM_REMOVE, PT_TOUCH, QS_ALLEVENTS, RI_MOUSE_HWHEEL,
+    RI_MOUSE_WHEEL, SC_MINIMIZE, SC_RESTORE, SIZE_MAXIMIZED, SWP_NOACTIVATE, SWP_NOMOVE,
+    SWP_NOSIZE, SWP_NOZORDER, WHEEL_DELTA, WINDOWPOS, WMSZ_BOTTOM, WMSZ_BOTTOMLEFT,
+    WMSZ_BOTTOMRIGHT, WMSZ_LEFT, WMSZ_RIGHT, WMSZ_TOP, WMSZ_TOPLEFT, WMSZ_TOPRIGHT,
+    WM_CAPTURECHANGED, WM_CLOSE, WM_CREATE, WM_DESTROY, WM_DPICHANGED, WM_ENTERSIZEMOVE,
+    WM_EXITSIZEMOVE, WM_GETMINMAXINFO, WM_IME_COMPOSITION, WM_IME_ENDCOMPOSITION,
     WM_IME_SETCONTEXT, WM_IME_STARTCOMPOSITION, WM_INPUT, WM_KEYDOWN, WM_KEYUP, WM_KILLFOCUS,
     WM_LBUTTONDOWN, WM_LBUTTONUP, WM_MBUTTONDOWN, WM_MBUTTONUP, WM_MENUCHAR, WM_MOUSEHWHEEL,
     WM_MOUSEMOVE, WM_MOUSEWHEEL, WM_NCACTIVATE, WM_NCCALCSIZE, WM_NCCREATE, WM_NCDESTROY,
@@ -127,6 +134,10 @@ pub(crate) enum ProcResult {
 pub struct EventLoop {
     window_target: ActiveEventLoop,
     msg_hook: Option<Box<dyn FnMut(*const c_void) -> bool + 'static>>,
+    // It is a timer used on timed waits.
+    // It is created lazily in case if we have `ControlFlow::WaitUntil`.
+    // Keep it as a field to avoid recreating it on every `ControlFlow::WaitUntil`.
+    high_resolution_timer: Option<OwnedHandle>,
 }
 
 pub(crate) struct PlatformSpecificEventLoopAttributes {
@@ -201,6 +212,7 @@ impl EventLoop {
         Ok(EventLoop {
             window_target: ActiveEventLoop { thread_id, thread_msg_target, runner_shared },
             msg_hook: attributes.msg_hook.take(),
+            high_resolution_timer: None,
         })
     }
 
@@ -245,17 +257,13 @@ impl EventLoop {
         }
 
         let exit_code = loop {
-            self.wait_and_dispatch_message(None);
-
-            if let Some(code) = self.exit_code() {
-                break code;
-            }
-
             self.dispatch_peeked_messages();
 
             if let Some(code) = self.exit_code() {
                 break code;
             }
+
+            self.wait_for_messages(None);
         };
 
         let runner = &self.window_target.runner_shared;
@@ -312,9 +320,8 @@ impl EventLoop {
             }
         }
 
-        self.wait_and_dispatch_message(timeout);
-
         if self.exit_code().is_none() {
+            self.wait_for_messages(timeout);
             self.dispatch_peeked_messages();
         }
 
@@ -343,107 +350,115 @@ impl EventLoop {
         status
     }
 
-    /// Wait for one message and dispatch it, optionally with a timeout
-    fn wait_and_dispatch_message(&mut self, timeout: Option<Duration>) {
-        fn get_msg_with_timeout(msg: &mut MSG, timeout: Option<Duration>) -> PumpStatus {
-            unsafe {
-                // A timeout of None means wait indefinitely (so we don't need to call SetTimer)
-                let timer_id = timeout.map(|timeout| SetTimer(0, 0, dur2timeout(timeout), None));
-                let get_status = GetMessageW(msg, 0, 0, 0);
-                if let Some(timer_id) = timer_id {
-                    KillTimer(0, timer_id);
-                }
-                // A return value of 0 implies `WM_QUIT`
-                if get_status == 0 {
-                    PumpStatus::Exit(0)
-                } else {
-                    PumpStatus::Continue
-                }
-            }
-        }
-
-        /// Fetch the next MSG either via PeekMessage or GetMessage depending on whether the
-        /// requested timeout is `ZERO` (and so we don't want to block)
-        ///
-        /// Returns `None` if no MSG was read, else a `Continue` or `Exit` status
-        fn wait_for_msg(msg: &mut MSG, timeout: Option<Duration>) -> Option<PumpStatus> {
-            if timeout == Some(Duration::ZERO) {
-                unsafe {
-                    if PeekMessageW(msg, 0, 0, 0, PM_REMOVE) != 0 {
-                        Some(PumpStatus::Continue)
-                    } else {
-                        None
-                    }
-                }
-            } else {
-                Some(get_msg_with_timeout(msg, timeout))
-            }
-        }
-
+    /// Waits until new event messages arrive to be peeked.
+    /// Doesn't peek messages itself.
+    ///
+    /// Parameter timeout is optional. This method would wait for the smaller timeout
+    /// between the argument and a timeout from control flow.
+    fn wait_for_messages(&mut self, timeout: Option<Duration>) {
         let runner = &self.window_target.runner_shared;
 
         // We aim to be consistent with the MacOS backend which has a RunLoop
         // observer that will dispatch AboutToWait when about to wait for
         // events, and NewEvents after the RunLoop wakes up.
         //
-        // We emulate similar behaviour by treating `GetMessage` as our wait
+        // We emulate similar behaviour by treating `MsgWaitForMultipleObjectsEx` as our wait
         // point and wake up point (when it returns) and we drain all other
         // pending messages via `PeekMessage` until we come back to "wait" via
-        // `GetMessage`
+        // `MsgWaitForMultipleObjectsEx`.
         //
         runner.prepare_wait();
 
-        let control_flow_timeout = match runner.control_flow() {
-            ControlFlow::Wait => None,
-            ControlFlow::Poll => Some(Duration::ZERO),
-            ControlFlow::WaitUntil(wait_deadline) => {
-                let start = Instant::now();
-                Some(wait_deadline.saturating_duration_since(start))
-            },
+        let timeout = {
+            let control_flow_timeout = match runner.control_flow() {
+                ControlFlow::Wait => None,
+                ControlFlow::Poll => Some(Duration::ZERO),
+                ControlFlow::WaitUntil(wait_deadline) => {
+                    let start = Instant::now();
+                    Some(wait_deadline.saturating_duration_since(start))
+                },
+            };
+            let timeout = min_timeout(timeout, control_flow_timeout);
+            if timeout == Some(Duration::ZERO) {
+                // Do not wait if we don't have time.
+                return;
+            }
+            // Now we decided to wait so need to do some clamping
+            // to avoid problems with overflow and calling WinAPI with invalid parameters.
+            timeout
+                .map(|t| t.min(FIFTY_DAYS))
+                // If timeout is less than minimally supported by Windows,
+                // increase it to that minimum. Who want less than microsecond delays anyway?
+                .map(|t| t.max(MIN_WAIT))
         };
-        let timeout = min_timeout(control_flow_timeout, timeout);
 
-        // # Safety
-        // The Windows API has no documented requirement for bitwise
-        // initializing a `MSG` struct (it can be uninitialized memory for the C
-        // API) and there's no API to construct or initialize a `MSG`. This
-        // is the simplest way avoid uninitialized memory in Rust
-        let mut msg = unsafe { mem::zeroed() };
-        let msg_status = wait_for_msg(&mut msg, timeout);
+        if timeout.is_some() && self.high_resolution_timer.is_none() {
+            self.high_resolution_timer = create_high_resolution_timer();
+        }
 
-        // Before we potentially exit, make sure to consistently emit an event for the wake up
-        runner.wakeup();
+        let high_resolution_timer: Option<RawHandle> =
+            self.high_resolution_timer.as_ref().map(OwnedHandle::as_raw_handle);
 
-        match msg_status {
-            None => {}, // No MSG to dispatch
-            Some(PumpStatus::Exit(code)) => {
-                runner.set_exit_code(code);
-            },
-            Some(PumpStatus::Continue) => {
-                unsafe {
-                    let handled = if let Some(callback) = self.msg_hook.as_deref_mut() {
-                        callback(&mut msg as *mut _ as *mut _)
-                    } else {
-                        false
-                    };
-                    if !handled {
-                        TranslateMessage(&msg);
-                        DispatchMessageW(&msg);
-                    }
-                }
+        let use_timer: bool;
+        if let (Some(handle), Some(timeout)) = (high_resolution_timer, timeout) {
+            let res = unsafe {
+                // Safety: handle can be Some only if we succeeded in creating high resolution
+                // timer. We properly clamped timeout so it can be used as argument
+                // to timer.
+                set_high_resolution_timer(handle, timeout)
+            };
+            if let Err(error_code) = res {
+                // We successfully got timer but failed to set it?
+                // Should be some bug in our code.
+                tracing::trace!("Failed to set high resolution timer: last error {}", error_code);
+                use_timer = false;
+            } else {
+                use_timer = true;
+            }
+        } else {
+            use_timer = false;
+        }
 
-                if let Err(payload) = runner.take_panic_error() {
-                    runner.reset_runner();
-                    panic::resume_unwind(payload);
-                }
-            },
+        unsafe {
+            // Either:
+            //  1. User wants to wait indefinely if timeout is not set.
+            //  2. We failed to get and set high resolution timer and we need something instead of
+            //     it.
+            let wait_duration_ms = timeout.map(dur2timeout).unwrap_or(INFINITE);
+
+            let mut raw_handles: [RawHandle; 1] = [ptr::null_mut()];
+            let (num_handles, handles_ptr) = if use_timer {
+                raw_handles[0] = high_resolution_timer.unwrap();
+                (1, raw_handles.as_ptr())
+            } else {
+                (0, ptr::null())
+            };
+
+            let result = MsgWaitForMultipleObjectsEx(
+                num_handles,
+                handles_ptr as *const _,
+                wait_duration_ms,
+                QS_ALLEVENTS,
+                MWMO_INPUTAVAILABLE,
+            );
+            if result == WAIT_FAILED {
+                // Well, nothing smart to do in such case.
+                // Treat it as spurious wake up.
+                tracing::warn!(
+                    "Failed to MsgWaitForMultipleObjectsEx: error code {}",
+                    GetLastError(),
+                );
+            }
         }
     }
 
     /// Dispatch all queued messages via `PeekMessageW`
     fn dispatch_peeked_messages(&mut self) {
         let runner = &self.window_target.runner_shared;
 
+        // Before we potentially exit, make sure to consistently emit an event for the wake up
+        runner.wakeup();
+
         // We generally want to continue dispatching all pending messages
         // but we also allow dispatching to be interrupted as a means to
         // ensure the `pump_events` won't indefinitely block an external
@@ -456,7 +471,7 @@ impl EventLoop {
         // initializing a `MSG` struct (it can be uninitialized memory for the C
         // API) and there's no API to construct or initialize a `MSG`. This
         // is the simplest way avoid uninitialized memory in Rust
-        let mut msg = unsafe { mem::zeroed() };
+        let mut msg: MSG = unsafe { mem::zeroed() };
 
         loop {
             unsafe {
@@ -668,6 +683,58 @@ fn dur2timeout(dur: Duration) -> u32 {
         .unwrap_or(INFINITE)
 }
 
+/// Set upper limit for waiting time to avoid overflows.
+/// I chose 50 days as a limit because it is used in dur2timeout.
+const FIFTY_DAYS: Duration = Duration::from_secs(50_u64 * 24 * 60 * 60);
+/// Waitable timers use 100 ns intervals to indicate due time.
+/// <https://learn.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-setwaitabletimer#parameters>
+/// And there is no point waiting using other ways for such small timings
+/// because they are even less precise (can overshoot by few ms).
+const MIN_WAIT: Duration = Duration::from_nanos(100);
+
+fn create_high_resolution_timer() -> Option<OwnedHandle> {
+    unsafe {
+        let handle: HANDLE = CreateWaitableTimerExW(
+            ptr::null(),
+            ptr::null(),
+            CREATE_WAITABLE_TIMER_HIGH_RESOLUTION,
+            TIMER_ALL_ACCESS,
+        );
+        // CREATE_WAITABLE_TIMER_HIGH_RESOLUTION is supported only after
+        // Win10 1803 but it is already default option for rustc
+        // (std uses it to implement `std::thread::sleep`).
+        if handle == 0 {
+            None
+        } else {
+            Some(OwnedHandle::from_raw_handle(handle as *mut c_void))
+        }
+    }
+}
+
+/// This function should not return error if parameters valid
+/// but there is no guarantee about that at MSDN docs
+/// so we return result of GetLastError if fail.
+/// ## Safety
+/// timer must be a valid timer handle created by create_high_resolution_timer.
+/// timeout divided by 100 nanoseconds must be more than 0 and less than i64::MAX.
+unsafe fn set_high_resolution_timer(timer: RawHandle, timeout: Duration) -> Result<(), u32> {
+    const INTERVAL_NS: u32 = MIN_WAIT.subsec_nanos();
+    const INTERVALS_IN_SEC: u64 = (Duration::from_secs(1).as_nanos() / INTERVAL_NS as u128) as u64;
+    let intervals_to_wait: u64 =
+        timeout.as_secs() * INTERVALS_IN_SEC + u64::from(timeout.subsec_nanos() / INTERVAL_NS);
+    debug_assert!(intervals_to_wait < i64::MAX as u64, "Must be called with smaller duration",);
+    // Use negative time to indicate relative time.
+    let due_time: i64 = -(intervals_to_wait as i64);
+    unsafe {
+        let set_result = SetWaitableTimer(timer as HANDLE, &due_time, 0, None, ptr::null(), FALSE);
+        if set_result != FALSE {
+            Ok(())
+        } else {
+            Err(GetLastError())
+        }
+    }
+}
+
 impl Drop for EventLoop {
     fn drop(&mut self) {
         unsafe {