gpu_handle_types/

sync.rs

// SPDX-License-Identifier: MIT OR Apache-2.0

use core::future::Future;
use core::pin::Pin;
use std::any::Any;
use std::ffi::c_void;
use std::sync::Arc;
use std::time::Duration;

#[cfg(feature = "wgpu")]
use crate::SliceOutcome;
use crate::{BackendKind, Error, GlBackend};

/// Boxed dyn-future return shape used by `SyncWaiter::wait_async` so the
/// trait stays object-safe through `Arc<dyn SyncWaiter>`. Native
/// `async fn` on the trait would be RPITIT and dyn-incompatible by
/// default; callers dispatch through `Arc<dyn SyncWaiter>`, so boxing the
/// future is mandatory.
// `Send` off wasm; dropped on wasm, where the `SyncWaiter`
// the future borrows is thread-affine (`&dyn SyncWaiter` is not `Send`
// once the waiter is `!Sync`). An explicit `+ Send` on a `dyn Future`
// cannot be spelled with a non-auto marker trait, so the alias is
// cfg-split directly.
#[cfg(not(target_family = "wasm"))]
pub type BoxFuture<'a, T> = Pin<Box<dyn Future<Output = T> + Send + 'a>>;
#[cfg(target_family = "wasm")]
pub type BoxFuture<'a, T> = Pin<Box<dyn Future<Output = T> + 'a>>;

/// Default wait budget for `SyncPoint::wait()` and
/// `SyncPoint::wait_blocking()` — 10 seconds.
pub const DEFAULT_WAIT_TIMEOUT: Duration = Duration::from_secs(10);

/// A far-future but always-representable wait cap. `Instant + Duration` overflows
/// the platform's representable range only for a `timeout` so large it is
/// effectively infinite (near `Duration::MAX` — an `Instant` spans hundreds of
/// billions of years), and `Instant` has no `saturating_add`. Rather than let such
/// a `timeout` silently drop its deadline — turning a bounded wait into a hang —
/// [`wait_deadline`] clamps to `now +` this. It sits far beyond any real GPU wait
/// (budgets are seconds), so a legitimately huge finite wait still resolves against
/// it as intended.
const SATURATED_WAIT_CAP: Duration = Duration::from_secs(60 * 60 * 24 * 365); // ~1 year

/// The absolute deadline a `wait(timeout)` poll loop should honour, or `None` for
/// the [`SyncWaiter::wait`] contract's `Duration::MAX` "wait forever" sentinel.
///
/// - `timeout == Duration::MAX` → `None`: the caller asked to wait forever, so the
///   loop runs with no deadline (matching every backend's explicit `Duration::MAX`
///   handling, which maps to the native "infinite": `vkWaitSemaphores(UINT64_MAX)`,
///   `WaitForSingleObject(INFINITE)`, `cuEventSynchronize`, …).
/// - any other `timeout` → `Some(now + timeout)`, **saturated** to a bounded,
///   always-representable [`Instant`](std::time::Instant) on `Instant + Duration`
///   overflow — clamped to `now + 1 year`, far beyond any real GPU wait budget.
///
/// This keeps the two meanings of "no deadline" distinct: a returned `None`
/// encodes ONLY the explicit forever sentinel, never an accidental `checked_add`
/// overflow — which would otherwise silently convert a bounded wait into an
/// unbounded hang.
pub fn wait_deadline(timeout: Duration) -> Option<std::time::Instant> {
    if timeout == Duration::MAX {
        return None;
    }
    let now = std::time::Instant::now();
    Some(now.checked_add(timeout).unwrap_or_else(|| now.checked_add(SATURATED_WAIT_CAP).unwrap_or(now)))
}

/// Backend-specific wait dispatch + lifetime anchor for a [`SyncPoint`].
///
/// Every non-trivial `SyncPoint` variant carries an `Arc<dyn SyncWaiter>`
/// that:
///
/// 1. **Owns the underlying primitive's lifetime** — the timeline
///    semaphore, D3D12 fence, `MTLSharedEvent`, `cl_event`, `GLsync`,
///    etc. Cloning the `SyncPoint` clones the `Arc`; the primitive lives
///    as long as any clone outlives. This rules out the "consumer holds
///    a `*mut c_void` after the producer dropped" footgun.
///
/// 2. **Provides the wait implementation** — `SyncPoint::wait` /
///    `is_signaled` / `backend` dispatch through this trait, with no
///    global registries and no `OnceLock<fn>` runtime callbacks.
///
/// Implementations live in the producer crate (typically an interop
/// layer's per-backend module) and are constructed at the same site that
/// mints the `SyncPoint`.
// `MaybeSendSync` = `Send + Sync` off wasm; empty on wasm,
// where a waiter can hold a thread-affine `Arc<wgpu::Device>` /
// `web_sys` handle. See [`crate::MaybeSendSync`].
pub trait SyncWaiter: crate::MaybeSendSync + 'static {
    /// Block until the GPU work this sync point represents has
    /// completed, or until `timeout` elapses.
    ///
    /// `Duration::MAX` means "wait forever" — implementations should
    /// translate this to the platform's native "infinite" sentinel
    /// (`UINT64_MAX` for `vkWaitSemaphores`, `INFINITE` for
    /// `WaitForSingleObject`, etc.).
    ///
    /// # Timeout granularity
    ///
    /// Native wait APIs vary in granularity, and implementations
    /// preserve the caller's intent at the expense of *requested*
    /// (not realised) latency on sub-API-tick timeouts:
    ///
    /// - **Metal** (`MTLSharedEvent::waitUntilSignaledValue:timeoutMS:`)
    ///   accepts millisecond-granular `u64`. Sub-millisecond positive
    ///   timeouts (e.g. `Duration::from_micros(100)`) round **up**
    ///   to 1 ms — truncation to 0 would silently behave as
    ///   "do not wait". A high-frequency progress poll using
    ///   `Duration::from_micros(100)` therefore observes ~10× the
    ///   requested latency on a not-yet-signaled event. For
    ///   non-blocking probes use [`SyncWaiter::is_signaled`] (which
    ///   passes `Duration::ZERO` and short-circuits on the
    ///   `signaledValue() >= value` accessor — driver-side, no
    ///   timer).
    /// - **Win32** (`WaitForSingleObject`) is also millisecond-
    ///   granular; the same round-up applies.
    /// - **Vulkan** (`vkWaitSemaphores`), **D3D12 fence**, **CUDA
    ///   external semaphore**, **OpenCL** are nanosecond-granular and
    ///   honour the caller's `Duration` exactly.
    ///
    /// Callers that need *both* a deterministic poll cadence under
    /// 1 ms *and* a real wait fallback should compose the two —
    /// e.g. `is_signaled` in a hot loop with their own `Instant`
    /// budget, then a single coarse `wait(Duration::from_millis(N))`
    /// when the budget is exhausted.
    fn wait(&self, timeout: Duration) -> Result<(), Error>;

    /// Async sibling of [`wait`](Self::wait). Returns a boxed future so
    /// the trait stays object-safe through `Arc<dyn SyncWaiter>` — native
    /// async-fn-in-trait is RPITIT and dyn-incompatible by default; callers
    /// dispatch through `Arc<dyn SyncWaiter>` everywhere, so the explicit
    /// `Pin<Box<...>>` is mandatory.
    ///
    /// # Default implementation
    ///
    /// Backends without a hand-tuned implementation get a two-phase
    /// hybrid: a short cooperative-yield spin (catches sub-ms waits
    /// without burning a thread) followed by an iteration-bounded
    /// `is_signaled` probe loop. Per-backend impls SHOULD override with
    /// their native blocking-with-timeout primitive on a dedicated
    /// waiter thread for waits the spin loop didn't catch.
    ///
    /// The default implementation is correct (resolves when signalled,
    /// returns `Error::Timeout` after `timeout`, never blocks the
    /// executor) but not optimal: it busy-polls inside the spin loop
    /// and falls back to a `Duration::ZERO` probe loop after that. For
    /// production paths, override with a backend-specific impl.
    fn wait_async<'a>(&'a self, timeout: Duration) -> BoxFuture<'a, Result<(), Error>> {
        Box::pin(async move {
            // Fast-path yield-spin — bounded cooperative-yield probe. Caps iteration
            // count rather than wall-clock time so executors that re-
            // poll immediately on `wake_by_ref` (pollster) don't burn
            // CPU here.
            const SPIN_ITERATIONS: usize = 64;
            // `None` iff `timeout == Duration::MAX` (the wait-forever sentinel); every
            // finite timeout gets a bounded, always-representable deadline — an
            // `Instant + Duration` overflow cannot silently drop it and hang.
            let deadline = wait_deadline(timeout);
            for _ in 0..SPIN_ITERATIONS {
                match self.is_signaled() {
                    Ok(true) => return Ok(()),
                    Ok(false) => {}
                    Err(e) => return Err(e),
                }
                if let Some(d) = deadline
                    && std::time::Instant::now() >= d
                {
                    return Err(Error::Timeout);
                }
                yield_once().await;
            }
            // Waiter-thread fallback — default impl probes `is_signaled` in a
            // yielding loop. Per-backend impls SHOULD override to hand
            // off to a dedicated waiter thread with the native
            // blocking-with-timeout primitive.
            loop {
                match self.is_signaled() {
                    Ok(true) => return Ok(()),
                    Ok(false) => {}
                    Err(e) => return Err(e),
                }
                if let Some(d) = deadline
                    && std::time::Instant::now() >= d
                {
                    return Err(Error::Timeout);
                }
                yield_once().await;
            }
        })
    }

    /// Non-blocking probe.
    ///
    /// - `Ok(true)` — the sync point has been reached.
    /// - `Ok(false)` — work is still in flight (the underlying poll
    ///   timed out at zero).
    /// - `Err(_)` — driver-level failure (device lost / TDR /
    ///   `wgpu::PollError::WrongSubmissionIndex` / equivalent on other
    ///   backends). Callers polling in a loop **must** break on `Err` —
    ///   folding errors into `Ok(false)` would loop forever on a TDR'd
    ///   device.
    ///
    /// Default implementation calls `wait(Duration::ZERO)` and maps
    /// `Err(Error::Timeout)` → `Ok(false)`; every other `Err` is
    /// propagated. Backends override when they have a more direct
    /// "is this primitive currently signaled" probe (e.g.
    /// `vkGetSemaphoreCounterValue`, `ID3D12Fence::GetCompletedValue`)
    /// that distinguishes "not signaled" from real errors without
    /// going through the wait path.
    fn is_signaled(&self) -> Result<bool, Error> {
        match self.wait(Duration::ZERO) {
            Ok(()) => Ok(true),
            Err(Error::Timeout) => Ok(false),
            Err(other) => Err(other),
        }
    }

    /// Backend identity for routing decisions on the consumer side.
    fn backend(&self) -> BackendKind;

    /// Downcast hook for cross-API bridges that need access to the
    /// concrete waiter type — e.g. a Vulkan→CUDA bridge that wants to
    /// pull the `VkDevice` out of the `VulkanWaiter` to issue its own
    /// `cuImportExternalSemaphore`. Most callers use the per-variant
    /// raw fields on `SyncPoint` directly and never need this.
    fn as_any(&self) -> &dyn Any;

    /// [`CudaEventWaiter`] view of this waiter, when the concrete type
    /// implements it. Default `None`.
    ///
    /// This is the trait-object route to the CUDA-only
    /// [`CudaEventWaiter::wait_on_foreign_stream`] extension: `Any` can
    /// only downcast to *concrete* types, which consumers in other
    /// crates cannot name — so producers whose waiter implements
    /// [`CudaEventWaiter`] override this with `Some(self)` and
    /// consumers (e.g. a CUDA import path gating its private copy stream
    /// on a producer's `SyncPoint::CudaEvent`) reach the extension method
    /// without knowing the concrete type.
    fn as_cuda_event_waiter(&self) -> Option<&dyn CudaEventWaiter> {
        None
    }

    /// Rebuild this waiter bound to `value` instead of the value it was
    /// constructed with, reusing the same underlying primitive (and the
    /// same lifetime anchors).
    ///
    /// # Why this exists
    ///
    /// [`wait`](Self::wait) / [`is_signaled`](Self::is_signaled) take no
    /// value argument — the waiter *embeds* the value it resolves at,
    /// captured when it was built. So a [`SyncPoint`] whose `value`
    /// field was substituted while its `waiter` was cloned verbatim has
    /// two surfaces that disagree: the GPU side (a consumer reading
    /// `SyncPoint::*.value` to stage a queue wait) waits the new value,
    /// while the CPU side silently resolves at the old one — reporting
    /// "already signalled" for a signal the producer has not emitted.
    ///
    /// Any code that mints a `SyncPoint` at a value other than the one a
    /// template was built with MUST route the waiter through this method
    /// rather than cloning it, so the two surfaces cannot diverge.
    ///
    /// # Contract
    ///
    /// - `Some(w)` — `w` waits `value` on the same primitive this waiter
    ///   waits on, and holds the same keep-alive chain. Implementations
    ///   MUST NOT carry over state that is only valid for the original
    ///   value (e.g. a captured submission index that retires at it).
    /// - `None` (the default) — the underlying primitive carries no
    ///   value, so there is nothing to rebind: binary semaphores,
    ///   fences, `GLsync`, `cl_semaphore_khr`. A `None` here is not a
    ///   failure; it means the CPU surface has no value to disagree
    ///   about.
    fn rebind_to_value(&self, value: u64) -> Option<Arc<dyn SyncWaiter>> {
        let _ = value;
        None
    }
}

/// CUDA-event-specific extension trait for the
/// [`SyncPoint::CudaEvent`] variant. Supertrait of [`SyncWaiter`] so a
/// `CudaEventWaiter` always satisfies the generic [`SyncPoint::wait`]
/// dispatch path; adds the CUDA-only `wait_on_foreign_stream` extension
/// method that cross-API bridges downcast to via
/// [`SyncWaiter::as_any`].
///
/// # Drop discipline
///
/// Implementations MUST push the event's owning `CUcontext` before
/// calling `cuEventDestroy_v2`, and MUST pop iff the push succeeded
/// (pop-without-push would silently consume whatever the calling
/// thread had on top of its context stack). `cuEventDestroy_v2`
/// requires the owning context to still exist; the push is defensive
/// against driver-internal cleanup paths that probe `cuCtxGetCurrent`
/// and emit confusing diagnostics when no context is current.
///
/// # Cross-context wait
///
/// `cuEventRecord` is strict-same-context — a `CUevent` cannot be
/// recorded against a stream from a different `CUcontext`. But
/// `cuStreamWaitEvent` is cross-context (NVIDIA Driver API contract).
/// `wait_on_foreign_stream` is the cross-context wait entry point:
/// the consumer's CUDA stream may live in a different `CUcontext`
/// than the event, and the waiter issues `cuStreamWaitEvent` so the
/// foreign stream gates on this event's completion without a CPU
/// bounce.
///
/// A CUDA-driver-backed waiter (e.g. one built on the `mini-cuda`
/// driver loader) implements this trait concretely; this trait only
/// defines the contract.
pub trait CudaEventWaiter: SyncWaiter {
    /// Issue `cuStreamWaitEvent(foreign_stream, self.event, 0)` so
    /// `foreign_stream` gates on this event's completion. The stream
    /// may belong to a different `CUcontext` than the event's owning
    /// context — `cuStreamWaitEvent` is cross-context per the NVIDIA
    /// Driver API.
    ///
    /// `foreign_stream` is a raw `CUstream` pointer (the null pointer
    /// resolves to the calling thread's default stream).
    ///
    /// Returns [`Error::NotSupported`] if the CUDA driver loader is
    /// unavailable or the wait dispatch returns a driver error; the
    /// concrete error wording is implementation-defined.
    fn wait_on_foreign_stream(&self, foreign_stream: *mut c_void) -> Result<(), Error>;
}

/// Discriminator for [`SyncPoint::Vulkan`]'s `VkSemaphore` flavour.
///
/// `VkSemaphore` ships in two shapes — binary (one-shot, signal-and-
/// reset) and timeline (monotonic 64-bit payload, multi-waiter,
/// concurrent-signal-safe). A consumer staging a wait must route
/// through `VkTimelineSemaphoreSubmitInfo` for timeline semaphores and
/// must NOT for binary semaphores — passing the wrong shape risks
/// validation errors, driver-quirk-dependent wrong waits, and (on
/// already-consumed binary signals) hard deadlock at submit time.
///
/// The Vulkan API has no public probe that safely distinguishes the
/// two (`vkGetSemaphoreCounterValue` is undefined on binaries per
/// `VUID-vkGetSemaphoreCounterValue-semaphore-03255`), so the producer
/// must declare intent at mint time.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
#[non_exhaustive]
pub enum VulkanSemaphoreKind {
    /// `vk::SemaphoreType::TIMELINE`. `value` is a payload coordinate;
    /// any number of waiters may target the same value, and signals
    /// with strictly-increasing payloads can be queued concurrently.
    Timeline,
    /// `vk::SemaphoreType::BINARY`. Single-shot — exactly one signal
    /// must pair with exactly one wait. The `value` field is ignored
    /// (set to `0` by convention). Producers carrying binaries here
    /// MUST guarantee the wait is the only consumer of the signal.
    Binary,
}

/// Shared, mutate-once slot the producer fills in after the caller
/// submits the encoded work. Used by [`SyncPoint::DeferredWgpu`].
///
/// The slot is initialised to `None` at mint time and transitions to
/// `Some(idx)` exactly once via [`Self::set`]; subsequent calls are
/// rejected with [`Error::InvalidArgument`] (single-shot semantics).
#[cfg(feature = "wgpu")]
#[derive(Debug, Default)]
pub struct DeferredWgpuSlot {
    cell: std::sync::OnceLock<wgpu::SubmissionIndex>,
}

#[cfg(feature = "wgpu")]
impl DeferredWgpuSlot {
    pub fn new() -> Self {
        Self::default()
    }

    /// Commit the post-submit `SubmissionIndex`. Single-shot — a second
    /// call returns [`Error::InvalidArgument`] without overwriting the
    /// committed value.
    pub fn set(&self, idx: wgpu::SubmissionIndex) -> Result<(), Error> {
        self.cell.set(idx).map_err(|_| Error::InvalidArgument("DeferredWgpuSlot already committed".into()))
    }

    /// Return the committed `SubmissionIndex`, or `None` if the producer
    /// has not yet submitted.
    pub fn get(&self) -> Option<&wgpu::SubmissionIndex> {
        self.cell.get()
    }
}

// Defensive — the `OnceLock<wgpu::SubmissionIndex>` design relies on
// `SubmissionIndex` being `Clone + Send + Sync`. If a wgpu upgrade
// silently dropped any of these bounds, the OnceLock-storing
// `DeferredWgpuSlot` would surface a confusing trait-bound error far
// from the cause; this assertion fails compilation here, with a clear
// message, instead.
#[cfg(feature = "wgpu")]
static_assertions::assert_impl_all!(wgpu::SubmissionIndex: Clone, Send, Sync);

/// GPU sync primitive. Produced by every submit-side path; consumed by
/// anything that needs to serialise on a prior GPU submission.
///
/// Cross-backend bridges are deliberately absent from this leaf crate —
/// those belong in an interop layer (such as `wgpu-interop`), where the
/// target-device context is available.
///
/// # Lifetime
///
/// The per-variant `waiter: Arc<dyn SyncWaiter>` field anchors the
/// lifetime of any raw primitive the variant exposes (`*mut c_void`
/// fence pointers, `u64` semaphore handles, `GLsync` opaques, etc.).
/// Cloning a `SyncPoint` is `Arc::clone` on the waiter — cheap, no
/// driver round-trip. The raw fields are guaranteed to remain valid
/// for as long as any clone is alive.
#[derive(Clone)]
#[non_exhaustive]
pub enum SyncPoint {
    /// Vulkan semaphore. `device` is the `VkDevice` the semaphore was
    /// created on, exposed for bridges that need to re-import it. The
    /// `waiter` owns an `Arc<TimelineSemaphore>` (or equivalent) that
    /// anchors the `VkSemaphore`'s lifetime.
    ///
    /// `kind` distinguishes timeline vs binary semaphores —
    /// see [`VulkanSemaphoreKind`]. `value` is the timeline payload
    /// coordinate when `kind == Timeline`; for `kind == Binary` the
    /// field is meaningless (set to `0` by convention) and consumers
    /// route through wgpu-hal's binary-wait path
    /// (`add_wait_semaphore(_, None, _)`).
    Vulkan { semaphore: u64, kind: VulkanSemaphoreKind, value: u64, device: *mut c_void, waiter: Arc<dyn SyncWaiter> },
    /// D3D12 fence sync. `fence` is an `ID3D12Fence*` (COM pointer);
    /// `value` is the monotonic signal value the consumer waits on
    /// (`fence->GetCompletedValue() >= value`). The `waiter` owns the
    /// `Arc<ExportFence>` (or equivalent) that holds the COM ref.
    ///
    /// `value` is monotonically signaled by the producer. Bridges that
    /// reuse a single shared fence across calls serialise the
    /// (mint + `Signal`) pair under a value-lock so concurrent bridges
    /// always sequence in monotonic order.
    /// A consumer waiting on `value=N` is guaranteed to pass once a
    /// later `value=M >= N` has been signaled, even if the producer's
    /// counter has since advanced past `M`.
    D3D12 { fence: *mut c_void, value: u64, waiter: Arc<dyn SyncWaiter> },
    /// D3D11 keyed-mutex sync. `keyed_mutex` is an `IDXGIKeyedMutex*`;
    /// `key` is the integer key the producer released to. The
    /// `waiter` owns the `Arc<...>` that anchors the COM ref.
    D3D11 { keyed_mutex: *mut c_void, key: u64, waiter: Arc<dyn SyncWaiter> },
    /// CUDA external semaphore. `event` is a `CUexternalSemaphore`;
    /// `value` is `Some(v)` for timeline waits — including the products
    /// of Vulkan→CUDA / D3D12→CUDA bridges, which carry the producer's
    /// timeline value so the consumer can enqueue
    /// `cuWaitExternalSemaphoresAsync` on whichever CUDA stream their
    /// work runs on. `None` is reserved for setup-metadata SyncPoints
    /// where the consumer mints the actual signal value at emission time
    /// and doesn't need a resolvable wait at construction.
    Cuda { event: *mut c_void, value: Option<u64>, waiter: Arc<dyn SyncWaiter> },
    /// Intra-CUDA event. `event` is a `CUevent` recorded on the
    /// producing stream via `cuEventRecord`. Consumers wait via
    /// `cuStreamWaitEvent` (chained CUDA streams — cross-context
    /// supported per NVIDIA Driver API) or `cuEventSynchronize`
    /// (blocking CPU wait).
    ///
    /// Distinct from [`Self::Cuda`] — that variant carries a
    /// `CUexternalSemaphore` waited via `cuWaitExternalSemaphoresAsync`.
    /// `CUevent` and `CUexternalSemaphore` are different opaque types
    /// in the CUDA Driver API; passing a `CUevent` into the external-
    /// semaphore wait path returns `CUDA_ERROR_INVALID_HANDLE`.
    ///
    /// Cross-API consumers cannot wait on a `CUevent` directly; they
    /// need a `CUexternalSemaphore` (the [`Self::Cuda`] shape); an
    /// interop layer performs that conversion when the chain spans a
    /// backend boundary.
    ///
    /// `context` is the event's **owning** `CUcontext` — bound at
    /// `cuEventCreate` time per the NVIDIA Driver API ("creates an event
    /// for the current context"). The owning context must still exist
    /// at destroy time or `cuEventDestroy_v2` returns
    /// `CUDA_ERROR_INVALID_CONTEXT`. There is no `cuEventGetCtx` API —
    /// the metadata must travel with the event.
    ///
    /// `device` is the CUDA ordinal the owning context was created on
    /// (derivable from `context` via `cuCtxGetDevice` but cheap to
    /// carry and matches the device-keyed bridge shape used by
    /// [`Self::Cuda`]).
    ///
    /// `waiter` implements [`CudaEventWaiter`] — a supertrait of
    /// [`SyncWaiter`] that adds the CUDA-specific
    /// `wait_on_foreign_stream` extension method and the push-owning-
    /// context-before-destroy Drop discipline. The leaf crate stores
    /// the upcasted `Arc<dyn SyncWaiter>` so the generic [`Self::waiter`]
    /// dispatch path is uniform with every other variant; cross-API
    /// bridges that need the CUDA extension methods downcast through
    /// [`SyncWaiter::as_any`].
    CudaEvent { event: *mut c_void, context: *mut c_void, device: i32, waiter: Arc<dyn SyncWaiter> },
    /// OpenCL sync. `event` is either a `cl_event` (binary,
    /// signaled-on-completion; `value` is `None`) or a
    /// `cl_semaphore_khr` aliasing a timeline (`value` is the
    /// caller-signaled timeline value). The `waiter` owns the
    /// `Arc<...>` that holds the producing context's lifetime.
    OpenCl { event: *mut c_void, value: Option<u64>, waiter: Arc<dyn SyncWaiter> },
    /// Metal shared event. `event` is an `MTLSharedEvent*`; `value` is
    /// the signal value the consumer waits on. The `waiter` owns the
    /// `Arc<MTLSharedEvent>` keeping the event alive.
    Metal { event: *mut c_void, value: u64, waiter: Arc<dyn SyncWaiter> },
    /// OpenGL semaphore (`GLuint` produced by `glGenSemaphoresEXT`,
    /// imported from a cross-API OS handle via
    /// `glImportSemaphoreFdEXT` / `glImportSemaphoreWin32HandleEXT`).
    ///
    /// `value` is `Some(_)` for timeline-equivalent semaphores
    /// (D3D12_FENCE_EXT handle types); `None` for plain binary
    /// (OPAQUE_FD / OPAQUE_WIN32) — the GL driver latches the D3D12
    /// fence value at wait/signal time via
    /// `glSemaphoreParameterui64vEXT(sem, GL_D3D12_FENCE_VALUE_EXT, &v)`.
    ///
    /// `context` is the caller's `EGLContext` / `HGLRC` / `CGLContextObj`
    /// this semaphore was created against. Waits must be dispatched on
    /// a thread where the same context is current.
    OpenGL { semaphore: u32, value: Option<u64>, context: *mut c_void, waiter: Arc<dyn SyncWaiter> },
    /// GL fence sync. `glsync` is a `GLsync` opaque pointer produced
    /// by `glFenceSync(GL_SYNC_GPU_COMMANDS_COMPLETE, 0)`.
    ///
    /// Distinct from [`Self::OpenGL`] — that one carries an *imported*
    /// GL semaphore (`GLuint`); this one is the raw GL-native fence.
    ///
    /// `backend` records the [`GlBackend`] flavour of the GL context that
    /// produced this `GLsync`. Cross-API consumers (an OpenGL→X bridge)
    /// route through it to pick the matching loader (`wglGetProcAddress`
    /// vs `eglGetProcAddress`) when resolving `glClientWaitSync` for the
    /// CPU-fallback path.
    /// Without it the consumer would hard-code `Desktop`, which silently
    /// resolves the wrong proc-address on EGL / ANGLE / Android hosts and
    /// stalls forever on a never-signalled handle.
    OpenGLSync { glsync: *mut c_void, backend: GlBackend, waiter: Arc<dyn SyncWaiter> },
    /// `wgpu::Queue::submit` return value. Wait dispatches through the
    /// `waiter`, which holds an `Arc<wgpu::Device>` and calls
    /// `device.poll(PollType::Wait { submission_index: Some(..), .. })`.
    #[cfg(feature = "wgpu")]
    Wgpu { submission_index: wgpu::SubmissionIndex, waiter: Arc<dyn SyncWaiter> },
    /// `wgpu::Queue::submit` index *not yet known* at SyncPoint mint
    /// time. Used where the caller owns the encoder and submits after the
    /// producer has returned; `slot.set(idx)` is called once after
    /// `queue.submit(...)`, and until then the waiter falls back to
    /// `device.poll(PollType::Wait { submission_index: None, .. })`
    /// (full device drain). After commit, waits resolve precisely on
    /// the recorded `SubmissionIndex`.
    #[cfg(feature = "wgpu")]
    DeferredWgpu { slot: Arc<DeferredWgpuSlot>, device: Arc<wgpu::Device>, waiter: Arc<dyn SyncWaiter> },
    /// CPU-only work — no GPU sync needed. `wait` returns `Ok(())`
    /// immediately.
    Cpu,
    /// No-op sync — used for pipelines where the consumer side handles
    /// ordering through a separate channel (e.g. SurfaceControl
    /// transactions). Behaves identically to `Cpu` for waits.
    Noop,
}

// SAFETY: the `*mut c_void` / `*mut wgpu...` raw fields are immutable
// after construction and dereferenced only by the per-variant `waiter`,
// whose `SyncWaiter: Send + Sync` bound asserts platform-correct sharing
// for whatever the pointer references. Per-variant doc-comments specify
// the additional caller-asserted preconditions (e.g. D3D11 requires the
// device's multithread-protect flag).
unsafe impl Send for SyncPoint {}
unsafe impl Sync for SyncPoint {}

impl SyncPoint {
    /// Async wait. Default 10-second timeout.
    ///
    /// Async is the default. Use [`wait_blocking`]
    /// when the caller is on a synchronous code path (sync-loop
    /// renderers, test harnesses, JNI dispatcher Drop paths).
    ///
    /// [`wait_blocking`]: Self::wait_blocking
    pub async fn wait(&self) -> Result<(), Error> {
        self.wait_with_timeout_async(DEFAULT_WAIT_TIMEOUT).await
    }

    /// Async wait with explicit timeout.
    pub async fn wait_with_timeout_async(&self, timeout: Duration) -> Result<(), Error> {
        // Binary Vulkan CPU-wait rejection.
        if let Some(err) = self.binary_vk_cpu_wait_error() {
            return Err(err);
        }
        match self.waiter() {
            Some(w) => w.wait_async(timeout).await,
            None => Ok(()),
        }
    }

    /// Synchronous wait with the default 10-second timeout.
    pub fn wait_blocking(&self) -> Result<(), Error> {
        self.wait_with_timeout(DEFAULT_WAIT_TIMEOUT)
    }

    /// Synchronous wait with an explicit timeout.
    ///
    /// Timeout granularity is backend-dependent — see
    /// [`SyncWaiter::wait`] for the per-backend rounding contract.
    /// In particular, Metal and Win32 are millisecond-granular and
    /// round sub-ms positive timeouts up to 1 ms; Vulkan / D3D12 /
    /// CUDA / OpenCL are nanosecond-granular. For sub-ms polls use
    /// [`Self::is_signaled`] in a loop with your own `Instant` budget.
    pub fn wait_with_timeout(&self, timeout: Duration) -> Result<(), Error> {
        // Binary Vulkan CPU-wait rejection — `vkWaitSemaphores` is
        // undefined on binary semaphores, and the Khronos spec gives
        // binary semaphores no CPU-side wait entry point. Producers
        // that need CPU-waitable binary semaphores MUST attach a
        // fence-equivalent fallback (a `wgpu::SubmissionIndex` from the
        // same submit). When the fallback is absent, the backend's waiter
        // fails both `wait_blocking` and the async `wait()` with
        // `Error::NotSupported` (see `binary_vk_cpu_wait_error`).
        if let Some(err) = self.binary_vk_cpu_wait_error() {
            return Err(err);
        }
        match self.waiter() {
            Some(w) => w.wait(timeout),
            None => Ok(()),
        }
    }

    /// Leaf-crate hook for rejecting a CPU wait on a binary
    /// `SyncPoint::Vulkan` whose producer did not attach a
    /// fence-equivalent fallback. Always `None`: whether the fallback is
    /// attached is visible only to the backend's waiter, which is where
    /// that half of the binary-Vulkan CPU-wait contract is enforced.
    fn binary_vk_cpu_wait_error(&self) -> Option<Error> {
        // The fence-equivalent fallback lives on the per-backend Vulkan
        // waiter impl. From the leaf crate's perspective we only see
        // `Arc<dyn SyncWaiter>` — the contract is that `wait` /
        // `wait_async` on a binary semaphore without fallback return
        // `Error::NotSupported`. Backends MUST honour that contract; we
        // cannot enforce the "fallback present" check here, so this
        // returns `None` rather than gate behaviour it can't observe.
        let _ = matches!(self, Self::Vulkan { kind: VulkanSemaphoreKind::Binary, .. });
        None
    }

    /// Sequence two sync points. Returns a `SyncPoint` whose wait
    /// completes only after both `self` and `then` have completed.
    ///
    /// Implemented by a small adapter waiter that
    /// drives both inner waits in sequence; cheap (one heap alloc for
    /// the adapter `Arc`).
    pub fn chain(self, then: SyncPoint) -> SyncPoint {
        // Trivial cases — skip the adapter entirely.
        if matches!(self, Self::Cpu | Self::Noop) {
            return then;
        }
        if matches!(then, Self::Cpu | Self::Noop) {
            return self;
        }
        let backend = self.backend();
        let waiter: Arc<dyn SyncWaiter> = Arc::new(ChainWaiter { first: self, then, backend });
        // The chain adapter looks like an opaque waiter; we expose it
        // through the `Cuda` shape (see `with_chain_waiter`) only because
        // every variant needs *some* per-variant carrier. The event/value
        // fields are unused — only the `waiter` is consulted.
        //
        // A dedicated `SyncPoint::Chained` variant would be cleaner.
        // Callers should treat the result as opaque (only use
        // `.wait()`, `.is_signaled()`, `.backend()`).
        Self::Cpu // intentionally fall back; ChainWaiter is used below
            .with_chain_waiter(waiter)
    }

    /// Internal helper to splice a `ChainWaiter` into a placeholder
    /// `SyncPoint::Cpu`. Used only by `chain`.
    fn with_chain_waiter(self, waiter: Arc<dyn SyncWaiter>) -> SyncPoint {
        // Encode the chain waiter as a `Cuda` variant with a sentinel
        // event pointer. The variant choice is arbitrary — consumers
        // dispatch on `.waiter().backend()` for routing decisions, so
        // the variant tag is invisible.
        //
        // This is a pragmatic placeholder in lieu of a dedicated
        // `SyncPoint::Chained` variant.
        let _ = self;
        Self::Cuda { event: core::ptr::null_mut(), value: None, waiter }
    }
}

// ─────────────────────────────────────────────────────────────────────────────
// Raw-handle reconstruction (cross-C / cross-instance transport carriers)
// ─────────────────────────────────────────────────────────────────────────────

/// A [`SyncWaiter`] for a [`SyncPoint`] **reconstructed from a raw foreign fence
/// handle** that has not yet been imported onto a waiting device.
///
/// A D3D12 fence / Vulkan semaphore / Metal shared event that crossed an API or
/// process boundary as a raw handle (e.g. a `#[repr(C)]` descriptor over a C ABI)
/// carries no device the leaf crate can drive a wait on — the real CPU/GPU wait
/// only becomes possible once a consumer **imports** the handle onto its own device
/// (the importer stages the device-side `vkWaitSemaphores` /
/// `ID3D12CommandQueue::Wait` / `encodeWaitForEvent` off the carried raw fields).
///
/// So a `SyncPoint` reconstructed via [`SyncPoint::from_raw_d3d12_fence`] /
/// [`SyncPoint::from_raw_vulkan_semaphore`] / [`SyncPoint::from_raw_metal_event`] is
/// a **transport carrier**: its raw fields are consumed by the importer, and its
/// `waiter` is this carrier. Calling [`SyncPoint::wait_blocking`] / `wait` /
/// `is_signaled` on the carrier (before import) returns [`Error::NotSupported`] —
/// fail-closed, never a silent "already signaled" — because the leaf crate cannot
/// wait a fence it has no device for. Import the handle first; the import path
/// produces the real, waitable `SyncPoint`.
#[derive(Debug)]
pub struct RawFenceCarrierWaiter {
    backend: BackendKind,
}

impl RawFenceCarrierWaiter {
    fn new(backend: BackendKind) -> Self {
        Self { backend }
    }

    fn not_imported() -> Error {
        Error::NotSupported(std::borrow::Cow::Borrowed(
            "SyncPoint reconstructed from a raw foreign fence handle is a transport \
                 carrier — import it onto a device before waiting",
        ))
    }
}

impl SyncWaiter for RawFenceCarrierWaiter {
    fn wait(&self, _timeout: Duration) -> Result<(), Error> {
        Err(Self::not_imported())
    }

    fn is_signaled(&self) -> Result<bool, Error> {
        Err(Self::not_imported())
    }

    fn backend(&self) -> BackendKind {
        self.backend
    }

    fn as_any(&self) -> &dyn Any {
        self
    }
}

impl SyncPoint {
    /// Reconstruct a [`SyncPoint::D3D12`] transport carrier from a raw
    /// `ID3D12Fence*` pointer + signal value that crossed an API / process boundary
    /// (e.g. a `#[repr(C)]` fence descriptor over a C ABI).
    ///
    /// `fence` is an `ID3D12Fence*` COM pointer the caller guarantees is live for
    /// the carrier's lifetime; `value` is the monotonic value the consumer waits for
    /// (`fence->GetCompletedValue() >= value`). The returned `SyncPoint` is a
    /// **carrier** ([`RawFenceCarrierWaiter`]) — its raw `(fence, value)` fields feed
    /// a device-side import (e.g. an interop layer's acquire-sync path); a CPU
    /// `wait()` on it before import returns [`Error::NotSupported`].
    pub fn from_raw_d3d12_fence(fence: *mut c_void, value: u64) -> SyncPoint {
        let waiter: Arc<dyn SyncWaiter> = Arc::new(RawFenceCarrierWaiter::new(BackendKind::D3D12));
        SyncPoint::D3D12 { fence, value, waiter }
    }

    /// Reconstruct a [`SyncPoint::Vulkan`] transport carrier from a raw
    /// `VkSemaphore` + `VkDevice` that crossed an API / process boundary.
    ///
    /// `semaphore` is the raw `VkSemaphore` (a 64-bit non-dispatchable handle);
    /// `kind` distinguishes timeline vs binary (see [`VulkanSemaphoreKind`]);
    /// `value` is the timeline coordinate (ignored for `Binary`); `device` is the
    /// `VkDevice*` the semaphore lives on (needed by an importer that re-exports it
    /// for a cross-device wait). The returned `SyncPoint` is a **carrier** — its raw
    /// fields feed a device-side import; a CPU `wait()` before import returns
    /// [`Error::NotSupported`].
    pub fn from_raw_vulkan_semaphore(
        semaphore: u64,
        kind: VulkanSemaphoreKind,
        value: u64,
        device: *mut c_void,
    ) -> SyncPoint {
        let waiter: Arc<dyn SyncWaiter> = Arc::new(RawFenceCarrierWaiter::new(BackendKind::Vulkan));
        SyncPoint::Vulkan { semaphore, kind, value, device, waiter }
    }

    /// Reconstruct a [`SyncPoint::Metal`] transport carrier from a raw
    /// `MTLSharedEvent*` + signal value that crossed an API / process boundary.
    ///
    /// `event` is an `MTLSharedEvent*` the caller guarantees is live for the
    /// carrier's lifetime; `value` is the value the consumer waits for. The returned
    /// `SyncPoint` is a **carrier** — its raw `(event, value)` fields feed a
    /// device-side import; a CPU `wait()` before import returns
    /// [`Error::NotSupported`].
    pub fn from_raw_metal_event(event: *mut c_void, value: u64) -> SyncPoint {
        let waiter: Arc<dyn SyncWaiter> = Arc::new(RawFenceCarrierWaiter::new(BackendKind::Metal));
        SyncPoint::Metal { event, value, waiter }
    }
}

/// Per-process waiter thread for [`SyncPoint::DeferredWgpu`].
///
/// The waiter thread is stateless — every per-instance datum (device
/// handle, deferred slot) travels inside the `SliceFn` closure — so one
/// thread serves every deferred sync point in the process.
#[cfg(feature = "wgpu")]
fn deferred_wgpu_waiter_thread() -> &'static crate::WaiterThread {
    static THREAD: std::sync::OnceLock<crate::WaiterThread> = std::sync::OnceLock::new();
    THREAD.get_or_init(|| crate::WaiterThread::new("wgpu-deferred"))
}

/// Backend-internal waiter for [`SyncPoint::DeferredWgpu`]. Until the
/// producer commits a `SubmissionIndex` via [`DeferredWgpuSlot::set`],
/// `wait` falls back to `device.poll(PollType::Wait { submission_index:
/// None, .. })` (full device drain). After commit, it passes
/// `submission_index: Some(idx)` and resolves on that precise index.
///
/// `wait_async` routes through [`run_hybrid_wait`] — the fast-path
/// yield-spin spins on `is_signaled` for sub-millisecond signals, the
/// waiter-thread fallback hands off to the
/// per-process waiter thread which issues blocking-with-timeout slices
/// of [`WAITER_SLICE`] on a dedicated thread.
#[cfg(feature = "wgpu")]
pub(crate) struct DeferredWgpuWaiter {
    slot: Arc<DeferredWgpuSlot>,
    device: Arc<wgpu::Device>,
}

#[cfg(feature = "wgpu")]
impl DeferredWgpuWaiter {
    pub(crate) fn new(slot: Arc<DeferredWgpuSlot>, device: Arc<wgpu::Device>) -> Self {
        Self { slot, device }
    }

    /// Single `device.poll(Wait { .. })` issuance keyed off the
    /// current commit state. Shared between `wait`, `is_signaled`, and
    /// the per-slice closure produced by `wait_async`.
    fn poll_slice(&self, timeout: Option<Duration>) -> SliceOutcome {
        let submission_index = self.slot.get().cloned();
        match self.device.poll(wgpu::PollType::Wait { submission_index, timeout }) {
            Ok(_) => SliceOutcome::Signaled,
            Err(wgpu::PollError::Timeout) => SliceOutcome::TimedOut,
            Err(e) => {
                log::warn!("SyncPoint::DeferredWgpu: device.poll returned {e:?}");
                SliceOutcome::Failed(Error::NotSupported("SyncPoint::DeferredWgpu: device.poll failed".into()))
            }
        }
    }
}

#[cfg(feature = "wgpu")]
impl SyncWaiter for DeferredWgpuWaiter {
    fn wait(&self, timeout: Duration) -> Result<(), Error> {
        let timeout_arg = if timeout == Duration::MAX { None } else { Some(timeout) };
        match self.poll_slice(timeout_arg) {
            SliceOutcome::Signaled => Ok(()),
            SliceOutcome::TimedOut => Err(Error::Timeout),
            SliceOutcome::Failed(e) => Err(e),
        }
    }

    fn wait_async<'a>(&'a self, timeout: Duration) -> BoxFuture<'a, Result<(), Error>> {
        let slot = self.slot.clone();
        let device = self.device.clone();
        let make_slice = move || -> crate::SliceFn {
            Box::new(move |slice: Duration| -> SliceOutcome {
                let submission_index = slot.get().cloned();
                match device.poll(wgpu::PollType::Wait { submission_index, timeout: Some(slice) }) {
                    Ok(_) => SliceOutcome::Signaled,
                    Err(wgpu::PollError::Timeout) => SliceOutcome::TimedOut,
                    Err(e) => {
                        log::warn!("SyncPoint::DeferredWgpu::wait_async: device.poll returned {e:?}");
                        SliceOutcome::Failed(Error::NotSupported(
                            "SyncPoint::DeferredWgpu::wait_async: device.poll failed".into(),
                        ))
                    }
                }
            })
        };
        Box::pin(crate::run_hybrid_wait(move || self.is_signaled(), deferred_wgpu_waiter_thread(), timeout, make_slice))
    }

    fn is_signaled(&self) -> Result<bool, Error> {
        match self.poll_slice(Some(Duration::ZERO)) {
            SliceOutcome::Signaled => Ok(true),
            SliceOutcome::TimedOut => Ok(false),
            SliceOutcome::Failed(e) => Err(e),
        }
    }

    fn backend(&self) -> BackendKind {
        BackendKind::Wgpu
    }

    fn as_any(&self) -> &dyn Any {
        self
    }
}

/// Construct a [`SyncPoint::DeferredWgpu`] complete with its waiter.
///
/// For producers that hand out a sync point *before* they submit: the
/// returned SyncPoint's clones all share the same `Arc<DeferredWgpuSlot>`,
/// so the producer-side `slot.set(idx)` at submit time is visible to every
/// clone.
#[cfg(feature = "wgpu")]
pub fn make_deferred_wgpu_sync_point(slot: Arc<DeferredWgpuSlot>, device: Arc<wgpu::Device>) -> SyncPoint {
    let waiter: Arc<dyn SyncWaiter> = Arc::new(DeferredWgpuWaiter::new(slot.clone(), device.clone()));
    SyncPoint::DeferredWgpu { slot, device, waiter }
}

struct ChainWaiter {
    first: SyncPoint,
    then: SyncPoint,
    backend: BackendKind,
}

impl SyncWaiter for ChainWaiter {
    fn wait(&self, timeout: Duration) -> Result<(), Error> {
        let start = std::time::Instant::now();
        self.first.wait_with_timeout(timeout)?;
        let elapsed = start.elapsed();
        let remaining = timeout.saturating_sub(elapsed);
        self.then.wait_with_timeout(remaining)
    }

    fn wait_async<'a>(&'a self, timeout: Duration) -> BoxFuture<'a, Result<(), Error>> {
        Box::pin(async move {
            let start = std::time::Instant::now();
            self.first.wait_with_timeout_async(timeout).await?;
            let elapsed = start.elapsed();
            let remaining = timeout.saturating_sub(elapsed);
            self.then.wait_with_timeout_async(remaining).await
        })
    }

    fn is_signaled(&self) -> Result<bool, Error> {
        Ok(self.first.is_signaled()? && self.then.is_signaled()?)
    }

    fn backend(&self) -> BackendKind {
        self.backend
    }

    fn as_any(&self) -> &dyn Any {
        self
    }
}

impl SyncPoint {
    /// Non-blocking probe.
    ///
    /// - `Ok(true)`  — sync point reached (or trivial `Cpu`/`Noop`).
    /// - `Ok(false)` — work still in flight.
    /// - `Err(_)`    — driver-level failure (device lost / TDR /
    ///   wrong-submission-index). See [`SyncWaiter::is_signaled`]
    ///   for the rationale on surfacing errors instead of folding
    ///   them into `false`.
    pub fn is_signaled(&self) -> Result<bool, Error> {
        match self.waiter() {
            Some(w) => w.is_signaled(),
            None => Ok(true),
        }
    }

    /// Backend identity. For the trivial variants (`Cpu`, `Noop`) this
    /// returns [`BackendKind::Cpu`]; otherwise it dispatches to the
    /// per-variant waiter.
    pub fn backend(&self) -> BackendKind {
        match self.waiter() {
            Some(w) => w.backend(),
            None => BackendKind::Cpu,
        }
    }

    /// Borrow the per-variant waiter, if any. `Cpu` and `Noop` return
    /// `None`.
    pub fn waiter(&self) -> Option<&Arc<dyn SyncWaiter>> {
        match self {
            Self::Vulkan { waiter, .. }
            | Self::D3D12 { waiter, .. }
            | Self::D3D11 { waiter, .. }
            | Self::Cuda { waiter, .. }
            | Self::CudaEvent { waiter, .. }
            | Self::OpenCl { waiter, .. }
            | Self::Metal { waiter, .. }
            | Self::OpenGL { waiter, .. }
            | Self::OpenGLSync { waiter, .. } => Some(waiter),
            #[cfg(feature = "wgpu")]
            Self::Wgpu { waiter, .. } => Some(waiter),
            #[cfg(feature = "wgpu")]
            Self::DeferredWgpu { waiter, .. } => Some(waiter),
            Self::Cpu | Self::Noop => None,
        }
    }
}

impl core::fmt::Debug for SyncPoint {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        match self {
            Self::Vulkan { semaphore, kind, value, .. } => f
                .debug_struct("Vulkan")
                .field("semaphore", semaphore)
                .field("kind", kind)
                .field("value", value)
                .finish(),
            Self::D3D12 { value, .. } => f.debug_struct("D3D12").field("value", value).finish(),
            Self::D3D11 { key, .. } => f.debug_struct("D3D11").field("key", key).finish(),
            Self::Cuda { value, .. } => f.debug_struct("Cuda").field("value", value).finish(),
            Self::CudaEvent { event, device, .. } => {
                f.debug_struct("CudaEvent").field("event", &(*event as usize)).field("device", device).finish()
            }
            Self::OpenCl { event, value, .. } => {
                f.debug_struct("OpenCl").field("event", &(*event as usize)).field("value", value).finish()
            }
            Self::Metal { value, .. } => f.debug_struct("Metal").field("value", value).finish(),
            Self::OpenGL { semaphore, value, .. } => {
                f.debug_struct("OpenGL").field("semaphore", semaphore).field("value", value).finish()
            }
            Self::OpenGLSync { glsync, backend, .. } => {
                f.debug_struct("OpenGLSync").field("glsync", &(*glsync as usize)).field("backend", backend).finish()
            }
            #[cfg(feature = "wgpu")]
            Self::Wgpu { .. } => f.write_str("SyncPoint::Wgpu"),
            #[cfg(feature = "wgpu")]
            Self::DeferredWgpu { slot, .. } => {
                f.debug_struct("DeferredWgpu").field("committed", &slot.get().is_some()).finish()
            }
            Self::Cpu => f.write_str("SyncPoint::Cpu"),
            Self::Noop => f.write_str("SyncPoint::Noop"),
        }
    }
}

/// Runtime-agnostic single-yield future. Returns `Pending` once then
/// resolves to `Ready(())` on the next poll, with `wake_by_ref` driving
/// the re-poll. Works under any executor (`pollster`, `tokio`, `smol`,
/// `futures::executor`).
///
/// Used by the default `SyncWaiter::wait_async` impl and by per-backend
/// overrides for their fast-path yield-spin loops.
pub async fn yield_once() {
    let mut yielded = false;
    core::future::poll_fn(|cx| {
        if yielded {
            core::task::Poll::Ready(())
        } else {
            yielded = true;
            cx.waker().wake_by_ref();
            core::task::Poll::Pending
        }
    })
    .await
}

/// Helper: convert a `Duration` to nanoseconds, saturating at
/// `u64::MAX` for "wait forever". Matches `vkWaitSemaphores`'
/// `UINT64_MAX` infinite-wait sentinel.
pub fn duration_to_ns(t: Duration) -> u64 {
    u64::try_from(t.as_nanos()).unwrap_or(u64::MAX)
}

/// Helper: convert a `Duration` to milliseconds, saturating at
/// `u32::MAX` for the Win32 `INFINITE` sentinel
/// (`WaitForSingleObject`, `WaitForMultipleObjects`).
pub fn duration_to_ms_u32(t: Duration) -> u32 {
    u32::try_from(t.as_millis()).unwrap_or(u32::MAX)
}