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use core::{cell::UnsafeCell, fmt};
use crate::lock::{rank, RwLock, RwLockReadGuard, RwLockWriteGuard};
/// A guard that provides read access to snatchable data.
pub struct SnatchGuard<'a>(#[expect(dead_code)] RwLockReadGuard<'a, ()>);
/// A guard that allows snatching the snatchable data.
pub struct ExclusiveSnatchGuard<'a>(#[expect(dead_code)] RwLockWriteGuard<'a, ()>);
/// A value that is mostly immutable but can be "snatched" if we need to destroy
/// it early.
///
/// In order to safely access the underlying data, the device's global snatchable
/// lock must be taken. To guarantee it, methods take a read or write guard of that
/// special lock.
pub struct Snatchable<T> {
value: UnsafeCell<Option<T>>,
}
impl<T> Snatchable<T> {
pub fn new(val: T) -> Self {
Snatchable {
value: UnsafeCell::new(Some(val)),
}
}
#[allow(dead_code)]
pub fn empty() -> Self {
Snatchable {
value: UnsafeCell::new(None),
}
}
/// Get read access to the value. Requires a the snatchable lock's read guard.
pub fn get<'a>(&'a self, _guard: &'a SnatchGuard) -> Option<&'a T> {
unsafe { (*self.value.get()).as_ref() }
}
/// Take the value. Requires a the snatchable lock's write guard.
pub fn snatch(&self, _guard: &mut ExclusiveSnatchGuard) -> Option<T> {
unsafe { (*self.value.get()).take() }
}
/// Take the value without a guard. This can only be used with exclusive access
/// to self, so it does not require locking.
///
/// Typically useful in a drop implementation.
pub fn take(&mut self) -> Option<T> {
self.value.get_mut().take()
}
}
// Can't safely print the contents of a snatchable object without holding
// the lock.
impl<T> fmt::Debug for Snatchable<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "<snatchable>")
}
}
unsafe impl<T> Sync for Snatchable<T> {}
use trace::LockTrace;
#[cfg(all(debug_assertions, feature = "std"))]
mod trace {
use core::{cell::Cell, fmt, panic::Location};
use std::{backtrace::Backtrace, thread};
pub(super) struct LockTrace {
purpose: &'static str,
caller: &'static Location<'static>,
backtrace: Backtrace,
}
impl fmt::Display for LockTrace {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(
f,
"a {} lock at {}\n{}",
self.purpose, self.caller, self.backtrace
)
}
}
impl LockTrace {
#[track_caller]
pub(super) fn enter(purpose: &'static str) {
let new = LockTrace {
purpose,
caller: Location::caller(),
backtrace: Backtrace::capture(),
};
if let Some(prev) = SNATCH_LOCK_TRACE.take() {
let current = thread::current();
let name = current.name().unwrap_or("<unnamed>");
panic!(
"thread '{name}' attempted to acquire a snatch lock recursively.\n\
- Currently trying to acquire {new}\n\
- Previously acquired {prev}",
);
} else {
SNATCH_LOCK_TRACE.set(Some(new));
}
}
pub(super) fn exit() {
SNATCH_LOCK_TRACE.take();
}
}
std::thread_local! {
static SNATCH_LOCK_TRACE: Cell<Option<LockTrace>> = const { Cell::new(None) };
}
}
#[cfg(not(all(debug_assertions, feature = "std")))]
mod trace {
pub(super) struct LockTrace {
_private: (),
}
impl LockTrace {
pub(super) fn enter(_purpose: &'static str) {}
pub(super) fn exit() {}
}
}
/// A Device-global lock for all snatchable data.
pub struct SnatchLock {
lock: RwLock<()>,
}
impl SnatchLock {
/// The safety of `Snatchable::get` and `Snatchable::snatch` rely on their using of the
/// right SnatchLock (the one associated to the same device). This method is unsafe
/// to force force sers to think twice about creating a SnatchLock. The only place this
/// method should be called is when creating the device.
pub unsafe fn new(rank: rank::LockRank) -> Self {
SnatchLock {
lock: RwLock::new(rank, ()),
}
}
/// Request read access to snatchable resources.
#[track_caller]
pub fn read(&self) -> SnatchGuard {
LockTrace::enter("read");
SnatchGuard(self.lock.read())
}
/// Request write access to snatchable resources.
///
/// This should only be called when a resource needs to be snatched. This has
/// a high risk of causing lock contention if called concurrently with other
/// wgpu work.
#[track_caller]
pub fn write(&self) -> ExclusiveSnatchGuard {
LockTrace::enter("write");
ExclusiveSnatchGuard(self.lock.write())
}
}
impl Drop for SnatchGuard<'_> {
fn drop(&mut self) {
LockTrace::exit();
}
}
impl Drop for ExclusiveSnatchGuard<'_> {
fn drop(&mut self) {
LockTrace::exit();
}
}