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#![allow(unused)]
use parking_lot::{RwLock, RwLockReadGuard, RwLockWriteGuard};
use std::{
backtrace::Backtrace,
cell::{Cell, RefCell, UnsafeCell},
panic::{self, Location},
thread,
};
/// A guard that provides read access to snatchable data.
pub struct SnatchGuard<'a>(RwLockReadGuard<'a, ()>);
/// A guard that allows snatching the snatchable data.
pub struct ExclusiveSnatchGuard<'a>(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)),
}
}
/// Get read access to the value. Requires a the snatchable lock's read guard.
pub fn get(&self, _guard: &SnatchGuard) -> Option<&T> {
unsafe { (*self.value.get()).as_ref() }
}
/// Get write access to the value. Requires a the snatchable lock's write guard.
pub fn get_mut(&self, _guard: &mut ExclusiveSnatchGuard) -> Option<&mut T> {
unsafe { (*self.value.get()).as_mut() }
}
/// Take the value. Requires a the snatchable lock's write guard.
pub fn snatch(&self, _guard: 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> std::fmt::Debug for Snatchable<T> {
fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
write!(f, "<snatchable>")
}
}
unsafe impl<T> Sync for Snatchable<T> {}
thread_local! {
static READ_LOCK_LOCATION: Cell<Option<(&'static Location<'static>, Backtrace)>> = const { Cell::new(None) };
}
/// 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() -> Self {
SnatchLock {
lock: RwLock::new(()),
}
}
/// Request read access to snatchable resources.
#[track_caller]
pub fn read(&self) -> SnatchGuard {
if cfg!(debug_assertions) {
let caller = Location::caller();
let backtrace = Backtrace::capture();
if let Some((prev, bt)) = READ_LOCK_LOCATION.take() {
let current = thread::current();
let name = current.name().unwrap_or("<unnamed>");
panic!(
"thread '{name}' attempted to acquire a snatch read lock recursively.\n
- {prev}\n{bt}\n
- {caller}\n{backtrace}"
);
} else {
READ_LOCK_LOCATION.set(Some((caller, backtrace)));
}
}
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.
pub fn write(&self) -> ExclusiveSnatchGuard {
ExclusiveSnatchGuard(self.lock.write())
}
}
impl Drop for SnatchGuard<'_> {
fn drop(&mut self) {
#[cfg(debug_assertions)]
READ_LOCK_LOCATION.take();
}
}
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