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use crate::sync::batch_semaphore::Semaphore;
use std::marker::PhantomData;
use std::{fmt, mem, ops};
/// RAII structure used to release the exclusive write access of a lock when
/// dropped.
///
/// This structure is created by [mapping] an [`RwLockWriteGuard`]. It is a
/// separate type from `RwLockWriteGuard` to disallow downgrading a mapped
/// guard, since doing so can cause undefined behavior.
///
/// [mapping]: method@crate::sync::RwLockWriteGuard::map
/// [`RwLockWriteGuard`]: struct@crate::sync::RwLockWriteGuard
#[clippy::has_significant_drop]
pub struct RwLockMappedWriteGuard<'a, T: ?Sized> {
// When changing the fields in this struct, make sure to update the
// `skip_drop` method.
#[cfg(all(tokio_unstable, feature = "tracing"))]
pub(super) resource_span: tracing::Span,
pub(super) permits_acquired: u32,
pub(super) s: &'a Semaphore,
pub(super) data: *mut T,
pub(super) marker: PhantomData<&'a mut T>,
}
#[allow(dead_code)] // Unused fields are still used in Drop.
struct Inner<'a, T: ?Sized> {
#[cfg(all(tokio_unstable, feature = "tracing"))]
resource_span: tracing::Span,
permits_acquired: u32,
s: &'a Semaphore,
data: *mut T,
}
impl<'a, T: ?Sized> RwLockMappedWriteGuard<'a, T> {
fn skip_drop(self) -> Inner<'a, T> {
let me = mem::ManuallyDrop::new(self);
// SAFETY: This duplicates the values in every field of the guard, then
// forgets the originals, so in the end no value is duplicated.
Inner {
#[cfg(all(tokio_unstable, feature = "tracing"))]
resource_span: unsafe { std::ptr::read(&me.resource_span) },
permits_acquired: me.permits_acquired,
s: me.s,
data: me.data,
}
}
/// Makes a new `RwLockMappedWriteGuard` for a component of the locked data.
///
/// This operation cannot fail as the `RwLockMappedWriteGuard` passed in already
/// locked the data.
///
/// This is an associated function that needs to be used as
/// `RwLockMappedWriteGuard::map(..)`. A method would interfere with methods
/// of the same name on the contents of the locked data.
///
/// This is an asynchronous version of [`RwLockWriteGuard::map`] from the
/// [`parking_lot` crate].
///
/// [`RwLockWriteGuard::map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockWriteGuard.html#method.map
///
/// # Examples
///
/// ```
/// use tokio::sync::{RwLock, RwLockWriteGuard};
///
/// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
/// struct Foo(u32);
///
/// # #[tokio::main]
/// # async fn main() {
/// let lock = RwLock::new(Foo(1));
///
/// {
/// let mut mapped = RwLockWriteGuard::map(lock.write().await, |f| &mut f.0);
/// *mapped = 2;
/// }
///
/// assert_eq!(Foo(2), *lock.read().await);
/// # }
/// ```
#[inline]
pub fn map<F, U: ?Sized>(mut this: Self, f: F) -> RwLockMappedWriteGuard<'a, U>
where
F: FnOnce(&mut T) -> &mut U,
{
let data = f(&mut *this) as *mut U;
let this = this.skip_drop();
RwLockMappedWriteGuard {
permits_acquired: this.permits_acquired,
s: this.s,
data,
marker: PhantomData,
#[cfg(all(tokio_unstable, feature = "tracing"))]
resource_span: this.resource_span,
}
}
/// Attempts to make a new [`RwLockMappedWriteGuard`] for a component of
/// the locked data. The original guard is returned if the closure returns
/// `None`.
///
/// This operation cannot fail as the `RwLockMappedWriteGuard` passed in already
/// locked the data.
///
/// This is an associated function that needs to be
/// used as `RwLockMappedWriteGuard::try_map(...)`. A method would interfere
/// with methods of the same name on the contents of the locked data.
///
/// This is an asynchronous version of [`RwLockWriteGuard::try_map`] from
/// the [`parking_lot` crate].
///
/// [`RwLockWriteGuard::try_map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockWriteGuard.html#method.try_map
///
/// # Examples
///
/// ```
/// use tokio::sync::{RwLock, RwLockWriteGuard};
///
/// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
/// struct Foo(u32);
///
/// # #[tokio::main]
/// # async fn main() {
/// let lock = RwLock::new(Foo(1));
///
/// {
/// let guard = lock.write().await;
/// let mut guard = RwLockWriteGuard::try_map(guard, |f| Some(&mut f.0)).expect("should not fail");
/// *guard = 2;
/// }
///
/// assert_eq!(Foo(2), *lock.read().await);
/// # }
/// ```
#[inline]
pub fn try_map<F, U: ?Sized>(
mut this: Self,
f: F,
) -> Result<RwLockMappedWriteGuard<'a, U>, Self>
where
F: FnOnce(&mut T) -> Option<&mut U>,
{
let data = match f(&mut *this) {
Some(data) => data as *mut U,
None => return Err(this),
};
let this = this.skip_drop();
Ok(RwLockMappedWriteGuard {
permits_acquired: this.permits_acquired,
s: this.s,
data,
marker: PhantomData,
#[cfg(all(tokio_unstable, feature = "tracing"))]
resource_span: this.resource_span,
})
}
// Note: No `downgrade`, `downgrade_map` nor `try_downgrade_map` because they would be unsound, as we're already
// potentially been mapped with internal mutability.
}
impl<T: ?Sized> ops::Deref for RwLockMappedWriteGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.data }
}
}
impl<T: ?Sized> ops::DerefMut for RwLockMappedWriteGuard<'_, T> {
fn deref_mut(&mut self) -> &mut T {
unsafe { &mut *self.data }
}
}
impl<'a, T: ?Sized> fmt::Debug for RwLockMappedWriteGuard<'a, T>
where
T: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
impl<'a, T: ?Sized> fmt::Display for RwLockMappedWriteGuard<'a, T>
where
T: fmt::Display,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(&**self, f)
}
}
impl<'a, T: ?Sized> Drop for RwLockMappedWriteGuard<'a, T> {
fn drop(&mut self) {
self.s.release(self.permits_acquired as usize);
#[cfg(all(tokio_unstable, feature = "tracing"))]
self.resource_span.in_scope(|| {
tracing::trace!(
target: "runtime::resource::state_update",
write_locked = false,
write_locked.op = "override",
)
});
}
}