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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 shared read access of a lock when
/// dropped.
///
/// This structure is created by the [`read`] method on
/// [`RwLock`].
///
/// [`read`]: method@crate::sync::RwLock::read
/// [`RwLock`]: struct@crate::sync::RwLock
#[clippy::has_significant_drop]
#[must_use = "if unused the RwLock will immediately unlock"]
pub struct RwLockReadGuard<'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) s: &'a Semaphore,
pub(super) data: *const T,
pub(super) marker: PhantomData<&'a 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,
s: &'a Semaphore,
data: *const T,
}
impl<'a, T: ?Sized> RwLockReadGuard<'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) },
s: me.s,
data: me.data,
}
}
/// Makes a new `RwLockReadGuard` for a component of the locked data.
///
/// This operation cannot fail as the `RwLockReadGuard` passed in already
/// locked the data.
///
/// This is an associated function that needs to be
/// used as `RwLockReadGuard::map(...)`. A method would interfere with
/// methods of the same name on the contents of the locked data.
///
/// This is an asynchronous version of [`RwLockReadGuard::map`] from the
/// [`parking_lot` crate].
///
/// [`RwLockReadGuard::map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockReadGuard.html#method.map
/// [`parking_lot` crate]: https://crates.io/crates/parking_lot
///
/// # Examples
///
/// ```
/// use tokio::sync::{RwLock, RwLockReadGuard};
///
/// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
/// struct Foo(u32);
///
/// # #[tokio::main]
/// # async fn main() {
/// let lock = RwLock::new(Foo(1));
///
/// let guard = lock.read().await;
/// let guard = RwLockReadGuard::map(guard, |f| &f.0);
///
/// assert_eq!(1, *guard);
/// # }
/// ```
#[inline]
pub fn map<F, U: ?Sized>(this: Self, f: F) -> RwLockReadGuard<'a, U>
where
F: FnOnce(&T) -> &U,
{
let data = f(&*this) as *const U;
let this = this.skip_drop();
RwLockReadGuard {
s: this.s,
data,
marker: PhantomData,
#[cfg(all(tokio_unstable, feature = "tracing"))]
resource_span: this.resource_span,
}
}
/// Attempts to make a new [`RwLockReadGuard`] for a component of the
/// locked data. The original guard is returned if the closure returns
/// `None`.
///
/// This operation cannot fail as the `RwLockReadGuard` passed in already
/// locked the data.
///
/// This is an associated function that needs to be used as
/// `RwLockReadGuard::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 [`RwLockReadGuard::try_map`] from the
/// [`parking_lot` crate].
///
/// [`RwLockReadGuard::try_map`]: https://docs.rs/lock_api/latest/lock_api/struct.RwLockReadGuard.html#method.try_map
/// [`parking_lot` crate]: https://crates.io/crates/parking_lot
///
/// # Examples
///
/// ```
/// use tokio::sync::{RwLock, RwLockReadGuard};
///
/// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
/// struct Foo(u32);
///
/// # #[tokio::main]
/// # async fn main() {
/// let lock = RwLock::new(Foo(1));
///
/// let guard = lock.read().await;
/// let guard = RwLockReadGuard::try_map(guard, |f| Some(&f.0)).expect("should not fail");
///
/// assert_eq!(1, *guard);
/// # }
/// ```
#[inline]
pub fn try_map<F, U: ?Sized>(this: Self, f: F) -> Result<RwLockReadGuard<'a, U>, Self>
where
F: FnOnce(&T) -> Option<&U>,
{
let data = match f(&*this) {
Some(data) => data as *const U,
None => return Err(this),
};
let this = this.skip_drop();
Ok(RwLockReadGuard {
s: this.s,
data,
marker: PhantomData,
#[cfg(all(tokio_unstable, feature = "tracing"))]
resource_span: this.resource_span,
})
}
}
impl<T: ?Sized> ops::Deref for RwLockReadGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
unsafe { &*self.data }
}
}
impl<'a, T: ?Sized> fmt::Debug for RwLockReadGuard<'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 RwLockReadGuard<'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 RwLockReadGuard<'a, T> {
fn drop(&mut self) {
self.s.release(1);
#[cfg(all(tokio_unstable, feature = "tracing"))]
self.resource_span.in_scope(|| {
tracing::trace!(
target: "runtime::resource::state_update",
current_readers = 1,
current_readers.op = "sub",
)
});
}
}