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use crate::sync::rwlock::RwLock;
use std::marker::PhantomData;
use std::sync::Arc;
use std::{fmt, mem, ops, ptr};
/// Owned RAII structure used to release the shared read access of a lock when
/// dropped.
///
/// This structure is created by the [`read_owned`] method on
/// [`RwLock`].
///
/// [`read_owned`]: method@crate::sync::RwLock::read_owned
/// [`RwLock`]: struct@crate::sync::RwLock
#[clippy::has_significant_drop]
pub struct OwnedRwLockReadGuard<T: ?Sized, U: ?Sized = T> {
// 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) lock: Arc<RwLock<T>>,
pub(super) data: *const U,
pub(super) _p: PhantomData<T>,
}
#[allow(dead_code)] // Unused fields are still used in Drop.
struct Inner<T: ?Sized, U: ?Sized> {
#[cfg(all(tokio_unstable, feature = "tracing"))]
resource_span: tracing::Span,
lock: Arc<RwLock<T>>,
data: *const U,
}
impl<T: ?Sized, U: ?Sized> OwnedRwLockReadGuard<T, U> {
fn skip_drop(self) -> Inner<T, U> {
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.
unsafe {
Inner {
#[cfg(all(tokio_unstable, feature = "tracing"))]
resource_span: ptr::read(&me.resource_span),
lock: ptr::read(&me.lock),
data: me.data,
}
}
}
/// Makes a new `OwnedRwLockReadGuard` for a component of the locked data.
/// This operation cannot fail as the `OwnedRwLockReadGuard` passed in
/// already locked the data.
///
/// This is an associated function that needs to be
/// used as `OwnedRwLockReadGuard::map(...)`. A method would interfere with
/// methods of the same name on the contents of the locked data.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
/// use tokio::sync::{RwLock, OwnedRwLockReadGuard};
///
/// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
/// struct Foo(u32);
///
/// # #[tokio::main]
/// # async fn main() {
/// let lock = Arc::new(RwLock::new(Foo(1)));
///
/// let guard = lock.read_owned().await;
/// let guard = OwnedRwLockReadGuard::map(guard, |f| &f.0);
///
/// assert_eq!(1, *guard);
/// # }
/// ```
#[inline]
pub fn map<F, V: ?Sized>(this: Self, f: F) -> OwnedRwLockReadGuard<T, V>
where
F: FnOnce(&U) -> &V,
{
let data = f(&*this) as *const V;
let this = this.skip_drop();
OwnedRwLockReadGuard {
lock: this.lock,
data,
_p: PhantomData,
#[cfg(all(tokio_unstable, feature = "tracing"))]
resource_span: this.resource_span,
}
}
/// Attempts to make a new [`OwnedRwLockReadGuard`] for a component of the
/// locked data. The original guard is returned if the closure returns
/// `None`.
///
/// This operation cannot fail as the `OwnedRwLockReadGuard` passed in
/// already locked the data.
///
/// This is an associated function that needs to be used as
/// `OwnedRwLockReadGuard::try_map(..)`. A method would interfere with
/// methods of the same name on the contents of the locked data.
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
/// use tokio::sync::{RwLock, OwnedRwLockReadGuard};
///
/// #[derive(Debug, Clone, Copy, PartialEq, Eq)]
/// struct Foo(u32);
///
/// # #[tokio::main]
/// # async fn main() {
/// let lock = Arc::new(RwLock::new(Foo(1)));
///
/// let guard = lock.read_owned().await;
/// let guard = OwnedRwLockReadGuard::try_map(guard, |f| Some(&f.0)).expect("should not fail");
///
/// assert_eq!(1, *guard);
/// # }
/// ```
#[inline]
pub fn try_map<F, V: ?Sized>(this: Self, f: F) -> Result<OwnedRwLockReadGuard<T, V>, Self>
where
F: FnOnce(&U) -> Option<&V>,
{
let data = match f(&*this) {
Some(data) => data as *const V,
None => return Err(this),
};
let this = this.skip_drop();
Ok(OwnedRwLockReadGuard {
lock: this.lock,
data,
_p: PhantomData,
#[cfg(all(tokio_unstable, feature = "tracing"))]
resource_span: this.resource_span,
})
}
}
impl<T: ?Sized, U: ?Sized> ops::Deref for OwnedRwLockReadGuard<T, U> {
type Target = U;
fn deref(&self) -> &U {
unsafe { &*self.data }
}
}
impl<T: ?Sized, U: ?Sized> fmt::Debug for OwnedRwLockReadGuard<T, U>
where
U: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, f)
}
}
impl<T: ?Sized, U: ?Sized> fmt::Display for OwnedRwLockReadGuard<T, U>
where
U: fmt::Display,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Display::fmt(&**self, f)
}
}
impl<T: ?Sized, U: ?Sized> Drop for OwnedRwLockReadGuard<T, U> {
fn drop(&mut self) {
self.lock.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",
)
});
}
}