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// Copyright 2024 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
//! Helper types, traits, and aliases for dealing with resource references.
use core::convert::Infallible as Never;
use crate::sync::{DynDebugReferences, MapRcNotifier, PrimaryRc, RcNotifier};
/// A context trait determining the types to be used for reference
/// notifications.
pub trait ReferenceNotifiers {
/// The receiver for shared reference destruction notifications.
type ReferenceReceiver<T: 'static>: 'static;
/// The notifier for shared reference destruction notifications.
type ReferenceNotifier<T: Send + 'static>: RcNotifier<T> + 'static;
/// Creates a new Notifier/Receiver pair for `T`.
///
/// `debug_references` is given to provide information on outstanding
/// references that caused the notifier to be requested.
fn new_reference_notifier<T: Send + 'static>(
debug_references: DynDebugReferences,
) -> (Self::ReferenceNotifier<T>, Self::ReferenceReceiver<T>);
}
/// A context trait that allows core to defer observing proper resource cleanup
/// to bindings.
///
/// This trait exists as a debug utility to expose resources that are not
/// removed from the system as expected.
pub trait DeferredResourceRemovalContext: ReferenceNotifiers {
/// Defers the removal of some resource `T` to bindings.
///
/// Bindings can watch `receiver` and notify when resource removal is not
/// completing in a timely manner.
fn defer_removal<T: Send + 'static>(&mut self, receiver: Self::ReferenceReceiver<T>);
/// A shorthand for [`defer_removal`] that takes a `ReferenceReceiver` from
/// the `Deferred` variant of a [`RemoveResourceResult`].
///
/// The default implementation is `track_caller` when the `instrumented`
/// feature is available so implementers can track the caller location when
/// needed.
#[cfg_attr(feature = "instrumented", track_caller)]
fn defer_removal_result<T: Send + 'static>(
&mut self,
result: RemoveResourceResultWithContext<T, Self>,
) {
match result {
// We don't need to do anything for synchronously removed resources.
RemoveResourceResult::Removed(_) => (),
RemoveResourceResult::Deferred(d) => self.defer_removal(d),
}
}
}
/// The result of removing some reference-counted resource from core.
#[derive(Debug)]
pub enum RemoveResourceResult<R, D> {
/// The resource was synchronously removed and no more references to it
/// exist.
Removed(R),
/// The resource was marked for destruction but there are still references
/// to it in existence. The provided receiver can be polled on to observe
/// resource destruction completion.
Deferred(D),
}
impl<R, D> RemoveResourceResult<R, D> {
/// Maps the `Removed` variant to a different type.
pub fn map_removed<N, F: FnOnce(R) -> N>(self, f: F) -> RemoveResourceResult<N, D> {
match self {
Self::Removed(r) => RemoveResourceResult::Removed(f(r)),
Self::Deferred(d) => RemoveResourceResult::Deferred(d),
}
}
/// Maps the `Deferred` variant to a different type.
pub fn map_deferred<N, F: FnOnce(D) -> N>(self, f: F) -> RemoveResourceResult<R, N> {
match self {
Self::Removed(r) => RemoveResourceResult::Removed(r),
Self::Deferred(d) => RemoveResourceResult::Deferred(f(d)),
}
}
}
impl<R> RemoveResourceResult<R, Never> {
/// A helper function to unwrap a [`RemoveResourceResult`] that can never be
/// [`RemoveResourceResult::Deferred`].
pub fn into_removed(self) -> R {
#[allow(unreachable_patterns)] // TODO(https://fxbug.dev/360335974)
match self {
Self::Removed(r) => r,
Self::Deferred(never) => match never {},
}
}
}
/// An alias for [`RemoveResourceResult`] that extracts the receiver type from
/// the bindings context.
pub type RemoveResourceResultWithContext<S, BT> =
RemoveResourceResult<S, <BT as ReferenceNotifiers>::ReferenceReceiver<S>>;
/// An extension of [`ReferenceNotifiers`] providing extra functionality.
pub trait ReferenceNotifiersExt: ReferenceNotifiers {
/// Unwraps a [`PrimaryRc`] if it has no pending references, otherwise
/// notifying with [`Self::ReferenceNotifier`].
///
/// S: Is the state held behind the [`PrimaryRc`].
/// O: Is the desired output state, once references have been destructed.
/// F: Is a function that converts S to O.
fn unwrap_or_notify_with_new_reference_notifier<
S: Send + Sync + 'static,
O: Send,
F: Send + Copy + 'static + FnOnce(S) -> O,
>(
primary: PrimaryRc<S>,
map: F,
) -> RemoveResourceResultWithContext<O, Self> {
let debug_references = PrimaryRc::debug_references(&primary).into_dyn();
match PrimaryRc::unwrap_or_notify_with(primary, || {
let (notifier, receiver) = Self::new_reference_notifier::<O>(debug_references);
let notifier = MapRcNotifier::new(notifier, map);
(notifier, receiver)
}) {
Ok(state) => RemoveResourceResult::Removed(map(state)),
Err(receiver) => RemoveResourceResult::Deferred(receiver),
}
}
}
impl<N: ReferenceNotifiers> ReferenceNotifiersExt for N {}