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// Copyright 2023 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.
use core::ops::{Deref, DerefMut};
/// Describes how to apply a lock type to the implementing type.
///
/// An implementation of `LockFor<L>` for some `Self` means that `L` is a valid
/// lock level for `Self`, and defines how to access the state in `Self` that is
/// under the lock indicated by `L`.
pub trait LockFor<L> {
/// The data produced by locking the state indicated by `L` in `Self`.
type Data;
/// A guard providing read and write access to the data.
type Guard<'l>: DerefMut<Target = Self::Data>
where
Self: 'l;
/// Locks `Self` for lock `L`.
fn lock(&self) -> Self::Guard<'_>;
}
/// Describes how to acquire reader and writer locks to the implementing type.
///
/// An implementation of `RwLockFor<L>` for some `Self` means that `L` is a
/// valid lock level for `T`, and defines how to access the state in `Self` that
/// is under the lock indicated by `L` in either read mode or write mode.
pub trait RwLockFor<L> {
/// The data produced by locking the state indicated by `L` in `Self`.
type Data;
/// A guard providing read access to the data.
type ReadGuard<'l>: Deref<Target = Self::Data>
where
Self: 'l;
/// A guard providing write access to the data.
type WriteGuard<'l>: DerefMut<Target = Self::Data>
where
Self: 'l;
/// Acquires a read lock on the data in `Self` indicated by `L`.
fn read_lock(&self) -> Self::ReadGuard<'_>;
/// Acquires a write lock on the data in `Self` indicated by `L`.
fn write_lock(&self) -> Self::WriteGuard<'_>;
}
/// Describes how to access state in `Self` that doesn't require locking.
///
/// `UnlockedAccess` allows access to some state in `Self` without acquiring
/// a lock. Unlike `Lock` and friends, the type parameter `A` in
/// `UnlockedAccess<A>` is used to provide a label for the state; it is
/// unrelated to the lock levels for `Self`.
///
/// In order for this crate to provide guarantees about lock ordering safety,
/// `UnlockedAccess` must only be implemented for accessing state that is
/// guaranteed to be accessible lock-free.
pub trait UnlockedAccess<A> {
/// The type of state being accessed.
type Data;
/// A guard providing read access to the data.
type Guard<'l>: Deref<Target = Self::Data>
where
Self: 'l;
/// How to access the state.
fn access(&self) -> Self::Guard<'_>;
}
/// Marks a type as offering ordered lock access for some inner type `T`.
///
/// This trait allows for types that are lock order sensitive to be defined in a
/// separate crate than the lock levels themselves while nudging local code away
/// from using the locks without regards for ordering.
///
/// The crate defining the lock levels can implement [`LockLevelFor`] to declare
/// the lock level to access the field exposed by this implementation.
pub trait OrderedLockAccess<T> {
/// The lock type that observes ordering.
///
/// This should be a type that implements either [`ExclusiveLock`] or
/// [`ReadWriteLock`].
type Lock;
/// Returns a borrow to the order-aware lock.
///
/// Note that this returns [`OrderedLockRef`] to further prevent out of
/// order lock usage. Once sealed into [`OrderedLockRef`], the borrow can
/// only be used via the blanket [`RwLockFor`] and [`LockFor`]
/// implementations provided by this crate.
fn ordered_lock_access(&self) -> OrderedLockRef<'_, Self::Lock>;
}
/// Marks a type as offering ordered lock access for some inner type `T`
/// *through* the [`OrderedLockAccess`] implementation of `Inner`.
///
/// See [`OrderedLockAccess`] for more details.
pub trait DelegatedOrderedLockAccess<T> {
/// The inner type acting as a proxy for ordered access to T.
type Inner: OrderedLockAccess<T> + 'static;
/// Returns the inner type.
fn delegate_ordered_lock_access(&self) -> &Self::Inner;
}
impl<T, O> OrderedLockAccess<T> for O
where
O: DelegatedOrderedLockAccess<T>,
{
type Lock = <O::Inner as OrderedLockAccess<T>>::Lock;
fn ordered_lock_access(&self) -> OrderedLockRef<'_, Self::Lock> {
self.delegate_ordered_lock_access().ordered_lock_access()
}
}
/// A borrowed order-aware lock.
pub struct OrderedLockRef<'a, T>(&'a T);
impl<'a, T> OrderedLockRef<'a, T> {
/// Creates a new `OrderedLockRef` with a borrow on `lock`.
pub fn new(lock: &'a T) -> Self {
Self(lock)
}
}
/// Declares a type as the lock level for some type `T` that exposes locked
/// state of type `Self::Data`.
///
/// If `T` implements [`OrderedLockAccess`] for `Self::Data`, then the
/// [`LockFor`] and [`RwLockFor`] traits can be used to gain access to the
/// protected state `Data` within `T` at lock level `Self`.
///
/// See [`OrderedLockAccess`] for more details.
pub trait LockLevelFor<T> {
/// The data type within `T` that this is a lock level for.
type Data;
}
/// Abstracts an exclusive lock (i.e. a Mutex).
pub trait ExclusiveLock<T>: 'static {
/// The guard type returned when locking the lock.
type Guard<'l>: DerefMut<Target = T>;
/// Locks this lock.
fn lock(&self) -> Self::Guard<'_>;
}
/// Abstracts a read write lock (i.e. an RwLock).
pub trait ReadWriteLock<T>: 'static {
/// The guard type returned when locking for reads (i.e. shared).
type ReadGuard<'l>: Deref<Target = T>;
/// The guard type returned when locking for writes (i.e. exclusive).
type WriteGuard<'l>: DerefMut<Target = T>;
/// Locks this lock for reading.
fn read_lock(&self) -> Self::ReadGuard<'_>;
/// Locks this lock for writing.
fn write_lock(&self) -> Self::WriteGuard<'_>;
}
impl<L, T> LockFor<L> for T
where
L: LockLevelFor<T>,
T: OrderedLockAccess<L::Data>,
T::Lock: ExclusiveLock<L::Data>,
{
type Data = L::Data;
type Guard<'l> = <T::Lock as ExclusiveLock<L::Data>>::Guard<'l>
where
Self: 'l;
fn lock(&self) -> Self::Guard<'_> {
let OrderedLockRef(lock) = self.ordered_lock_access();
lock.lock()
}
}
impl<L, T> RwLockFor<L> for T
where
L: LockLevelFor<T>,
T: OrderedLockAccess<L::Data>,
T::Lock: ReadWriteLock<L::Data>,
{
type Data = L::Data;
type ReadGuard<'l> = <T::Lock as ReadWriteLock<L::Data>>::ReadGuard<'l>
where
Self: 'l;
type WriteGuard<'l> = <T::Lock as ReadWriteLock<L::Data>>::WriteGuard<'l>
where
Self: 'l;
fn read_lock(&self) -> Self::ReadGuard<'_> {
let OrderedLockRef(lock) = self.ordered_lock_access();
lock.read_lock()
}
fn write_lock(&self) -> Self::WriteGuard<'_> {
let OrderedLockRef(lock) = self.ordered_lock_access();
lock.write_lock()
}
}
/// Declares a type that is an [`UnlockedAccess`] marker for some field `Data`
/// within `T`.
///
/// This is the equivalent of [`LockLevelFor`] for [`UnlockedAccess`], but given
/// unlocked access is freely available through borrows the foreign type can
/// safely expose a getter.
pub trait UnlockedAccessMarkerFor<T> {
/// The data type within `T` that this an unlocked access marker for.
type Data: 'static;
/// Retrieves `Self::Data` from `T`.
fn unlocked_access(t: &T) -> &Self::Data;
}
impl<L, T> UnlockedAccess<L> for T
where
L: UnlockedAccessMarkerFor<T>,
{
type Data = <L as UnlockedAccessMarkerFor<T>>::Data;
type Guard<'l> = &'l <L as UnlockedAccessMarkerFor<T>>::Data
where
Self: 'l;
fn access(&self) -> Self::Guard<'_> {
L::unlocked_access(self)
}
}
#[cfg(test)]
mod example {
//! Example implementations of the traits in this crate.
use std::sync::{Mutex, MutexGuard, RwLock, RwLockReadGuard, RwLockWriteGuard};
use super::*;
enum LockLevel {}
impl<T> LockFor<LockLevel> for Mutex<T> {
type Data = T;
type Guard<'l> = MutexGuard<'l, T> where Self: 'l;
fn lock(&self) -> Self::Guard<'_> {
self.lock().unwrap()
}
}
impl<T> RwLockFor<LockLevel> for RwLock<T> {
type Data = T;
type ReadGuard<'l> = RwLockReadGuard<'l, T> where Self: 'l;
type WriteGuard<'l> = RwLockWriteGuard<'l, T> where Self: 'l;
fn read_lock(&self) -> Self::ReadGuard<'_> {
self.read().unwrap()
}
fn write_lock(&self) -> Self::WriteGuard<'_> {
self.write().unwrap()
}
}
}