add InteriorMutableWorld with resource access methods

Author: jakobhellermann

crates/bevy_ecs/src/world/interior_mutable_world.rs

@@ -0,0 +1,157 @@
+#![warn(unsafe_op_in_unsafe_fn)]
+
+use super::{Mut, World};
+use crate::{
+    change_detection::{MutUntyped, Ticks},
+    component::ComponentId,
+    system::Resource,
+};
+use bevy_ptr::Ptr;
+use bevy_ptr::UnsafeCellDeref;
+use std::any::TypeId;
+
+/// Variant of the [`World`] where resource and component accesses takes a `&World`, and the responsibility to avoid
+/// aliasing violations are given to the caller instead of being checked at compile-time by rust's unique XOR shared rule.
+///
+/// ### Rationale
+/// In rust, having a `&mut World` means that there are absolutely no other references to the safe world alive at the same time,
+/// without exceptions. Not even unsafe code can change this.
+///
+/// But there are situations where careful shared mutable access through a type is possible and safe. For this, rust provides the [`UnsafeCell`](std::cell::UnsafeCell)
+/// escape hatch, which allows you to get a `*mut T` from a `&UnsafeCell<T>` and around which safe abstractions can be built.
+///
+/// Access to resources and components can be done uniquely using [`World::resource_mut`] and [`World::entity_mut`], and shared using [`World::resource`] and [`World::entity`].
+/// These methods use lifetimes to check at compile time that no aliasing rules are being broken.
+///
+/// This alone is not enough to implement bevy systems where multiple systems can access *disjoint* parts of the world concurrently. For this, bevy stores all values of
+/// resources and components (and [`ComponentTicks`](crate::component::ComponentTicks)) in [`UnsafeCell`](std::cell::UnsafeCell)s, and carefully validates disjoint access patterns using
+/// APIs like [`System::component_access`](crate::system::System::component_access).
+///
+/// A system then can be executed using [`System::run_unsafe`](crate::system::System::run_unsafe) with a `&World` and use methods with interior mutability to access resource values.
+/// access resource values.
+///
+/// ### Example Usage
+///
+/// [`InteriorMutableWorld`] can be used as a building block for writing APIs that safely allow disjoint access into the world.
+/// In the following example, the world is split into a resource access half and a component access half, where each one can
+/// safely hand out mutable references.
+///
+/// ```
+/// use bevy_ecs::world::World;
+/// use bevy_ecs::change_detection::Mut;
+/// use bevy_ecs::system::Resource;
+/// use bevy_ecs::world::interior_mutable_world::InteriorMutableWorld;
+///
+/// // INVARIANT: existance of this struct means that users of it are the only ones being able to access resources in the world
+/// struct OnlyResourceAccessWorld<'w>(InteriorMutableWorld<'w>);
+/// // INVARIANT: existance of this struct means that users of it are the only ones being able to access components in the world
+/// struct OnlyComponentAccessWorld<'w>(InteriorMutableWorld<'w>);
+///
+/// impl<'w> OnlyResourceAccessWorld<'w> {
+///     fn get_resource_mut<T: Resource>(&mut self) -> Option<Mut<'w, T>> {
+///         // SAFETY: resource access is allowed through this InteriorMutableWorld
+///         unsafe { self.0.get_resource_mut::<T>() }
+///     }
+/// }
+/// // impl<'w> OnlyComponentAccessWorld<'w> {
+/// //     ...
+/// // }
+///
+/// // the two interior mutable worlds borrow from the `&mut World`, so it cannot be accessed while they are live
+/// fn split_world_access(world: &mut World) -> (OnlyResourceAccessWorld<'_>, OnlyComponentAccessWorld<'_>) {
+///     let resource_access = OnlyResourceAccessWorld(unsafe { world.as_interior_mutable() });
+///     let component_access = OnlyComponentAccessWorld(unsafe { world.as_interior_mutable() });
+///     (resource_access, component_access)
+/// }
+/// ```
+#[derive(Copy, Clone)]
+pub struct InteriorMutableWorld<'w>(&'w World);
+
+impl<'w> InteriorMutableWorld<'w> {
+    pub(crate) fn new(world: &'w World) -> Self {
+        InteriorMutableWorld(world)
+    }
+
+    /// Gets a reference to the [`&World`](crate::world::World) this [`InteriorMutableWorld`] belongs to.
+    /// This can be used to call methods like [`World::read_change_tick`] which aren't exposed here but don't perform any accesses.
+    ///
+    /// **Note**: You *must not* hand out a `&World` reference to arbitrary safe code when the [`InteriorMutableWorld`] is currently
+    /// being used for mutable accesses.
+    pub unsafe fn world(&self) -> &'w World {
+        self.0
+    }
+
+    /// Gets a reference to the resource of the given type if it exists
+    #[inline]
+    pub unsafe fn get_resource<R: Resource>(&self) -> Option<&'w R> {
+        self.0.get_resource::<R>()
+    }
+
+    /// Gets a pointer to the resource with the id [`ComponentId`] if it exists.
+    /// The returned pointer must not be used to modify the resource, and must not be
+    /// dereferenced after the borrow of the [`World`] ends.
+    ///
+    /// **You should prefer to use the typed API [`InteriorMutableWorld::get_resource`] where possible and only
+    /// use this in cases where the actual types are not known at compile time.**
+    #[inline]
+    pub unsafe fn get_resource_by_id(&self, component_id: ComponentId) -> Option<Ptr<'w>> {
+        self.0.get_resource_by_id(component_id)
+    }
+
+    /// Gets a mutable reference to the resource of the given type if it exists
+    #[inline]
+    pub unsafe fn get_resource_mut<R: Resource>(&self) -> Option<Mut<'w, R>> {
+        let component_id = self.0.components.get_resource_id(TypeId::of::<R>())?;
+        unsafe { self.0.get_resource_unchecked_mut_with_id(component_id) }
+    }
+
+    /// Gets a pointer to the resource with the id [`ComponentId`] if it exists.
+    /// The returned pointer may be used to modify the resource, as long as the mutable borrow
+    /// of the [`InteriorMutableWorld`] is still valid.
+    ///
+    /// **You should prefer to use the typed API [`InteriorMutableWorld::get_resource_mut`] where possible and only
+    /// use this in cases where the actual types are not known at compile time.**
+    #[inline]
+    pub unsafe fn get_resource_mut_by_id(
+        &self,
+        component_id: ComponentId,
+    ) -> Option<MutUntyped<'w>> {
+        let info = self.0.components.get_info(component_id)?;
+        if !info.is_send_and_sync() {
+            self.0.validate_non_send_access_untyped(info.name());
+        }
+
+        let (ptr, ticks) = self.0.get_resource_with_ticks(component_id)?;
+
+        // SAFE: This function has exclusive access to the world so nothing aliases `ticks`.
+        let ticks = Ticks {
+            // SAFETY:
+            // - index is in-bounds because the column is initialized and non-empty
+            // - no other reference to the ticks of the same row can exist at the same time
+            component_ticks: unsafe { ticks.deref_mut() },
+            last_change_tick: self.0.last_change_tick(),
+            change_tick: self.0.read_change_tick(),
+        };
+
+        Some(MutUntyped {
+            // SAFETY: This function has exclusive access to the world so nothing aliases `ptr`.
+            value: unsafe { ptr.assert_unique() },
+            ticks,
+        })
+    }
+
+    /// Gets a reference to the non-send resource of the given type, if it exists.
+    /// Otherwise returns [None]
+    #[inline]
+    pub unsafe fn get_non_send_resource<R: 'static>(&self) -> Option<&R> {
+        self.0.get_non_send_resource::<R>()
+    }
+    /// Gets a mutable reference to the non-send resource of the given type, if it exists.
+    /// Otherwise returns [None]
+    #[inline]
+    pub unsafe fn get_non_send_resource_mut<R: 'static>(&mut self) -> Option<Mut<'w, R>> {
+        let component_id = self.0.components.get_resource_id(TypeId::of::<R>())?;
+        // SAFETY: safety requirement is deferred to the caller
+        unsafe { self.0.get_non_send_unchecked_mut_with_id(component_id) }
+    }
+}

crates/bevy_ecs/src/world/mod.rs

@@ -1,4 +1,5 @@
 mod entity_ref;
+pub mod interior_mutable_world;
 mod spawn_batch;
 mod world_cell;
 
@@ -31,6 +32,8 @@ mod identifier;
 
 pub use identifier::WorldId;
 
+use self::interior_mutable_world::InteriorMutableWorld;
+
 /// Stores and exposes operations on [entities](Entity), [components](Component), resources,
 /// and their associated metadata.
 ///
@@ -103,6 +106,10 @@ impl World {
         self.id
     }
 
+    pub fn as_interior_mutable(&self) -> InteriorMutableWorld<'_> {
+        InteriorMutableWorld::new(self)
+    }
+
     /// Retrieves this world's [Entities] collection
     #[inline]
     pub fn entities(&self) -> &Entities {
@@ -1429,7 +1436,7 @@ impl World {
 }
 
 impl World {
-    /// Gets a resource to the resource with the id [`ComponentId`] if it exists.
+    /// Gets a pointer to the resource with the id [`ComponentId`] if it exists.
     /// The returned pointer must not be used to modify the resource, and must not be
     /// dereferenced after the immutable borrow of the [`World`] ends.
     ///
@@ -1444,7 +1451,7 @@ impl World {
         self.storages.resources.get(component_id)?.get_data()
     }
 
-    /// Gets a resource to the resource with the id [`ComponentId`] if it exists.
+    /// Gets a pointer to the resource with the id [`ComponentId`] if it exists.
     /// The returned pointer may be used to modify the resource, as long as the mutable borrow
     /// of the [`World`] is still valid.
     ///