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use crate::system::{SystemBuffer, SystemMeta};
use std::{
fmt::Debug,
mem::MaybeUninit,
panic::{self, AssertUnwindSafe},
ptr::{addr_of_mut, NonNull},
};
use bevy_ptr::{OwningPtr, Unaligned};
use bevy_utils::tracing::warn;
use crate::world::{Command, World};
use super::DeferredWorld;
struct CommandMeta {
/// SAFETY: The `value` must point to a value of type `T: Command`,
/// where `T` is some specific type that was used to produce this metadata.
///
/// `world` is optional to allow this one function pointer to perform double-duty as a drop.
///
/// Advances `cursor` by the size of `T` in bytes.
consume_command_and_get_size:
unsafe fn(value: OwningPtr<Unaligned>, world: Option<NonNull<World>>, cursor: &mut usize),
}
/// Densely and efficiently stores a queue of heterogenous types implementing [`Command`].
//
// NOTE: [`CommandQueue`] is implemented via a `Vec<MaybeUninit<u8>>` instead of a `Vec<Box<dyn Command>>`
// as an optimization. Since commands are used frequently in systems as a way to spawn
// entities/components/resources, and it's not currently possible to parallelize these
// due to mutable [`World`] access, maximizing performance for [`CommandQueue`] is
// preferred to simplicity of implementation.
#[derive(Default)]
pub struct CommandQueue {
// This buffer densely stores all queued commands.
//
// For each command, one `CommandMeta` is stored, followed by zero or more bytes
// to store the command itself. To interpret these bytes, a pointer must
// be passed to the corresponding `CommandMeta.apply_command_and_get_size` fn pointer.
pub(crate) bytes: Vec<MaybeUninit<u8>>,
pub(crate) cursor: usize,
pub(crate) panic_recovery: Vec<MaybeUninit<u8>>,
}
/// Wraps pointers to a [`CommandQueue`], used internally to avoid stacked borrow rules when
/// partially applying the world's command queue recursively
#[derive(Clone)]
pub(crate) struct RawCommandQueue {
pub(crate) bytes: NonNull<Vec<MaybeUninit<u8>>>,
pub(crate) cursor: NonNull<usize>,
pub(crate) panic_recovery: NonNull<Vec<MaybeUninit<u8>>>,
}
// CommandQueue needs to implement Debug manually, rather than deriving it, because the derived impl just prints
// [core::mem::maybe_uninit::MaybeUninit<u8>, core::mem::maybe_uninit::MaybeUninit<u8>, ..] for every byte in the vec,
// which gets extremely verbose very quickly, while also providing no useful information.
// It is not possible to soundly print the values of the contained bytes, as some of them may be padding or uninitialized (#4863)
// So instead, the manual impl just prints the length of vec.
impl Debug for CommandQueue {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
f.debug_struct("CommandQueue")
.field("len_bytes", &self.bytes.len())
.finish_non_exhaustive()
}
}
// SAFETY: All commands [`Command`] implement [`Send`]
unsafe impl Send for CommandQueue {}
// SAFETY: `&CommandQueue` never gives access to the inner commands.
unsafe impl Sync for CommandQueue {}
impl CommandQueue {
/// Push a [`Command`] onto the queue.
#[inline]
pub fn push<C>(&mut self, command: C)
where
C: Command,
{
// SAFETY: self is guaranteed to live for the lifetime of this method
unsafe {
self.get_raw().push(command);
}
}
/// Execute the queued [`Command`]s in the world after applying any commands in the world's internal queue.
/// This clears the queue.
#[inline]
pub fn apply(&mut self, world: &mut World) {
// flush the previously queued entities
world.flush_entities();
// flush the world's internal queue
world.flush_commands();
// SAFETY: A reference is always a valid pointer
unsafe {
self.get_raw().apply_or_drop_queued(Some(world.into()));
}
}
/// Take all commands from `other` and append them to `self`, leaving `other` empty
pub fn append(&mut self, other: &mut CommandQueue) {
self.bytes.append(&mut other.bytes);
}
/// Returns false if there are any commands in the queue
#[inline]
pub fn is_empty(&self) -> bool {
self.cursor >= self.bytes.len()
}
/// Returns a [`RawCommandQueue`] instance sharing the underlying command queue.
pub(crate) fn get_raw(&mut self) -> RawCommandQueue {
// SAFETY: self is always valid memory
unsafe {
RawCommandQueue {
bytes: NonNull::new_unchecked(addr_of_mut!(self.bytes)),
cursor: NonNull::new_unchecked(addr_of_mut!(self.cursor)),
panic_recovery: NonNull::new_unchecked(addr_of_mut!(self.panic_recovery)),
}
}
}
}
impl RawCommandQueue {
/// Returns a new `RawCommandQueue` instance, this must be manually dropped.
pub(crate) fn new() -> Self {
// SAFETY: Pointers returned by `Box::into_raw` are guaranteed to be non null
unsafe {
Self {
bytes: NonNull::new_unchecked(Box::into_raw(Box::default())),
cursor: NonNull::new_unchecked(Box::into_raw(Box::new(0usize))),
panic_recovery: NonNull::new_unchecked(Box::into_raw(Box::default())),
}
}
}
/// Returns true if the queue is empty.
///
/// # Safety
///
/// * Caller ensures that `bytes` and `cursor` point to valid memory
pub unsafe fn is_empty(&self) -> bool {
// SAFETY: Pointers are guaranteed to be valid by requirements on `.clone_unsafe`
(unsafe { *self.cursor.as_ref() }) >= (unsafe { self.bytes.as_ref() }).len()
}
/// Push a [`Command`] onto the queue.
///
/// # Safety
///
/// * Caller ensures that `self` has not outlived the underlying queue
#[inline]
pub unsafe fn push<C>(&mut self, command: C)
where
C: Command,
{
// Stores a command alongside its metadata.
// `repr(C)` prevents the compiler from reordering the fields,
// while `repr(packed)` prevents the compiler from inserting padding bytes.
#[repr(C, packed)]
struct Packed<T: Command> {
meta: CommandMeta,
command: T,
}
let meta = CommandMeta {
consume_command_and_get_size: |command, world, cursor| {
*cursor += std::mem::size_of::<C>();
// SAFETY: According to the invariants of `CommandMeta.consume_command_and_get_size`,
// `command` must point to a value of type `C`.
let command: C = unsafe { command.read_unaligned() };
match world {
// Apply command to the provided world...
Some(mut world) => {
// SAFETY: Caller ensures pointer is not null
let world = unsafe { world.as_mut() };
command.apply(world);
// The command may have queued up world commands, which we flush here to ensure they are also picked up.
// If the current command queue already the World Command queue, this will still behave appropriately because the global cursor
// is still at the current `stop`, ensuring only the newly queued Commands will be applied.
world.flush();
}
// ...or discard it.
None => drop(command),
}
},
};
// SAFETY: There are no outstanding references to self.bytes
let bytes = unsafe { self.bytes.as_mut() };
let old_len = bytes.len();
// Reserve enough bytes for both the metadata and the command itself.
bytes.reserve(std::mem::size_of::<Packed<C>>());
// Pointer to the bytes at the end of the buffer.
// SAFETY: We know it is within bounds of the allocation, due to the call to `.reserve()`.
let ptr = unsafe { bytes.as_mut_ptr().add(old_len) };
// Write the metadata into the buffer, followed by the command.
// We are using a packed struct to write them both as one operation.
// SAFETY: `ptr` must be non-null, since it is within a non-null buffer.
// The call to `reserve()` ensures that the buffer has enough space to fit a value of type `C`,
// and it is valid to write any bit pattern since the underlying buffer is of type `MaybeUninit<u8>`.
unsafe {
ptr.cast::<Packed<C>>()
.write_unaligned(Packed { meta, command });
}
// Extend the length of the buffer to include the data we just wrote.
// SAFETY: The new length is guaranteed to fit in the vector's capacity,
// due to the call to `.reserve()` above.
unsafe {
bytes.set_len(old_len + std::mem::size_of::<Packed<C>>());
}
}
/// If `world` is [`Some`], this will apply the queued [commands](`Command`).
/// If `world` is [`None`], this will drop the queued [commands](`Command`) (without applying them).
/// This clears the queue.
///
/// # Safety
///
/// * Caller ensures that `self` has not outlived the underlying queue
#[inline]
pub(crate) unsafe fn apply_or_drop_queued(&mut self, world: Option<NonNull<World>>) {
// SAFETY: If this is the command queue on world, world will not be dropped as we have a mutable reference
// If this is not the command queue on world we have exclusive ownership and self will not be mutated
let start = *self.cursor.as_ref();
let stop = self.bytes.as_ref().len();
let mut local_cursor = start;
// SAFETY: we are setting the global cursor to the current length to prevent the executing commands from applying
// the remaining commands currently in this list. This is safe.
*self.cursor.as_mut() = stop;
while local_cursor < stop {
// SAFETY: The cursor is either at the start of the buffer, or just after the previous command.
// Since we know that the cursor is in bounds, it must point to the start of a new command.
let meta = unsafe {
self.bytes
.as_mut()
.as_mut_ptr()
.add(local_cursor)
.cast::<CommandMeta>()
.read_unaligned()
};
// Advance to the bytes just after `meta`, which represent a type-erased command.
local_cursor += std::mem::size_of::<CommandMeta>();
// Construct an owned pointer to the command.
// SAFETY: It is safe to transfer ownership out of `self.bytes`, since the increment of `cursor` above
// guarantees that nothing stored in the buffer will get observed after this function ends.
// `cmd` points to a valid address of a stored command, so it must be non-null.
let cmd = unsafe {
OwningPtr::<Unaligned>::new(std::ptr::NonNull::new_unchecked(
self.bytes.as_mut().as_mut_ptr().add(local_cursor).cast(),
))
};
let result = panic::catch_unwind(AssertUnwindSafe(|| {
// SAFETY: The data underneath the cursor must correspond to the type erased in metadata,
// since they were stored next to each other by `.push()`.
// For ZSTs, the type doesn't matter as long as the pointer is non-null.
// This also advances the cursor past the command. For ZSTs, the cursor will not move.
// At this point, it will either point to the next `CommandMeta`,
// or the cursor will be out of bounds and the loop will end.
unsafe { (meta.consume_command_and_get_size)(cmd, world, &mut local_cursor) };
}));
if let Err(payload) = result {
// local_cursor now points to the location _after_ the panicked command.
// Add the remaining commands that _would have_ been applied to the
// panic_recovery queue.
//
// This uses `current_stop` instead of `stop` to account for any commands
// that were queued _during_ this panic.
//
// This is implemented in such a way that if apply_or_drop_queued() are nested recursively in,
// an applied Command, the correct command order will be retained.
let panic_recovery = self.panic_recovery.as_mut();
let bytes = self.bytes.as_mut();
let current_stop = bytes.len();
panic_recovery.extend_from_slice(&bytes[local_cursor..current_stop]);
bytes.set_len(start);
*self.cursor.as_mut() = start;
// This was the "top of the apply stack". If we are _not_ at the top of the apply stack,
// when we call`resume_unwind" the caller "closer to the top" will catch the unwind and do this check,
// until we reach the top.
if start == 0 {
bytes.append(panic_recovery);
}
panic::resume_unwind(payload);
}
}
// Reset the buffer: all commands past the original `start` cursor have been applied.
// SAFETY: we are setting the length of bytes to the original length, minus the length of the original
// list of commands being considered. All bytes remaining in the Vec are still valid, unapplied commands.
unsafe {
self.bytes.as_mut().set_len(start);
*self.cursor.as_mut() = start;
};
}
}
impl Drop for CommandQueue {
fn drop(&mut self) {
if !self.bytes.is_empty() {
warn!("CommandQueue has un-applied commands being dropped.");
}
// SAFETY: A reference is always a valid pointer
unsafe { self.get_raw().apply_or_drop_queued(None) };
}
}
impl SystemBuffer for CommandQueue {
#[inline]
fn apply(&mut self, _system_meta: &SystemMeta, world: &mut World) {
#[cfg(feature = "trace")]
let _span_guard = _system_meta.commands_span.enter();
self.apply(world);
}
#[inline]
fn queue(&mut self, _system_meta: &SystemMeta, mut world: DeferredWorld) {
world.commands().append(self);
}
}
#[cfg(test)]
mod test {
use super::*;
use crate as bevy_ecs;
use crate::system::Resource;
use std::{
panic::AssertUnwindSafe,
sync::{
atomic::{AtomicU32, Ordering},
Arc,
},
};
struct DropCheck(Arc<AtomicU32>);
impl DropCheck {
fn new() -> (Self, Arc<AtomicU32>) {
let drops = Arc::new(AtomicU32::new(0));
(Self(drops.clone()), drops)
}
}
impl Drop for DropCheck {
fn drop(&mut self) {
self.0.fetch_add(1, Ordering::Relaxed);
}
}
impl Command for DropCheck {
fn apply(self, _: &mut World) {}
}
#[test]
fn test_command_queue_inner_drop() {
let mut queue = CommandQueue::default();
let (dropcheck_a, drops_a) = DropCheck::new();
let (dropcheck_b, drops_b) = DropCheck::new();
queue.push(dropcheck_a);
queue.push(dropcheck_b);
assert_eq!(drops_a.load(Ordering::Relaxed), 0);
assert_eq!(drops_b.load(Ordering::Relaxed), 0);
let mut world = World::new();
queue.apply(&mut world);
assert_eq!(drops_a.load(Ordering::Relaxed), 1);
assert_eq!(drops_b.load(Ordering::Relaxed), 1);
}
/// Asserts that inner [commands](`Command`) are dropped on early drop of [`CommandQueue`].
/// Originally identified as an issue in [#10676](https://github.com/bevyengine/bevy/issues/10676)
#[test]
fn test_command_queue_inner_drop_early() {
let mut queue = CommandQueue::default();
let (dropcheck_a, drops_a) = DropCheck::new();
let (dropcheck_b, drops_b) = DropCheck::new();
queue.push(dropcheck_a);
queue.push(dropcheck_b);
assert_eq!(drops_a.load(Ordering::Relaxed), 0);
assert_eq!(drops_b.load(Ordering::Relaxed), 0);
drop(queue);
assert_eq!(drops_a.load(Ordering::Relaxed), 1);
assert_eq!(drops_b.load(Ordering::Relaxed), 1);
}
struct SpawnCommand;
impl Command for SpawnCommand {
fn apply(self, world: &mut World) {
world.spawn_empty();
}
}
#[test]
fn test_command_queue_inner() {
let mut queue = CommandQueue::default();
queue.push(SpawnCommand);
queue.push(SpawnCommand);
let mut world = World::new();
queue.apply(&mut world);
assert_eq!(world.entities().len(), 2);
// The previous call to `apply` cleared the queue.
// This call should do nothing.
queue.apply(&mut world);
assert_eq!(world.entities().len(), 2);
}
// This has an arbitrary value `String` stored to ensure
// when then command gets pushed, the `bytes` vector gets
// some data added to it.
#[allow(dead_code)]
struct PanicCommand(String);
impl Command for PanicCommand {
fn apply(self, _: &mut World) {
panic!("command is panicking");
}
}
#[test]
fn test_command_queue_inner_panic_safe() {
std::panic::set_hook(Box::new(|_| {}));
let mut queue = CommandQueue::default();
queue.push(PanicCommand("I panic!".to_owned()));
queue.push(SpawnCommand);
let mut world = World::new();
let _ = std::panic::catch_unwind(AssertUnwindSafe(|| {
queue.apply(&mut world);
}));
// Even though the first command panicked, it's still ok to push
// more commands.
queue.push(SpawnCommand);
queue.push(SpawnCommand);
queue.apply(&mut world);
assert_eq!(world.entities().len(), 3);
}
#[test]
fn test_command_queue_inner_nested_panic_safe() {
std::panic::set_hook(Box::new(|_| {}));
#[derive(Resource, Default)]
struct Order(Vec<usize>);
let mut world = World::new();
world.init_resource::<Order>();
fn add_index(index: usize) -> impl Command {
move |world: &mut World| world.resource_mut::<Order>().0.push(index)
}
world.commands().add(add_index(1));
world.commands().add(|world: &mut World| {
world.commands().add(add_index(2));
world.commands().add(PanicCommand("I panic!".to_owned()));
world.commands().add(add_index(3));
world.flush_commands();
});
world.commands().add(add_index(4));
let _ = std::panic::catch_unwind(AssertUnwindSafe(|| {
world.flush_commands();
}));
world.commands().add(add_index(5));
world.flush_commands();
assert_eq!(&world.resource::<Order>().0, &[1, 2, 3, 4, 5]);
}
// NOTE: `CommandQueue` is `Send` because `Command` is send.
// If the `Command` trait gets reworked to be non-send, `CommandQueue`
// should be reworked.
// This test asserts that Command types are send.
fn assert_is_send_impl(_: impl Send) {}
fn assert_is_send(command: impl Command) {
assert_is_send_impl(command);
}
#[test]
fn test_command_is_send() {
assert_is_send(SpawnCommand);
}
#[allow(dead_code)]
struct CommandWithPadding(u8, u16);
impl Command for CommandWithPadding {
fn apply(self, _: &mut World) {}
}
#[cfg(miri)]
#[test]
fn test_uninit_bytes() {
let mut queue = CommandQueue::default();
queue.push(CommandWithPadding(0, 0));
let _ = format!("{:?}", queue.bytes);
}
}