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use core::cell::Cell;
use core::fmt;
use core::future::Future;
use core::mem::ManuallyDrop;
use core::ops;
use core::pin::Pin;
use core::ptr;
use core::task::{Context, Poll};
use crate::runtime::{AnyObjError, RawStr};
/// Bitflag which if set indicates that the accessed value is an external
/// reference (exclusive or not).
const IS_REF_MASK: isize = 1isize;
/// Sentinel value to indicate that access is taken.
const TAKEN: isize = (isize::max_value() ^ IS_REF_MASK) >> 1;
/// Panic if we reach this number of shared accesses and we try to add one more,
/// since it's the largest we can support.
const MAX_USES: isize = 0b11isize.rotate_right(2);
/// An error raised while downcasting.
#[derive(Debug)]
#[allow(missing_docs)]
#[non_exhaustive]
pub enum AccessError {
UnexpectedType { expected: RawStr, actual: RawStr },
NotAccessibleRef { error: NotAccessibleRef },
NotAccessibleMut { error: NotAccessibleMut },
NotAccessibleTake { error: NotAccessibleTake },
AnyObjError { error: AnyObjError },
}
cfg_std! {
impl std::error::Error for AccessError {
fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
match self {
AccessError::NotAccessibleRef { error, .. } => Some(error),
AccessError::NotAccessibleMut { error, .. } => Some(error),
AccessError::NotAccessibleTake { error, .. } => Some(error),
AccessError::AnyObjError { error, .. } => Some(error),
_ => None,
}
}
}
}
impl fmt::Display for AccessError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
match self {
AccessError::UnexpectedType { expected, actual } => write!(
f,
"Expected data of type `{expected}`, but found `{actual}`",
),
AccessError::NotAccessibleRef { error } => error.fmt(f),
AccessError::NotAccessibleMut { error } => error.fmt(f),
AccessError::NotAccessibleTake { error } => error.fmt(f),
AccessError::AnyObjError { error } => error.fmt(f),
}
}
}
impl From<NotAccessibleRef> for AccessError {
#[inline]
fn from(error: NotAccessibleRef) -> Self {
AccessError::NotAccessibleRef { error }
}
}
impl From<NotAccessibleMut> for AccessError {
#[inline]
fn from(error: NotAccessibleMut) -> Self {
AccessError::NotAccessibleMut { error }
}
}
impl From<NotAccessibleTake> for AccessError {
#[inline]
fn from(error: NotAccessibleTake) -> Self {
AccessError::NotAccessibleTake { error }
}
}
impl From<AnyObjError> for AccessError {
#[inline]
fn from(source: AnyObjError) -> Self {
AccessError::AnyObjError { error: source }
}
}
/// Error raised when tried to access for shared access but it was not
/// accessible.
#[derive(Debug)]
pub struct NotAccessibleRef(Snapshot);
impl fmt::Display for NotAccessibleRef {
#[inline]
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "Cannot read, value is {}", self.0)
}
}
cfg_std! {
impl std::error::Error for NotAccessibleRef {}
}
/// Error raised when tried to access for exclusive access but it was not
/// accessible.
#[derive(Debug)]
pub struct NotAccessibleMut(Snapshot);
impl fmt::Display for NotAccessibleMut {
#[inline]
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "Cannot write, value is {}", self.0)
}
}
cfg_std! {
impl std::error::Error for NotAccessibleMut {}
}
/// Error raised when tried to access the guarded data for taking.
///
/// This requires exclusive access, but it's a scenario we structure separately
/// for diagnostics purposes.
#[derive(Debug)]
pub struct NotAccessibleTake(Snapshot);
impl fmt::Display for NotAccessibleTake {
#[inline]
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "Cannot take, value is {}", self.0)
}
}
cfg_std! {
impl std::error::Error for NotAccessibleTake {}
}
/// The kind of access to perform.
#[derive(Debug, Clone, Copy)]
pub(crate) enum AccessKind {
/// Access a reference.
Any,
/// Access something owned.
Owned,
}
/// Snapshot that can be used to indicate how the value was being accessed at
/// the time of an error.
#[derive(Debug)]
#[repr(transparent)]
struct Snapshot(isize);
impl fmt::Display for Snapshot {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self.0 >> 1 {
0 => write!(f, "fully accessible")?,
1 => write!(f, "exclusively accessed")?,
TAKEN => write!(f, "moved")?,
n if n < 0 => write!(f, "shared by {}", -n)?,
n => write!(f, "invalidly marked ({})", n)?,
}
if self.0 & IS_REF_MASK == 1 {
write!(f, " (ref)")?;
}
Ok(())
}
}
/// Access flags.
///
/// These accomplish the following things:
/// * Indicates if a value is a reference.
/// * Indicates if a value is exclusively held.
/// * Indicates if a value is shared, and if so by how many.
///
/// It has the following bit-pattern (assume isize is 16 bits for simplicity):
///
/// ```text
/// S0000000_00000000_00000000_0000000F
/// | ||
/// '-- Sign bit and number base ----'|
/// Reference Flag -'
///
/// The reference flag is the LSB, and the rest is treated as a signed number
/// with the following properties:
/// * If the value is `0`, it is not being accessed.
/// * If the value is `1`, it is being exclusively accessed.
/// * If the value is negative `n`, it is being shared accessed by `-n` uses.
///
/// This means that the maximum number of accesses for a 64-bit `isize` is
/// `(1 << 62) - 1` uses.
///
/// ```
#[repr(transparent)]
pub(crate) struct Access(Cell<isize>);
impl Access {
/// Construct a new default access.
pub(crate) const fn new(is_ref: bool) -> Self {
let initial = if is_ref { 1 } else { 0 };
Self(Cell::new(initial))
}
/// Test if access is guarding a reference.
#[inline]
pub(crate) fn is_ref(&self) -> bool {
self.0.get() & IS_REF_MASK != 0
}
/// Test if we can have shared access without modifying the internal count.
#[inline]
pub(crate) fn is_shared(&self) -> bool {
self.get() <= 0
}
/// Test if we can have exclusive access without modifying the internal
/// count.
#[inline]
pub(crate) fn is_exclusive(&self) -> bool {
self.get() == 0
}
/// Test if the data has been taken.
#[inline]
pub(crate) fn is_taken(&self) -> bool {
self.get() == TAKEN
}
/// Mark that we want shared access to the given access token.
///
/// # Safety
///
/// The returned guard must not outlive the access token that created it.
#[inline]
pub(crate) unsafe fn shared(
&self,
kind: AccessKind,
) -> Result<AccessGuard<'_>, NotAccessibleRef> {
if let AccessKind::Owned = kind {
if self.is_ref() {
return Err(NotAccessibleRef(Snapshot(self.0.get())));
}
}
let state = self.get();
if state == MAX_USES {
crate::alloc::abort();
}
let n = state.wrapping_sub(1);
if n >= 0 {
return Err(NotAccessibleRef(Snapshot(self.0.get())));
}
self.set(n);
Ok(AccessGuard(self))
}
/// Mark that we want exclusive access to the given access token.
///
/// # Safety
///
/// The returned guard must not outlive the access token that created it.
#[inline]
pub(crate) unsafe fn exclusive(
&self,
kind: AccessKind,
) -> Result<AccessGuard<'_>, NotAccessibleMut> {
if let AccessKind::Owned = kind {
if self.is_ref() {
return Err(NotAccessibleMut(Snapshot(self.0.get())));
}
}
let n = self.get();
if n != 0 {
return Err(NotAccessibleMut(Snapshot(self.0.get())));
}
self.set(n.wrapping_add(1));
Ok(AccessGuard(self))
}
/// Mark that we want to mark the given access as "taken".
///
/// I.e. whatever guarded data is no longer available.
///
/// # Safety
///
/// The returned guard must not outlive the access token that created it.
#[inline]
pub(crate) unsafe fn take(&self, kind: AccessKind) -> Result<RawTakeGuard, NotAccessibleTake> {
if let AccessKind::Owned = kind {
if self.is_ref() {
return Err(NotAccessibleTake(Snapshot(self.0.get())));
}
}
let state = self.get();
if state != 0 {
return Err(NotAccessibleTake(Snapshot(self.0.get())));
}
self.set(TAKEN);
Ok(RawTakeGuard { access: self })
}
/// Release the current access level.
#[inline]
fn release(&self) {
let b = self.get();
let b = if b < 0 {
debug_assert!(b < 0);
b.wrapping_add(1)
} else {
debug_assert_eq!(b, 1, "borrow value should be exclusive (0)");
b.wrapping_sub(1)
};
self.set(b);
}
/// Untake the current access.
#[inline]
fn release_take(&self) {
let b = self.get();
debug_assert_eq!(b, TAKEN, "borrow value should be TAKEN ({})", TAKEN);
self.set(0);
}
/// Get the current value of the flag.
#[inline]
fn get(&self) -> isize {
self.0.get() >> 1
}
/// Set the current value of the flag.
#[inline]
fn set(&self, value: isize) {
self.0.set(self.0.get() & IS_REF_MASK | value << 1);
}
}
impl fmt::Debug for Access {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "{}", Snapshot(self.get()))
}
}
/// Guard for a data borrowed from a slot in the virtual machine.
///
/// These guards are necessary, since we need to guarantee certain forms of
/// access depending on what we do. Releasing the guard releases the access.
pub struct BorrowRef<'a, T: ?Sized + 'a> {
data: &'a T,
guard: AccessGuard<'a>,
}
impl<'a, T: ?Sized> BorrowRef<'a, T> {
/// Construct a new shared guard.
///
/// # Safety
///
/// since this has implications for releasing access, the caller must
/// ensure that access has been acquired correctly using e.g.
/// [Access::shared]. Otherwise access can be release incorrectly once
/// this guard is dropped.
pub(crate) fn new(data: &'a T, access: &'a Access) -> Self {
Self {
data,
guard: AccessGuard(access),
}
}
/// Map the reference.
///
/// # Examples
///
/// ```
/// use rune::runtime::{BorrowRef, Shared};
///
/// let vec = Shared::<Vec<u32>>::new(vec![1, 2, 3, 4])?;
/// let vec = vec.borrow_ref()?;
/// let value: BorrowRef<[u32]> = BorrowRef::map(vec, |vec| &vec[0..2]);
///
/// assert_eq!(&*value, &[1u32, 2u32][..]);
/// # Ok::<_, rune::support::Error>(())
/// ```
pub fn map<M, U: ?Sized>(this: Self, m: M) -> BorrowRef<'a, U>
where
M: FnOnce(&T) -> &U,
{
BorrowRef {
data: m(this.data),
guard: this.guard,
}
}
/// Try to map the reference to a projection.
///
/// # Examples
///
/// ```
/// use rune::runtime::{BorrowRef, Shared};
///
/// let vec = Shared::<Vec<u32>>::new(vec![1, 2, 3, 4])?;
/// let vec = vec.borrow_ref()?;
/// let mut value: Option<BorrowRef<[u32]>> = BorrowRef::try_map(vec, |vec| vec.get(0..2));
///
/// assert_eq!(value.as_deref(), Some(&[1u32, 2u32][..]));
/// # Ok::<_, rune::support::Error>(())
/// ```
pub fn try_map<M, U: ?Sized>(this: Self, m: M) -> Option<BorrowRef<'a, U>>
where
M: FnOnce(&T) -> Option<&U>,
{
Some(BorrowRef {
data: m(this.data)?,
guard: this.guard,
})
}
}
impl<T: ?Sized> ops::Deref for BorrowRef<'_, T> {
type Target = T;
fn deref(&self) -> &Self::Target {
self.data
}
}
impl<T: ?Sized> fmt::Debug for BorrowRef<'_, T>
where
T: fmt::Debug,
{
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, fmt)
}
}
/// A guard around some specific access access.
#[repr(transparent)]
pub struct AccessGuard<'a>(&'a Access);
impl AccessGuard<'_> {
/// Convert into a raw guard which does not have a lifetime associated with
/// it. Droping the raw guard will release the resource.
///
/// # Safety
///
/// Since we're losing track of the lifetime, caller must ensure that the
/// access outlives the guard.
pub unsafe fn into_raw(self) -> RawAccessGuard {
RawAccessGuard(ptr::NonNull::from(ManuallyDrop::new(self).0))
}
}
impl Drop for AccessGuard<'_> {
fn drop(&mut self) {
self.0.release();
}
}
/// A raw guard around some level of access.
#[repr(transparent)]
pub struct RawAccessGuard(ptr::NonNull<Access>);
impl Drop for RawAccessGuard {
fn drop(&mut self) {
unsafe { self.0.as_ref().release() }
}
}
/// A taken access guard.
///
/// This is created with [Access::take], and must not outlive the [Access]
/// instance it was created from.
pub(crate) struct RawTakeGuard {
access: *const Access,
}
impl Drop for RawTakeGuard {
fn drop(&mut self) {
unsafe { (*self.access).release_take() }
}
}
/// Guard for data exclusively borrowed from a slot in the virtual machine.
///
/// These guards are necessary, since we need to guarantee certain forms of
/// access depending on what we do. Releasing the guard releases the access.
pub struct BorrowMut<'a, T: ?Sized> {
data: &'a mut T,
guard: AccessGuard<'a>,
}
impl<'a, T: ?Sized> BorrowMut<'a, T> {
/// Construct a new exclusive guard.
///
/// # Safety
///
/// since this has implications for releasing access, the caller must
/// ensure that access has been acquired correctly using e.g.
/// [Access::exclusive]. Otherwise access can be release incorrectly once
/// this guard is dropped.
pub(crate) unsafe fn new(data: &'a mut T, access: &'a Access) -> Self {
Self {
data,
guard: AccessGuard(access),
}
}
/// Map the mutable reference.
///
/// # Examples
///
/// ```
/// use rune::runtime::{BorrowMut, Shared};
///
/// let vec = Shared::<Vec<u32>>::new(vec![1, 2, 3, 4])?;
/// let vec = vec.borrow_mut()?;
/// let value: BorrowMut<[u32]> = BorrowMut::map(vec, |vec| &mut vec[0..2]);
///
/// assert_eq!(&*value, &mut [1u32, 2u32][..]);
/// # Ok::<_, rune::support::Error>(())
/// ```
pub fn map<M, U: ?Sized>(this: Self, m: M) -> BorrowMut<'a, U>
where
M: FnOnce(&mut T) -> &mut U,
{
BorrowMut {
data: m(this.data),
guard: this.guard,
}
}
/// Try to map the mutable reference to a projection.
///
/// # Examples
///
/// ```
/// use rune::runtime::{BorrowMut, Shared};
///
/// let vec = Shared::<Vec<u32>>::new(vec![1, 2, 3, 4])?;
/// let vec = vec.borrow_mut()?;
/// let mut value: Option<BorrowMut<[u32]>> = BorrowMut::try_map(vec, |vec| vec.get_mut(0..2));
///
/// assert_eq!(value.as_deref_mut(), Some(&mut [1u32, 2u32][..]));
/// # Ok::<_, rune::support::Error>(())
/// ```
pub fn try_map<M, U: ?Sized>(this: Self, m: M) -> Option<BorrowMut<'a, U>>
where
M: FnOnce(&mut T) -> Option<&mut U>,
{
Some(BorrowMut {
data: m(this.data)?,
guard: this.guard,
})
}
}
impl<T: ?Sized> ops::Deref for BorrowMut<'_, T> {
type Target = T;
fn deref(&self) -> &Self::Target {
self.data
}
}
impl<T: ?Sized> ops::DerefMut for BorrowMut<'_, T> {
fn deref_mut(&mut self) -> &mut Self::Target {
self.data
}
}
impl<T: ?Sized> fmt::Debug for BorrowMut<'_, T>
where
T: fmt::Debug,
{
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&**self, fmt)
}
}
impl<F> Future for BorrowMut<'_, F>
where
F: Unpin + Future,
{
type Output = F::Output;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
// NB: inner Future is Unpin.
let this = self.get_mut();
Pin::new(&mut **this).poll(cx)
}
}
#[cfg(test)]
mod tests {
use super::{Access, AccessKind};
#[test]
fn test_non_ref() {
unsafe {
let access = Access::new(false);
assert!(!access.is_ref());
assert!(access.is_shared());
assert!(access.is_exclusive());
let guard = access.shared(AccessKind::Any).unwrap();
assert!(!access.is_ref());
assert!(access.is_shared());
assert!(!access.is_exclusive());
drop(guard);
assert!(!access.is_ref());
assert!(access.is_shared());
assert!(access.is_exclusive());
let guard = access.exclusive(AccessKind::Any).unwrap();
assert!(!access.is_ref());
assert!(!access.is_shared());
assert!(!access.is_exclusive());
drop(guard);
assert!(!access.is_ref());
assert!(access.is_shared());
assert!(access.is_exclusive());
}
}
#[test]
fn test_ref() {
unsafe {
let access = Access::new(true);
assert!(access.is_ref());
assert!(access.is_shared());
assert!(access.is_exclusive());
let guard = access.shared(AccessKind::Any).unwrap();
assert!(access.is_ref());
assert!(access.is_shared());
assert!(!access.is_exclusive());
drop(guard);
assert!(access.is_ref());
assert!(access.is_shared());
assert!(access.is_exclusive());
let guard = access.exclusive(AccessKind::Any).unwrap();
assert!(access.is_ref());
assert!(!access.is_shared());
assert!(!access.is_exclusive());
drop(guard);
assert!(access.is_ref());
assert!(access.is_shared());
assert!(access.is_exclusive());
}
}
}