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use std::cmp::{Ordering, PartialOrd};
use std::fmt;
use crate::borrow_tracker::tree_borrows::diagnostics::TransitionError;
use crate::borrow_tracker::tree_borrows::tree::AccessRelatedness;
use crate::borrow_tracker::AccessKind;
/// The activation states of a pointer.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
enum PermissionPriv {
/// represents: a local reference that has not yet been written to;
/// allows: child reads, foreign reads, foreign writes if type is freeze;
/// rejects: child writes (Active), foreign writes (Disabled, except if type is not freeze).
/// special case: behaves differently when protected to adhere more closely to noalias
Reserved { ty_is_freeze: bool },
/// represents: a unique pointer;
/// allows: child reads, child writes;
/// rejects: foreign reads (Frozen), foreign writes (Disabled).
Active,
/// represents: a shared pointer;
/// allows: all read accesses;
/// rejects child writes (UB), foreign writes (Disabled).
Frozen,
/// represents: a dead pointer;
/// allows: all foreign accesses;
/// rejects: all child accesses (UB).
Disabled,
}
use PermissionPriv::*;
impl PartialOrd for PermissionPriv {
/// PermissionPriv is ordered as follows:
/// - Reserved(_) < Active < Frozen < Disabled;
/// - different kinds of `Reserved` (with or without interior mutability)
/// are incomparable to each other.
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
use Ordering::*;
Some(match (self, other) {
(a, b) if a == b => Equal,
(Disabled, _) => Greater,
(_, Disabled) => Less,
(Frozen, _) => Greater,
(_, Frozen) => Less,
(Active, _) => Greater,
(_, Active) => Less,
(Reserved { .. }, Reserved { .. }) => return None,
})
}
}
impl PermissionPriv {
/// Check if `self` can be the initial state of a pointer.
fn is_initial(&self) -> bool {
matches!(self, Reserved { ty_is_freeze: _ } | Frozen)
}
}
/// This module controls how each permission individually reacts to an access.
/// Although these functions take `protected` as an argument, this is NOT because
/// we check protector violations here, but because some permissions behave differently
/// when protected.
mod transition {
use super::*;
/// A child node was read-accessed: UB on Disabled, noop on the rest.
fn child_read(state: PermissionPriv, _protected: bool) -> Option<PermissionPriv> {
Some(match state {
Disabled => return None,
// The inner data `ty_is_freeze` of `Reserved` is always irrelevant for Read
// accesses, since the data is not being mutated. Hence the `{ .. }`
readable @ (Reserved { .. } | Active | Frozen) => readable,
})
}
/// A non-child node was read-accessed: noop on non-protected Reserved, advance to Frozen otherwise.
fn foreign_read(state: PermissionPriv, protected: bool) -> Option<PermissionPriv> {
Some(match state {
// Non-writeable states just ignore foreign reads.
non_writeable @ (Frozen | Disabled) => non_writeable,
// Writeable states are more tricky, and depend on whether things are protected.
// The inner data `ty_is_freeze` of `Reserved` is always irrelevant for Read
// accesses, since the data is not being mutated. Hence the `{ .. }`
res @ Reserved { .. } =>
if protected {
// Someone else read, make sure we won't write.
// We could make this `Disabled` but it doesn't look like we get anything out of that extra UB.
Frozen
} else {
// Before activation and without protectors, foreign reads are fine.
// That's the entire point of 2-phase borrows.
res
},
Active =>
if protected {
// We wrote, someone else reads -- that's bad.
// (If this is initialized, this move-to-protected will mean insta-UB.)
Disabled
} else {
// We don't want to disable here to allow read-read reordering: it is crucial
// that the foreign read does not invalidate future reads through this tag.
Frozen
},
})
}
/// A child node was write-accessed: `Reserved` must become `Active` to obtain
/// write permissions, `Frozen` and `Disabled` cannot obtain such permissions and produce UB.
fn child_write(state: PermissionPriv, _protected: bool) -> Option<PermissionPriv> {
Some(match state {
// A write always activates the 2-phase borrow, even with interior
// mutability
Reserved { .. } | Active => Active,
Frozen | Disabled => return None,
})
}
/// A non-child node was write-accessed: this makes everything `Disabled` except for
/// non-protected interior mutable `Reserved` which stay the same.
fn foreign_write(state: PermissionPriv, protected: bool) -> Option<PermissionPriv> {
Some(match state {
cell @ Reserved { ty_is_freeze: false } if !protected => cell,
_ => Disabled,
})
}
/// Dispatch handler depending on the kind of access and its position.
pub(super) fn perform_access(
kind: AccessKind,
rel_pos: AccessRelatedness,
child: PermissionPriv,
protected: bool,
) -> Option<PermissionPriv> {
match (kind, rel_pos.is_foreign()) {
(AccessKind::Write, true) => foreign_write(child, protected),
(AccessKind::Read, true) => foreign_read(child, protected),
(AccessKind::Write, false) => child_write(child, protected),
(AccessKind::Read, false) => child_read(child, protected),
}
}
}
/// Public interface to the state machine that controls read-write permissions.
/// This is the "private `enum`" pattern.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd)]
pub struct Permission {
inner: PermissionPriv,
}
/// Transition from one permission to the next.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct PermTransition {
from: PermissionPriv,
to: PermissionPriv,
}
impl Permission {
/// Check if `self` can be the initial state of a pointer.
pub fn is_initial(&self) -> bool {
self.inner.is_initial()
}
/// Default initial permission of the root of a new tree.
pub fn new_active() -> Self {
Self { inner: Active }
}
/// Default initial permission of a reborrowed mutable reference.
pub fn new_reserved(ty_is_freeze: bool) -> Self {
Self { inner: Reserved { ty_is_freeze } }
}
/// Default initial permission of a reborrowed shared reference
pub fn new_frozen() -> Self {
Self { inner: Frozen }
}
pub fn is_active(self) -> bool {
matches!(self.inner, Active)
}
// Leave `interior_mut` as `None` if interior mutability
// is irrelevant.
pub fn is_reserved(self, interior_mut: Option<bool>) -> bool {
match (interior_mut, self.inner) {
(None, Reserved { .. }) => true,
(Some(b1), Reserved { ty_is_freeze: b2 }) => b1 == b2,
_ => false,
}
}
pub fn is_frozen(self) -> bool {
matches!(self.inner, Frozen)
}
pub fn is_disabled(self) -> bool {
matches!(self.inner, Disabled)
}
/// Apply the transition to the inner PermissionPriv.
pub fn perform_access(
kind: AccessKind,
rel_pos: AccessRelatedness,
old_perm: Self,
protected: bool,
) -> Option<PermTransition> {
let old_state = old_perm.inner;
transition::perform_access(kind, rel_pos, old_state, protected)
.map(|new_state| PermTransition { from: old_state, to: new_state })
}
}
impl PermTransition {
/// All transitions created through normal means (using `perform_access`)
/// should be possible, but the same is not guaranteed by construction of
/// transitions inferred by diagnostics. This checks that a transition
/// reconstructed by diagnostics is indeed one that could happen.
fn is_possible(self) -> bool {
self.from <= self.to
}
pub fn from(from: Permission, to: Permission) -> Option<Self> {
let t = Self { from: from.inner, to: to.inner };
t.is_possible().then_some(t)
}
pub fn is_noop(self) -> bool {
self.from == self.to
}
/// Extract result of a transition (checks that the starting point matches).
pub fn applied(self, starting_point: Permission) -> Option<Permission> {
(starting_point.inner == self.from).then_some(Permission { inner: self.to })
}
/// Extract starting point of a transition
pub fn started(self) -> Permission {
Permission { inner: self.from }
}
/// Determines if this transition would disable the permission.
pub fn produces_disabled(self) -> bool {
self.to == Disabled
}
}
pub mod diagnostics {
use super::*;
impl fmt::Display for PermissionPriv {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(
f,
"{}",
match self {
Reserved { ty_is_freeze: true } => "Reserved",
Reserved { ty_is_freeze: false } => "Reserved (interior mutable)",
Active => "Active",
Frozen => "Frozen",
Disabled => "Disabled",
}
)
}
}
impl fmt::Display for PermTransition {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "from {} to {}", self.from, self.to)
}
}
impl fmt::Display for Permission {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "{}", self.inner)
}
}
impl Permission {
/// Abbreviated name of the permission (uniformly 3 letters for nice alignment).
pub fn short_name(self) -> &'static str {
// Make sure there are all of the same length as each other
// and also as `diagnostics::DisplayFmtPermission.uninit` otherwise
// alignment will be incorrect.
match self.inner {
Reserved { ty_is_freeze: true } => "Res",
Reserved { ty_is_freeze: false } => "Re*",
Active => "Act",
Frozen => "Frz",
Disabled => "Dis",
}
}
}
impl PermTransition {
/// Readable explanation of the consequences of an event.
/// Fits in the sentence "This accessed caused {trans.summary()}".
///
/// Important: for the purposes of this explanation, `Reserved` is considered
/// to have write permissions, because that's what the diagnostics care about
/// (otherwise `Reserved -> Frozen` would be considered a noop).
pub fn summary(&self) -> &'static str {
assert!(self.is_possible());
match (self.from, self.to) {
(_, Active) => "the first write to a 2-phase borrowed mutable reference",
(_, Frozen) => "a loss of write permissions",
(Frozen, Disabled) => "a loss of read permissions",
(_, Disabled) => "a loss of read and write permissions",
(old, new) =>
unreachable!("Transition from {old:?} to {new:?} should never be possible"),
}
}
/// Determines whether `self` is a relevant transition for the error `err`.
/// `self` will be a transition that happened to a tag some time before
/// that tag caused the error.
///
/// Irrelevant events:
/// - modifications of write permissions when the error is related to read permissions
/// (on failed reads and protected `Frozen -> Disabled`, ignore `Reserved -> Active`,
/// `Reserved -> Frozen`, and `Active -> Frozen`)
/// - all transitions for attempts to deallocate strongly protected tags
///
/// # Panics
///
/// This function assumes that its arguments apply to the same location
/// and that they were obtained during a normal execution. It will panic otherwise.
/// - all transitions involved in `self` and `err` should be increasing
/// (Reserved < Active < Frozen < Disabled);
/// - between `self` and `err` the permission should also be increasing,
/// so all permissions inside `err` should be greater than `self.1`;
/// - `Active` and `Reserved` cannot cause an error due to insufficient permissions,
/// so `err` cannot be a `ChildAccessForbidden(_)` of either of them;
/// - `err` should not be `ProtectedDisabled(Disabled)`, because the protected
/// tag should not have been `Disabled` in the first place (if this occurs it means
/// we have unprotected tags that become protected)
pub(in super::super) fn is_relevant(&self, err: TransitionError) -> bool {
// NOTE: `super::super` is the visibility of `TransitionError`
assert!(self.is_possible());
if self.is_noop() {
return false;
}
match err {
TransitionError::ChildAccessForbidden(insufficient) => {
// Show where the permission was gained then lost,
// but ignore unrelated permissions.
// This eliminates transitions like `Active -> Frozen`
// when the error is a failed `Read`.
match (self.to, insufficient.inner) {
(Frozen, Frozen) => true,
(Active, Frozen) => true,
(Disabled, Disabled) => true,
// A pointer being `Disabled` is a strictly stronger source of
// errors than it being `Frozen`. If we try to access a `Disabled`,
// then where it became `Frozen` (or `Active`) is the least of our concerns for now.
(Active | Frozen, Disabled) => false,
// `Active` and `Reserved` have all permissions, so a
// `ChildAccessForbidden(Reserved | Active)` can never exist.
(_, Active) | (_, Reserved { .. }) =>
unreachable!("this permission cannot cause an error"),
// No transition has `Reserved` as its `.to` unless it's a noop.
(Reserved { .. }, _) => unreachable!("self is a noop transition"),
// All transitions produced in normal executions (using `apply_access`)
// change permissions in the order `Reserved -> Active -> Frozen -> Disabled`.
// We assume that the error was triggered on the same location that
// the transition `self` applies to, so permissions found must be increasing
// in the order `self.from < self.to <= insufficient.inner`
(Disabled, Frozen) =>
unreachable!("permissions between self and err must be increasing"),
}
}
TransitionError::ProtectedDisabled(before_disabled) => {
// Show how we got to the starting point of the forbidden transition,
// but ignore what came before.
// This eliminates transitions like `Reserved -> Active`
// when the error is a `Frozen -> Disabled`.
match (self.to, before_disabled.inner) {
// We absolutely want to know where it was activated.
(Active, Active) => true,
// And knowing where it became Frozen is also important.
(Frozen, Frozen) => true,
// If the error is a transition `Frozen -> Disabled`, then we don't really
// care whether before that was `Reserved -> Active -> Frozen` or
// `Reserved -> Frozen` or even `Frozen` directly.
// The error will only show either
// - created as Frozen, then Frozen -> Disabled is forbidden
// - created as Reserved, later became Frozen, then Frozen -> Disabled is forbidden
// In both cases the `Reserved -> Active` part is inexistant or irrelevant.
(Active, Frozen) => false,
(_, Disabled) =>
unreachable!(
"permission that results in Disabled should not itself be Disabled in the first place"
),
// No transition has `Reserved` as its `.to` unless it's a noop.
(Reserved { .. }, _) => unreachable!("self is a noop transition"),
// Permissions only evolve in the order `Reserved -> Active -> Frozen -> Disabled`,
// so permissions found must be increasing in the order
// `self.from < self.to <= forbidden.from < forbidden.to`.
(Disabled, Reserved { .. } | Active | Frozen)
| (Frozen, Reserved { .. } | Active)
| (Active, Reserved { .. }) =>
unreachable!("permissions between self and err must be increasing"),
}
}
// We don't care because protectors evolve independently from
// permissions.
TransitionError::ProtectedDealloc => false,
}
}
/// Endpoint of a transition.
/// Meant only for diagnostics, use `applied` in non-diagnostics
/// code, which also checks that the starting point matches the current state.
pub fn endpoint(&self) -> Permission {
Permission { inner: self.to }
}
}
}
#[cfg(test)]
mod propagation_optimization_checks {
pub use super::*;
use crate::borrow_tracker::tree_borrows::exhaustive::{precondition, Exhaustive};
impl Exhaustive for PermissionPriv {
fn exhaustive() -> Box<dyn Iterator<Item = Self>> {
Box::new(
vec![Active, Frozen, Disabled]
.into_iter()
.chain(bool::exhaustive().map(|ty_is_freeze| Reserved { ty_is_freeze })),
)
}
}
impl Exhaustive for Permission {
fn exhaustive() -> Box<dyn Iterator<Item = Self>> {
Box::new(PermissionPriv::exhaustive().map(|inner| Self { inner }))
}
}
impl Exhaustive for AccessKind {
fn exhaustive() -> Box<dyn Iterator<Item = Self>> {
use AccessKind::*;
Box::new(vec![Read, Write].into_iter())
}
}
impl Exhaustive for AccessRelatedness {
fn exhaustive() -> Box<dyn Iterator<Item = Self>> {
use AccessRelatedness::*;
Box::new(vec![This, StrictChildAccess, AncestorAccess, DistantAccess].into_iter())
}
}
#[test]
// For any kind of access, if we do it twice the second should be a no-op.
// Even if the protector has disappeared.
fn all_transitions_idempotent() {
use transition::*;
for old in PermissionPriv::exhaustive() {
for (old_protected, new_protected) in <(bool, bool)>::exhaustive() {
// Protector can't appear out of nowhere: either the permission was
// created with a protector (`old_protected = true`) and it then may
// or may not lose it (`new_protected = false`, resp. `new_protected = true`),
// or it didn't have one upon creation and never will
// (`old_protected = new_protected = false`).
// We thus eliminate from this test and all other tests
// the case where the tag is initially unprotected and later becomes protected.
precondition!(old_protected || !new_protected);
for (access, rel_pos) in <(AccessKind, AccessRelatedness)>::exhaustive() {
if let Some(new) = perform_access(access, rel_pos, old, old_protected) {
assert_eq!(
new,
perform_access(access, rel_pos, new, new_protected).unwrap()
);
}
}
}
}
}
#[test]
#[rustfmt::skip]
fn foreign_read_is_noop_after_foreign_write() {
use transition::*;
let old_access = AccessKind::Write;
let new_access = AccessKind::Read;
for old in PermissionPriv::exhaustive() {
for [old_protected, new_protected] in <[bool; 2]>::exhaustive() {
precondition!(old_protected || !new_protected);
for rel_pos in AccessRelatedness::exhaustive() {
precondition!(rel_pos.is_foreign());
if let Some(new) = perform_access(old_access, rel_pos, old, old_protected) {
assert_eq!(
new,
perform_access(new_access, rel_pos, new, new_protected).unwrap()
);
}
}
}
}
}
#[test]
// Check that all transitions are consistent with the order on PermissionPriv,
// i.e. Reserved -> Active -> Frozen -> Disabled
fn permissionpriv_partialord_is_reachability() {
let reach = {
let mut reach = rustc_data_structures::fx::FxHashSet::default();
// One-step transitions + reflexivity
for start in PermissionPriv::exhaustive() {
reach.insert((start, start));
for (access, rel) in <(AccessKind, AccessRelatedness)>::exhaustive() {
for prot in bool::exhaustive() {
if let Some(end) = transition::perform_access(access, rel, start, prot) {
reach.insert((start, end));
}
}
}
}
// Transitive closure
let mut finished = false;
while !finished {
finished = true;
for [start, mid, end] in <[PermissionPriv; 3]>::exhaustive() {
if reach.contains(&(start, mid))
&& reach.contains(&(mid, end))
&& !reach.contains(&(start, end))
{
finished = false;
reach.insert((start, end));
}
}
}
reach
};
// Check that it matches `<`
for [p1, p2] in <[PermissionPriv; 2]>::exhaustive() {
let le12 = p1 <= p2;
let reach12 = reach.contains(&(p1, p2));
assert!(
le12 == reach12,
"`{p1} reach {p2}` ({reach12}) does not match `{p1} <= {p2}` ({le12})"
);
}
}
}