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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::AccessKind;
/// The activation states of a pointer.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
enum PermissionPriv {
/// represents: a local mutable reference that has not yet been written to;
/// allows: child reads, foreign reads;
/// affected by: child writes (becomes Active),
/// rejects: foreign writes (Disabled).
///
/// `ReservedFrz` is mostly for types that are `Freeze` (no interior mutability).
/// If the type has interior mutability, see `ReservedIM` instead.
/// (Note: since the discovery of `tests/fail/tree_borrows/reservedim_spurious_write.rs`,
/// we also use `ReservedFreeze` for mutable references that were retagged with a protector
/// independently of interior mutability)
///
/// special case: behaves differently when protected, which is where `conflicted`
/// is relevant
/// - `conflicted` is set on foreign reads,
/// - `conflicted` must not be set on child writes (there is UB otherwise).
///
/// This is so that the behavior of `Reserved` adheres to the rules of `noalias`:
/// - foreign-read then child-write is UB due to `conflicted`,
/// - child-write then foreign-read is UB since child-write will activate and then
/// foreign-read disables a protected `Active`, which is UB.
ReservedFrz { conflicted: bool },
/// Alternative version of `ReservedFrz` made for types with interior mutability.
/// allows: child reads, foreign reads, foreign writes (extra);
/// affected by: child writes (becomes Active);
/// rejects: nothing.
ReservedIM,
/// 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 by the reflexive transitive closure of
/// `Reserved(conflicted=false) < Reserved(conflicted=true) < Active < Frozen < Disabled`.
/// `Reserved` that have incompatible `ty_is_freeze` are incomparable to each other.
/// This ordering matches the reachability by transitions, as asserted by the exhaustive test
/// `permissionpriv_partialord_is_reachability`.
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,
(ReservedIM, ReservedIM) => Equal,
(ReservedFrz { conflicted: c1 }, ReservedFrz { conflicted: c2 }) => {
// `bool` is ordered such that `false <= true`, so this works as intended.
c1.cmp(c2)
}
// Versions of `Reserved` with different interior mutability are incomparable with each
// other.
(ReservedIM, ReservedFrz { .. }) | (ReservedFrz { .. }, ReservedIM) => return None,
})
}
}
impl PermissionPriv {
/// Check if `self` can be the initial state of a pointer.
fn is_initial(&self) -> bool {
matches!(self, ReservedFrz { conflicted: false } | Frozen | ReservedIM)
}
/// Reject `ReservedIM` that cannot exist in the presence of a protector.
fn compatible_with_protector(&self) -> bool {
!matches!(self, ReservedIM)
}
}
/// 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 @ (ReservedFrz { .. } | ReservedIM | Active | Frozen) => readable,
})
}
/// A non-child node was read-accessed: keep `Reserved` but mark it as `conflicted` if it
/// is protected; invalidate `Active`.
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 `{ .. }`
// Someone else read. To make sure we won't write before function exit,
// we set the "conflicted" flag, which will disallow writes while we are protected.
ReservedFrz { .. } if protected => ReservedFrz { conflicted: true },
// Before activation and without protectors, foreign reads are fine.
// That's the entire point of 2-phase borrows.
res @ (ReservedFrz { .. } | ReservedIM) => {
// Even though we haven't checked `ReservedIM if protected` separately,
// it is a state that cannot occur because under a protector we only
// create `ReservedFrz` never `ReservedIM`.
assert!(!protected);
res
}
Active =>
if protected {
// We wrote, someone else reads -- that's bad.
// (Since Active is always 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 {
// If the `conflicted` flag is set, then there was a foreign read during
// the function call that is still ongoing (still `protected`),
// this is UB (`noalias` violation).
ReservedFrz { conflicted: true } if protected => return None,
// A write always activates the 2-phase borrow, even with interior
// mutability
ReservedFrz { .. } | ReservedIM | 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> {
// There is no explicit dependency on `protected`, but recall that interior mutable
// types receive a `ReservedFrz` instead of `ReservedIM` when retagged under a protector,
// so the result of this function does indirectly depend on (past) protector status.
Some(match state {
res @ ReservedIM => {
// We can never create a `ReservedIM` under a protector, only `ReservedFrz`.
assert!(!protected);
res
}
_ => 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()
}
/// Check if `self` is the terminal state of a pointer (is `Disabled`).
pub fn is_disabled(&self) -> bool {
self.inner == Disabled
}
/// Check if `self` is the post-child-write state of a pointer (is `Active`).
pub fn is_active(&self) -> bool {
self.inner == Active
}
/// Default initial permission of the root of a new tree at inbounds positions.
/// Must *only* be used for the root, this is not in general an "initial" permission!
pub fn new_active() -> Self {
Self { inner: Active }
}
/// Default initial permission of a reborrowed mutable reference that is either
/// protected or not interior mutable.
fn new_reserved_frz() -> Self {
Self { inner: ReservedFrz { conflicted: false } }
}
/// Default initial permission of an unprotected interior mutable reference.
fn new_reserved_im() -> Self {
Self { inner: ReservedIM }
}
/// Wrapper around `new_reserved_frz` and `new_reserved_im` that decides
/// which to call based on the interior mutability and the retag kind (whether there
/// is a protector is relevant because being protected takes priority over being
/// interior mutable)
pub fn new_reserved(ty_is_freeze: bool, protected: bool) -> Self {
if ty_is_freeze || protected { Self::new_reserved_frz() } else { Self::new_reserved_im() }
}
/// Default initial permission of a reborrowed shared reference.
pub fn new_frozen() -> Self {
Self { inner: Frozen }
}
/// Default initial permission of the root of a new tree at out-of-bounds positions.
/// Must *only* be used for the root, this is not in general an "initial" permission!
pub fn new_disabled() -> Self {
Self { inner: Disabled }
}
/// Reject `ReservedIM` that cannot exist in the presence of a protector.
pub fn compatible_with_protector(&self) -> bool {
self.inner.compatible_with_protector()
}
/// 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 })
}
/// During a provenance GC, we want to compact the tree.
/// For this, we want to merge nodes upwards if they have a singleton parent.
/// But we need to be careful: If the parent is Frozen, and the child is Reserved,
/// we can not do such a merge. In general, such a merge is possible if the parent
/// allows similar accesses, and in particular if the parent never causes UB on its
/// own. This is enforced by a test, namely `tree_compacting_is_sound`. See that
/// test for more information.
/// This method is only sound if the parent is not protected. We never attempt to
/// remove protected parents.
pub fn can_be_replaced_by_child(self, child: Self) -> bool {
match (self.inner, child.inner) {
// ReservedIM can be replaced by anything, as it allows all
// transitions.
(ReservedIM, _) => true,
// Reserved (as parent, where conflictedness does not matter)
// can be replaced by all but ReservedIM,
// since ReservedIM alone would survive foreign writes
(ReservedFrz { .. }, ReservedIM) => false,
(ReservedFrz { .. }, _) => true,
// Active can not be replaced by something surviving
// foreign reads and then remaining writable.
(Active, ReservedIM) => false,
(Active, ReservedFrz { .. }) => false,
// Replacing a state by itself is always okay, even if the child state is protected.
(Active, Active) => true,
// Active can be replaced by Frozen, since it is not protected.
(Active, Frozen) => true,
(Active, Disabled) => true,
// Frozen can only be replaced by Disabled (and itself).
(Frozen, Frozen) => true,
(Frozen, Disabled) => true,
(Frozen, _) => false,
// Disabled can not be replaced by anything else.
(Disabled, Disabled) => true,
(Disabled, _) => false,
}
}
}
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 {
ReservedFrz { conflicted: false } => "Reserved",
ReservedFrz { conflicted: true } => "Reserved (conflicted)",
ReservedIM => "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 {
ReservedFrz { conflicted: false } => "Res ",
ReservedFrz { conflicted: true } => "ResC",
ReservedIM => "ReIM",
Active => "Act ",
Frozen => "Frz ",
Disabled => "Dis ",
}
}
}
impl PermTransition {
/// Readable explanation of the consequences of an event.
/// Fits in the sentence "This transition corresponds to {trans.summary()}".
pub fn summary(&self) -> &'static str {
assert!(self.is_possible());
assert!(!self.is_noop());
match (self.from, self.to) {
(_, Active) => "the first write to a 2-phase borrowed mutable reference",
(_, Frozen) => "a loss of write permissions",
(ReservedFrz { conflicted: false }, ReservedFrz { conflicted: true }) =>
"a temporary loss of write permissions until function exit",
(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(conflicted=false) -> Reserved(conflicted=true)`, 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(conflicted=false)` 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,
(
ReservedFrz { conflicted: true, .. },
ReservedFrz { conflicted: true, .. },
) => 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` or `Reserved`) is the least
// of our concerns for now.
(ReservedFrz { conflicted: true } | Active | Frozen, Disabled) => false,
(ReservedFrz { conflicted: true }, Frozen) => false,
// `Active` and `Reserved` have all permissions, so a
// `ChildAccessForbidden(Reserved | Active)` can never exist.
(_, Active) | (_, ReservedFrz { conflicted: false }) =>
unreachable!("this permission cannot cause an error"),
// No transition has `Reserved { conflicted: false }` or `ReservedIM`
// as its `.to` unless it's a noop.
(ReservedFrz { conflicted: false } | ReservedIM, _) =>
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`
(Active | Frozen | Disabled, ReservedFrz { .. } | ReservedIM)
| (Disabled, Frozen)
| (ReservedFrz { .. }, ReservedIM) =>
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/frozen/marked
// conflicted.
(Active, Active) => true,
(Frozen, Frozen) => true,
(
ReservedFrz { conflicted: true, .. },
ReservedFrz { conflicted: true, .. },
) => true,
// If the error is a transition `Frozen -> Disabled`, then we don't really
// care whether before that was `Reserved -> Active -> Frozen` or
// `Frozen` directly.
// The error will only show either
// - created as Reserved { conflicted: false },
// then Reserved { .. } -> Disabled is forbidden
// - created as Reserved { conflicted: false },
// then Active -> Disabled is forbidden
// A potential `Reserved { conflicted: false }
// -> Reserved { conflicted: true }` is inexistant or irrelevant,
// and so is the `Reserved { conflicted: false } -> Active`
(Active, Frozen) => false,
(ReservedFrz { conflicted: true }, _) => false,
(_, Disabled) =>
unreachable!(
"permission that results in Disabled should not itself be Disabled in the first place"
),
// No transition has `Reserved { conflicted: false }` or `ReservedIM` as its `.to`
// unless it's a noop.
(ReservedFrz { conflicted: false } | ReservedIM, _) =>
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, ReservedFrz { .. } | ReservedIM | Active | Frozen)
| (Frozen, ReservedFrz { .. } | ReservedIM | Active)
| (Active, ReservedFrz { .. } | ReservedIM) =>
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)]
impl Permission {
pub fn is_reserved_frz_with_conflicted(&self, expected_conflicted: bool) -> bool {
match self.inner {
ReservedFrz { conflicted } => conflicted == expected_conflicted,
_ => false,
}
}
}
#[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, ReservedIM]
.into_iter()
.chain(<bool>::exhaustive().map(|conflicted| ReservedFrz { conflicted })),
)
}
}
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);
if old_protected {
precondition!(old.compatible_with_protector());
}
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);
if old_protected {
precondition!(old.compatible_with_protector());
}
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 prot {
precondition!(start.compatible_with_protector());
}
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})"
);
}
}
}