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BTreeMap: enforce the panic rule imposed by replace #75071

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Aug 8, 2020
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BTreeMap: enforce the panic rule imposed by replace #75071

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104 changes: 51 additions & 53 deletions library/alloc/src/collections/btree/navigate.rs
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Original file line number Diff line number Diff line change
@@ -1,3 +1,5 @@
use core::intrinsics;
use core::mem;
use core::ptr;

use super::node::{marker, ForceResult::*, Handle, NodeRef};
Expand Down Expand Up @@ -79,16 +81,24 @@ def_next_kv_uncheched_dealloc! {unsafe fn next_kv_unchecked_dealloc: right_kv}
def_next_kv_uncheched_dealloc! {unsafe fn next_back_kv_unchecked_dealloc: left_kv}

/// This replaces the value behind the `v` unique reference by calling the
/// relevant function.
/// relevant function, and returns a result obtained along the way.
///
/// Safety: The change closure must not panic.
/// If a panic occurs in the `change` closure, the entire process will be aborted.
#[inline]
unsafe fn replace<T, R>(v: &mut T, change: impl FnOnce(T) -> (T, R)) -> R {
fn replace<T, R>(v: &mut T, change: impl FnOnce(T) -> (T, R)) -> R {
struct PanicGuard;
impl Drop for PanicGuard {
fn drop(&mut self) {
intrinsics::abort()
}
}
let guard = PanicGuard;
let value = unsafe { ptr::read(v) };
let (new_value, ret) = change(value);
unsafe {
ptr::write(v, new_value);
}
mem::forget(guard);
ret
}

Expand All @@ -97,26 +107,22 @@ impl<'a, K, V> Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Ed
/// key and value in between.
/// Unsafe because the caller must ensure that the leaf edge is not the last one in the tree.
pub unsafe fn next_unchecked(&mut self) -> (&'a K, &'a V) {
unsafe {
replace(self, |leaf_edge| {
let kv = leaf_edge.next_kv();
let kv = unwrap_unchecked(kv.ok());
(kv.next_leaf_edge(), kv.into_kv())
})
}
replace(self, |leaf_edge| {
let kv = leaf_edge.next_kv();
let kv = unsafe { unwrap_unchecked(kv.ok()) };
(kv.next_leaf_edge(), kv.into_kv())
})
}

/// Moves the leaf edge handle to the previous leaf edge and returns references to the
/// key and value in between.
/// Unsafe because the caller must ensure that the leaf edge is not the first one in the tree.
pub unsafe fn next_back_unchecked(&mut self) -> (&'a K, &'a V) {
unsafe {
replace(self, |leaf_edge| {
let kv = leaf_edge.next_back_kv();
let kv = unwrap_unchecked(kv.ok());
(kv.next_back_leaf_edge(), kv.into_kv())
})
}
replace(self, |leaf_edge| {
let kv = leaf_edge.next_back_kv();
let kv = unsafe { unwrap_unchecked(kv.ok()) };
(kv.next_back_leaf_edge(), kv.into_kv())
})
}
}

Expand All @@ -127,16 +133,14 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge
/// - The caller must ensure that the leaf edge is not the last one in the tree.
/// - Using the updated handle may well invalidate the returned references.
pub unsafe fn next_unchecked(&mut self) -> (&'a mut K, &'a mut V) {
unsafe {
let kv = replace(self, |leaf_edge| {
let kv = leaf_edge.next_kv();
let kv = unwrap_unchecked(kv.ok());
(ptr::read(&kv).next_leaf_edge(), kv)
});
// Doing the descend (and perhaps another move) invalidates the references
// returned by `into_kv_mut`, so we have to do this last.
kv.into_kv_mut()
}
let kv = replace(self, |leaf_edge| {
let kv = leaf_edge.next_kv();
let kv = unsafe { unwrap_unchecked(kv.ok()) };
(unsafe { ptr::read(&kv) }.next_leaf_edge(), kv)
});
// Doing the descend (and perhaps another move) invalidates the references
// returned by `into_kv_mut`, so we have to do this last.
kv.into_kv_mut()
}

/// Moves the leaf edge handle to the previous leaf and returns references to the
Expand All @@ -145,16 +149,14 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge
/// - The caller must ensure that the leaf edge is not the first one in the tree.
/// - Using the updated handle may well invalidate the returned references.
pub unsafe fn next_back_unchecked(&mut self) -> (&'a mut K, &'a mut V) {
unsafe {
let kv = replace(self, |leaf_edge| {
let kv = leaf_edge.next_back_kv();
let kv = unwrap_unchecked(kv.ok());
(ptr::read(&kv).next_back_leaf_edge(), kv)
});
// Doing the descend (and perhaps another move) invalidates the references
// returned by `into_kv_mut`, so we have to do this last.
kv.into_kv_mut()
}
let kv = replace(self, |leaf_edge| {
let kv = leaf_edge.next_back_kv();
let kv = unsafe { unwrap_unchecked(kv.ok()) };
(unsafe { ptr::read(&kv) }.next_back_leaf_edge(), kv)
});
// Doing the descend (and perhaps another move) invalidates the references
// returned by `into_kv_mut`, so we have to do this last.
kv.into_kv_mut()
}
}

Expand All @@ -172,14 +174,12 @@ impl<K, V> Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge> {
/// call this method again subject to both preconditions listed in the first point,
/// or call counterpart `next_back_unchecked` subject to its preconditions.
pub unsafe fn next_unchecked(&mut self) -> (K, V) {
unsafe {
replace(self, |leaf_edge| {
let kv = next_kv_unchecked_dealloc(leaf_edge);
let k = ptr::read(kv.reborrow().into_kv().0);
let v = ptr::read(kv.reborrow().into_kv().1);
(kv.next_leaf_edge(), (k, v))
})
}
replace(self, |leaf_edge| {
let kv = unsafe { next_kv_unchecked_dealloc(leaf_edge) };
let k = unsafe { ptr::read(kv.reborrow().into_kv().0) };
let v = unsafe { ptr::read(kv.reborrow().into_kv().1) };
(kv.next_leaf_edge(), (k, v))
})
}

/// Moves the leaf edge handle to the previous leaf edge and returns the key
Expand All @@ -195,14 +195,12 @@ impl<K, V> Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge> {
/// call this method again subject to both preconditions listed in the first point,
/// or call counterpart `next_unchecked` subject to its preconditions.
pub unsafe fn next_back_unchecked(&mut self) -> (K, V) {
unsafe {
replace(self, |leaf_edge| {
let kv = next_back_kv_unchecked_dealloc(leaf_edge);
let k = ptr::read(kv.reborrow().into_kv().0);
let v = ptr::read(kv.reborrow().into_kv().1);
(kv.next_back_leaf_edge(), (k, v))
})
}
replace(self, |leaf_edge| {
let kv = unsafe { next_back_kv_unchecked_dealloc(leaf_edge) };
let k = unsafe { ptr::read(kv.reborrow().into_kv().0) };
let v = unsafe { ptr::read(kv.reborrow().into_kv().1) };
(kv.next_back_leaf_edge(), (k, v))
})
}
}

Expand Down