rust: explain why the current `OwningDirstateMap` is unsound

See inline comments.

Differential Revision: https://phab.mercurial-scm.org/D12428

rust/hg-core/src/dirstate_tree/owning.rs

@@ -1,105 +1,142 b''
 use super::dirstate_map::DirstateMap;
 use stable_deref_trait::StableDeref;
 use std::ops::Deref;
 
+/*
+// /!\ This is unsound and can cause use after free. It will be fixed in the
+// next patch
+
+// If we change `value` from its current use of `HgPathBuf` to `&HgPath`,
+// nothing here tells that `value` will outlive `OwningDirstateMap`
+pub fn copy_map_insert<'a,'owned>(
+    &'owned mut self,
+    key: &HgPath,
+    value: &'a HgPath,
+) -> Result<Option<HgPathBuf>, DirstateV2ParseError> {
+    // `'local` is smaller than `'a` here
+    let map: &'local mut DirstateMap<'local> = self.get_map_mut();
+    let node: &'local mut Node<'local> = DirstateMap::get_or_insert_node(
+        map.on_disk,
+        &mut map.unreachable_bytes,
+        &mut map.root,
+        &key,
+        WithBasename::to_cow_owned,
+        |_ancestor| {},
+    )?;
+    if node.copy_source.is_none() {
+        map.nodes_with_copy_source_count += 1
+    }
+    Ok(node.copy_source.replace(value.into()).map(Cow::into_owned))
+    // and right here ----------^^^^^^^^^^^^
+    // we are storing `&'a HgPath` in `Node<'local>` which is possible
+    // because to the compiler, `'a` is longer than ``local`.
+    // It is wrong because nothing proves that `&'a HgPath` will outlive `self`.
+}
+
+// All of this is caused by the wrong cast of the DirstateMap pointer that
+// fakes the lifetime of `DirstateMap` and ensures the compiler that it lives
+// as long as `on_disk`, which is only true for its immutable data.
+// This will be fixed in the next commit.
+*/
+
 /// Keep a `DirstateMap<'on_disk>` next to the `on_disk` buffer that it
 /// borrows.
 ///
 /// This is similar to [`OwningRef`] which is more limited because it
 /// represents exactly one `&T` reference next to the value it borrows, as
 /// opposed to a struct that may contain an arbitrary number of references in
 /// arbitrarily-nested data structures.
 ///
 /// [`OwningRef`]: https://docs.rs/owning_ref/0.4.1/owning_ref/struct.OwningRef.html
 pub struct OwningDirstateMap {
     /// Owned handle to a bytes buffer with a stable address.
     ///
     /// See <https://docs.rs/owning_ref/0.4.1/owning_ref/trait.StableAddress.html>.
     on_disk: Box<dyn Deref<Target = [u8]> + Send>,
 
     /// Pointer for `Box<DirstateMap<'on_disk>>`, typed-erased because the
     /// language cannot represent a lifetime referencing a sibling field.
     /// This is not quite a self-referencial struct (moving this struct is not
     /// a problem as it doesn’t change the address of the bytes buffer owned
     /// by `on_disk`) but touches similar borrow-checker limitations.
     ptr: *mut (),
 }
 
 impl OwningDirstateMap {
     pub fn new_empty<OnDisk>(on_disk: OnDisk) -> Self
     where
         OnDisk: Deref<Target = [u8]> + StableDeref + Send + 'static,
     {
         let on_disk = Box::new(on_disk);
         let bytes: &'_ [u8] = &on_disk;
         let map = DirstateMap::empty(bytes);
 
         // Like in `bytes` above, this `'_` lifetime parameter borrows from
         // the bytes buffer owned by `on_disk`.
         let ptr: *mut DirstateMap<'_> = Box::into_raw(Box::new(map));
 
         // Erase the pointed type entirely in order to erase the lifetime.
         let ptr: *mut () = ptr.cast();
 
         Self { on_disk, ptr }
     }
 
     pub fn get_pair_mut<'a>(
         &'a mut self,
     ) -> (&'a [u8], &'a mut DirstateMap<'a>) {
         // SAFETY: We cast the type-erased pointer back to the same type it had
         // in `new`, except with a different lifetime parameter. This time we
         // connect the lifetime to that of `self`. This cast is valid because
         // `self` owns the same `on_disk` whose buffer `DirstateMap`
         // references. That buffer has a stable memory address because our
         // `Self::new_empty` counstructor requires `StableDeref`.
         let ptr: *mut DirstateMap<'a> = self.ptr.cast();
         // SAFETY: we dereference that pointer, connecting the lifetime of the
         // new `&mut` to that of `self`. This is valid because the
         // raw pointer is to a boxed value, and `self` owns that box.
         (&self.on_disk, unsafe { &mut *ptr })
     }
 
     pub fn get_map_mut<'a>(&'a mut self) -> &'a mut DirstateMap<'a> {
         self.get_pair_mut().1
     }
 
     pub fn get_map<'a>(&'a self) -> &'a DirstateMap<'a> {
         // SAFETY: same reasoning as in `get_pair_mut` above.
         let ptr: *mut DirstateMap<'a> = self.ptr.cast();
         unsafe { &*ptr }
     }
 
     pub fn on_disk<'a>(&'a self) -> &'a [u8] {
         &self.on_disk
     }
 }
 
 impl Drop for OwningDirstateMap {
     fn drop(&mut self) {
         // Silence a "field is never read" warning, and demonstrate that this
         // value is still alive.
         let _: &Box<dyn Deref<Target = [u8]> + Send> = &self.on_disk;
         // SAFETY: this cast is the same as in `get_mut`, and is valid for the
         // same reason. `self.on_disk` still exists at this point, drop glue
         // will drop it implicitly after this `drop` method returns.
         let ptr: *mut DirstateMap<'_> = self.ptr.cast();
         // SAFETY: `Box::from_raw` takes ownership of the box away from `self`.
         // This is fine because drop glue does nothing for `*mut ()` and we’re
         // in `drop`, so `get` and `get_mut` cannot be called again.
         unsafe { drop(Box::from_raw(ptr)) }
     }
 }
 
 fn _static_assert_is_send<T: Send>() {}
 
 fn _static_assert_fields_are_send() {
     _static_assert_is_send::<Box<DirstateMap<'_>>>();
 }
 
 // SAFETY: we don’t get this impl implicitly because `*mut (): !Send` because
 // thread-safety of raw pointers is unknown in the general case. However this
 // particular raw pointer represents a `Box<DirstateMap<'on_disk>>` that we
 // own. Since that `Box` is `Send` as shown in above, it is sound to mark
 // this struct as `Send` too.
 unsafe impl Send for OwningDirstateMap {}