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view rust/hg-core/src/revlog.rs @ 49000:dd6b67d5c256 stable
rust: fix unsound `OwningDirstateMap`
As per the previous patch, `OwningDirstateMap` is unsound. Self-referential
structs are difficult to implement correctly in Rust since the compiler is
free to move structs around as much as it wants to. They are also very rarely
needed in practice, so the state-of-the-art on how they should be done within
the Rust rules is still a bit new.
The crate `ouroboros` is an attempt at providing a safe way (in the Rust sense)
of declaring self-referential structs. It is getting a lot attention and was
improved very quickly when soundness issues were found in the past: rather than
relying on our own (limited) review circle, we might as well use the de-facto
common crate to fix this problem. This will give us a much better chance of
finding issues should any new ones be discovered as well as the benefit of
fewer `unsafe` APIs of our own.
I was starting to think about how I would present a safe API to the old struct
but soon realized that the callback-based approach was already done in
`ouroboros`, along with a lot more care towards refusing incorrect structs.
In short: we don't return a mutable reference to the `DirstateMap` anymore, we
expect users of its API to pass a `FnOnce` that takes the map as an argument.
This allows our `OwningDirstateMap` to control the input and output lifetimes
of the code that modifies it to prevent such issues.
Changing to `ouroboros` meant changing every API with it, but it is relatively
low churn in the end. It correctly identified the example buggy modification of
`copy_map_insert` outlined in the previous patch as violating the borrow rules.
Differential Revision: https://phab.mercurial-scm.org/D12429
| author | Rapha?l Gom?s <rgomes@octobus.net> |
|---|---|
| date | Tue, 05 Apr 2022 10:55:28 +0200 |
| parents | 4d2a5ca060e3 |
| children |
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// Copyright 2018-2020 Georges Racinet <georges.racinet@octobus.net> // and Mercurial contributors // // This software may be used and distributed according to the terms of the // GNU General Public License version 2 or any later version. //! Mercurial concepts for handling revision history pub mod node; pub mod nodemap; mod nodemap_docket; pub mod path_encode; pub use node::{FromHexError, Node, NodePrefix}; pub mod changelog; pub mod filelog; pub mod index; pub mod manifest; pub mod patch; pub mod revlog; /// Mercurial revision numbers /// /// As noted in revlog.c, revision numbers are actually encoded in /// 4 bytes, and are liberally converted to ints, whence the i32 pub type Revision = i32; /// Marker expressing the absence of a parent /// /// Independently of the actual representation, `NULL_REVISION` is guaranteed /// to be smaller than all existing revisions. pub const NULL_REVISION: Revision = -1; /// Same as `mercurial.node.wdirrev` /// /// This is also equal to `i32::max_value()`, but it's better to spell /// it out explicitely, same as in `mercurial.node` #[allow(clippy::unreadable_literal)] pub const WORKING_DIRECTORY_REVISION: Revision = 0x7fffffff; pub const WORKING_DIRECTORY_HEX: &str = "ffffffffffffffffffffffffffffffffffffffff"; /// The simplest expression of what we need of Mercurial DAGs. pub trait Graph { /// Return the two parents of the given `Revision`. /// /// Each of the parents can be independently `NULL_REVISION` fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError>; } #[derive(Clone, Debug, PartialEq)] pub enum GraphError { ParentOutOfRange(Revision), WorkingDirectoryUnsupported, } /// The Mercurial Revlog Index /// /// This is currently limited to the minimal interface that is needed for /// the [`nodemap`](nodemap/index.html) module pub trait RevlogIndex { /// Total number of Revisions referenced in this index fn len(&self) -> usize; fn is_empty(&self) -> bool { self.len() == 0 } /// Return a reference to the Node or `None` if rev is out of bounds /// /// `NULL_REVISION` is not considered to be out of bounds. fn node(&self, rev: Revision) -> Option<&Node>; }