tor/doc/HACKING/GettingStartedRust.md

Hacking on Rust in Tor

Getting Started

Please read or review our documentation on Rust coding standards (.../doc/HACKING/CodingStandardsRust.md) before doing anything.

Please also read the Rust Code of Conduct. We aim to follow the good example set by the Rust community and be excellent to one another. Let's be careful with each other, so we can be memory-safe together!

Next, please contact us before rewriting anything! Rust in Tor is still an experiment. It is an experiment that we very much want to see succeed, so we're going slowly and carefully. For the moment, it's also a completely volunteer-driven effort: while many, if not most, of us are paid to work on Tor, we are not yet funded to write Rust code for Tor. Please be patient with the other people who are working on getting more Rust code into Tor, because they are graciously donating their free time to contribute to this effort.

Resources for learning Rust

Beginning resources

The primary resource for learning Rust is The Book. If you'd like to start writing Rust immediately, without waiting for anything to install, there is an interactive browser-based playground.

Advanced resources

If you're interested in playing with various Rust compilers and viewing a very nicely displayed output of the generated assembly, there is the Godbolt compiler explorer

For learning how to write unsafe Rust, read The Rustonomicon.

For learning everything you ever wanted to know about Rust macros, there is The Little Book of Rust Macros.

For learning more about FFI and Rust, see Jake Goulding's Rust FFI Omnibus.

Compiling Tor with Rust enabled

You will need to run the configure script with the --enable-rust flag to explicitly build with Rust. Additionally, you will need to specify where to fetch Rust dependencies, as we allow for either fetching dependencies from Cargo or specifying a local directory.

Fetch dependencies from Cargo

./configure --enable-rust --enable-cargo-online-mode

Using a local dependency cache

You'll need the following Rust dependencies (as of this writing):

libc==0.2.39

We vendor our Rust dependencies in a separate repo using cargo-vendor. To use them, do:

git submodule init
git submodule update

To specify the local directory containing the dependencies, (assuming you are in the top level of the repository) configure tor with:

TOR_RUST_DEPENDENCIES='path_to_dependencies_directory' ./configure --enable-rust

(Note that TOR_RUST_DEPENDENCIES must be the full path to the directory; it cannot be relative.)

Assuming you used the above git submodule commands and you're in the topmost directory of the repository, this would be:

TOR_RUST_DEPENDENCIES=`pwd`/src/ext/rust/crates ./configure --enable-rust

Identifying which modules to rewrite

The places in the Tor codebase that are good candidates for porting to Rust are:

1. loosely coupled to other Tor submodules,
2. have high test coverage, and
3. would benefit from being implemented in a memory safe language.

Help in either identifying places such as this, or working to improve existing areas of the C codebase by adding regression tests and simplifying dependencies, would be really helpful.

Furthermore, as submodules in C are implemented in Rust, this is a good opportunity to refactor, add more tests, and split modules into smaller areas of responsibility.

A good first step is to build a module-level callgraph to understand how interconnected your target module is.

git clone https://git.torproject.org/user/nickm/calltool.git
cd tor
CFLAGS=0 ./configure
../calltool/src/main.py module_callgraph

The output will tell you each module name, along with a set of every module that the module calls. Modules which call fewer other modules are better targets.

Writing your Rust module

Strive to change the C API as little as possible.

We are currently targeting Rust nightly, for now. We expect this to change moving forward, as we understand more about which nightly features we need. It is on our TODO list to try to cultivate good standing with various distro maintainers of rustc and cargo, in order to ensure that whatever version we solidify on is readily available.

If parts of your Rust code needs to stay in sync with C code (such as handling enums across the FFI boundary), annonotate these places in a comment structured as follows:

/// C_RUST_COUPLED: <name_of_c_object>

Where can be an enum, struct, constant, etc. Then, do the same in the C code, to note that rust will need to be changed when the C does.

Adding your Rust module to Tor's build system

1. Your translation of the C module should live in its own crate(s) in the .../tor/src/rust/ directory.
2. Add your crate to .../tor/src/rust/Cargo.toml, in the [workspace.members] section.
3. Add your crate's files to src/rust/include.am

If your crate should be available to C (rather than just being included as a dependency of other Rust modules):

1. Declare the crate as a dependency of tor_rust in src/rust/tor_util/Cargo.toml and include it in src/rust/tor_rust/lib.rs

How to test your Rust code

Everything should be tested full stop. Even non-public functionality.

Be sure to edit .../tor/src/test/test_rust.sh to add the name of your crate to the crates variable! This will ensure that cargo test is run on your crate.

Configure Tor's build system to build with Rust enabled:

./configure --enable-fatal-warnings --enable-rust --enable-cargo-online-mode

Tor's test should be run by doing:

make check

Tor's integration tests should also pass:

make test-stem

Submitting a patch

Please follow the instructions in .../doc/HACKING/GettingStarted.md.