Mercurial > hg
view rust/README.rst @ 48793:6e559391f96e
tracked-key: remove the dual write and rename to tracked-hint
The dual-write approach was mostly useless. As explained in the previous version
of the help, the key had to be read twice before we could cache a value.
However this "read twice" limitation actually also apply to any usage of the
key. If some operation wants to rely of the "same value == same tracked set"
property it would need to read the value before, and after running that
operation (or at least, after, in all cases). So it cannot be sure the operation
it did is "valid" until checking the key after the operation. As a resultat such
operation can only be read-only or rollbackable.
This reduce the utility of the "same value == same tracked set" a lot.
So it seems simpler to drop the double write and to update the documentation to
highlight that this file does not garantee race-free operation. As a result the
"key" is demoted to a "hint".
Documentation is updated accordingly.
Differential Revision: https://phab.mercurial-scm.org/D12201
author | Pierre-Yves David <pierre-yves.david@octobus.net> |
---|---|
date | Thu, 17 Feb 2022 07:34:49 +0100 |
parents | 799fdf4cca80 |
children | 649ff7f86f96 |
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=================== Mercurial Rust Code =================== This directory contains various Rust code for the Mercurial project. Rust is not required to use (or build) Mercurial, but using it improves performance in some areas. There are currently four independent Rust projects: - chg. An implementation of chg, in Rust instead of C. - hgcli. A project that provides a (mostly) self-contained "hg" binary, for ease of deployment and a bit of speed, using PyOxidizer. See hgcli/README.md. - hg-core (and hg-cpython): implementation of some functionality of mercurial in Rust, e.g. ancestry computations in revision graphs, status or pull discovery. The top-level ``Cargo.toml`` file defines a workspace containing these crates. - rhg: a pure Rust implementation of Mercurial, with a fallback mechanism for unsupported invocations. It reuses the logic `hg-core` but completely forgoes interaction with Python. See `rust/rhg/README.md` for more details. Using Rust code =============== Local use (you need to clean previous build artifacts if you have built without rust previously):: $ make PURE=--rust local # to use ./hg $ ./tests/run-tests.py --rust # to run all tests $ ./hg debuginstall | grep -i rust # to validate rust is in use checking Rust extensions (installed) checking module policy (rust+c-allow) If the environment variable ``HGWITHRUSTEXT=cpython`` is set, the Rust extension will be used by default unless ``--no-rust``. One day we may use this environment variable to switch to new experimental binding crates like a hypothetical ``HGWITHRUSTEXT=hpy``. Special features ================ You might want to check the `features` section in ``hg-cpython/Cargo.toml``. It may contain features that might be interesting to try out. To use features from the Makefile, use the `HG_RUST_FEATURES` environment variable: for instance `HG_RUST_FEATURES="some-feature other-feature"` Profiling ========= Setting the environment variable ``RUST_LOG=trace`` will make hg print a few high level rust-related performance numbers. It can also indicate why the rust code cannot be used (say, using lookarounds in hgignore). Creating a ``.cargo/config`` file with the following content enables debug information in optimized builds. This make profiles more informative with source file name and line number for Rust stack frames and (in some cases) stack frames for Rust functions that have been inlined. [profile.release] debug = true ``py-spy`` (https://github.com/benfred/py-spy) can be used to construct a single profile with rust functions and python functions (as opposed to ``hg --profile``, which attributes time spent in rust to some unlucky python code running shortly after the rust code, and as opposed to tools for native code like ``perf``, which attribute time to the python interpreter instead of python functions). Example usage: $ make PURE=--rust local # Don't forget to recompile after a code change $ py-spy record --native --output /tmp/profile.svg -- ./hg ... Developing Rust =============== The current version of Rust in use is ``1.48.0``, because it's what Debian stable has. You can use ``rustup override set 1.48.0`` at the root of the repo to make it easier on you. Go to the ``hg-cpython`` folder:: $ cd rust/hg-cpython Or, only the ``hg-core`` folder. Be careful not to break compatibility:: $ cd rust/hg-core Simply run:: $ cargo build --release It is possible to build without ``--release``, but it is not recommended if performance is of any interest: there can be an order of magnitude of degradation when removing ``--release``. For faster builds, you may want to skip code generation:: $ cargo check For even faster typing:: $ cargo c You can run only the rust-specific tests (as opposed to tests of mercurial as a whole) with:: $ cargo test --all Formatting the code ------------------- We use ``rustfmt`` to keep the code formatted at all times. For now, we are using the nightly version because it has been stable enough and provides comment folding. To format the entire Rust workspace:: $ cargo +nightly fmt This requires you to have the nightly toolchain installed.