annotate rust/README.rst @ 43271:99394e6c5d12

rust-dirstate-status: add first Rust implementation of `dirstate.status`

Note: This patch also added the rayon crate as a Cargo dependency. It will
help us immensely in making Rust code parallel and easy to maintain. It is
a stable, well-known, and supported crate maintained by people on the Rust
team.

The current `dirstate.status` method has grown over the years through bug
reports and new features to the point where it got too big and too complex.

This series does not yet improve the logic, but adds a Rust fast-path to speed
up certain cases.

Tested on mozilla-try-2019-02-18 with zstd compression:
    - `hg diff` on an empty working copy:
        - c: 1.64(+-)0.04s
        - rust+c before this change: 2.84(+-)0.1s
        - rust+c: 849(+-)40ms
    - `hg commit` when creating a file:
        - c: 5.960s
        - rust+c before this change: 5.828s
        - rust+c: 4.668s
    - `hg commit` when updating a file:
        - c: 4.866s
        - rust+c before this change: 4.371s
        - rust+c: 3.855s
    - `hg status -mard`
        - c: 1.82(+-)0.04s
        - rust+c before this change: 2.64(+-)0.1s
        - rust+c: 896(+-)30ms

The numbers are clear: the current Rust `dirstatemap` implementation is super
slow, its performance needs to be addressed.
This will be done in a future series, immediately after this one, with the goal
of getting Rust to be at least to the speed of the Python + C implementation
in all cases before the 5.2 freeze. At worse, we gate dirstatemap to only be used
in those cases.

Cases where the fast-path is not executed:
    - for commands that need ignore support (`status`, for example)
    - if subrepos are found (should not be hard to add, but winter is coming)
    - any other matcher than an `alwaysmatcher`, like patterns, etc.
    - with extensions like `sparse` and `fsmonitor`

The next step after this is to rethink the logic to be closer to
Jane Street's Valentin Gatien-Baron's Rust fast-path which does a lot less
work when possible.

Differential Revision: https://phab.mercurial-scm.org/D7058

===================
Mercurial Rust Code
===================

This directory contains various Rust code for the Mercurial project.

The top-level ``Cargo.toml`` file defines a workspace containing
all primary Mercurial crates.

Building
========

To build the Rust components::

   $ cargo build

If you prefer a non-debug / release configuration::

   $ cargo build --release

Features
--------

The following Cargo features are available:

localdev (default)
   Produce files that work with an in-source-tree build.

   In this mode, the build finds and uses a ``python2.7`` binary from
   ``PATH``. The ``hg`` binary assumes it runs from ``rust/target/<target>hg``
   and it finds Mercurial files at ``dirname($0)/../../../``.

Build Mechanism
---------------

The produced ``hg`` binary is *bound* to a CPython installation. The
binary links against and loads a CPython library that is discovered
at build time (by a ``build.rs`` Cargo build script). The Python
standard library defined by this CPython installation is also used.

Finding the appropriate CPython installation to use is done by
the ``python27-sys`` crate's ``build.rs``. Its search order is::

1. ``PYTHON_SYS_EXECUTABLE`` environment variable.
2. ``python`` executable on ``PATH``
3. ``python2`` executable on ``PATH``
4. ``python2.7`` executable on ``PATH``

Additional verification of the found Python will be performed by our
``build.rs`` to ensure it meets Mercurial's requirements.

Details about the build-time configured Python are built into the
produced ``hg`` binary. This means that a built ``hg`` binary is only
suitable for a specific, well-defined role. These roles are controlled
by Cargo features (see above).

Running
=======

The ``hgcli`` crate produces an ``hg`` binary. You can run this binary
via ``cargo run``::

   $ cargo run --manifest-path hgcli/Cargo.toml

Or directly::

   $ target/debug/hg
   $ target/release/hg

You can also run the test harness with this binary::

   $ ./run-tests.py --with-hg ../rust/target/debug/hg

.. note::

   Integration with the test harness is still preliminary. Remember to
   ``cargo build`` after changes because the test harness doesn't yet
   automatically build Rust code.