view rust/README.rst @ 39604:335ae4d0a552

bundlerepo: dynamically create repository type from base repository

Previously, bundlerepository inherited from localrepo.localrepository.
You simply instantiated a bundlerepository and its __init__ called
localrepo.localrepository.__init__. Things were simple.

Unfortunately, this strategy is limiting because it assumes that
the base repository is a localrepository instance. And it assumes
various properties of localrepository, such as the arguments its
__init__ takes. And it prevents us from changing behavior of
localrepository.__init__ without also having to change derived classes.

Previous and ongoing work to abstract storage revealed these
limitations.

This commit changes the initialization strategy of bundle repositories
to dynamically create a type to represent the repository. Instead of
a static type, we instantiate a new local repo instance via
localrepo.instance(). We then combine its __class__ with
bundlerepository to produce a new type. This ensures that no matter
how localrepo.instance() decides to create a repository object, we
can derive a bundle repo object from it. i.e. localrepo.instance()
could return a type that isn't a localrepository and it would "just
work."

Well, it would "just work" if bundlerepository's custom implementations
only accessed attributes in the documented repository interface. I'm
pretty sure it violates the interface contract in a handful of
places. But we can worry about that another day. This change gets us
closer to doing more clever things around instantiating repository
instances without having to worry about teaching bundlerepository about
them.

.. api::

   ``bundlerepo.bundlerepository`` is no longer usable on its own.

   The class is combined with the class of the base repository it is
   associated with at run-time.

   New bundlerepository instances can be obtained by calling
   ``bundlerepo.instance()`` or ``bundlerepo.makebundlerepository()``.

Differential Revision: https://phab.mercurial-scm.org/D4555

===================
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.