rust/README.rst

wireprotov2: add phases to "changesetdata" command

This commit teaches the "changesetdata" wire protocol command
to emit the phase state for each changeset.

This is a different approach from existing phase transfer in a
few ways. Previously, if there are no new revisions (or we're
not using bundle2), we perform a "listkeys" request to retrieve
phase heads. And when revision data is being transferred
with bundle2, phases data is encoded in a standalone bundle2 part.
In both cases, phases data is logically decoupled from the changeset
data and is encountered/applied after changeset revision data
is received.

The new wire protocol purposefully tries to more tightly associate
changeset metadata (phases, bookmarks, obsolescence markers, etc)
with the changeset revision and index data itself, rather than
have it live as a separate entity that must be fetched and
processed separately. I reckon that one reason we didn't do this
before was it was difficult to add new data types/fields without
breaking existing consumers. By using CBOR maps to transfer
changeset data and putting clients in control of what fields are
requested / present in those maps, we can easily add additional
changeset data while maintaining backwards compatibility. I believe
this to be a superior approach to the problem.

That being said, for performance reasons, we may need to resort
to alternative mechanisms for transferring data like phases. But
for now, I think giving the wire protocol the ability to transfer
changeset metadata next to the changeset itself is a powerful feature
because it is a raw, changeset-centric data API. And if you build
simple APIs for accessing the fundamental units of repository data,
you enable client-side experimentation (partial clone, etc). If it
turns out that we need specialized APIs or mechanisms for transferring
data like phases, we can build in those APIs later. For now, I'd
like to see how far we can get on simple APIs.

It's worth noting that when phase data is being requested, the
server will also emit changeset records for nodes in the bases
specified by the "noderange" argument. This is to ensure that
phase-only updates for nodes the client has are available to the
client, even if no new changesets will be transferred.

Differential Revision: https://phab.mercurial-scm.org/D4483

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