This commit demonstrates the feasibility of integrating Rust into
Mercurial.
There are two significant components of this commit:
1) Establish a Rust `hg` binary.
2) Establish a new mechanism for producing standalone Mercurial
distributions.
These functions will likely get split up before this commit lands.
If you are familiar with Rust, the contents of the added rust/
directory should be pretty straightforward. We create an "hgcli"
package that implements a binary application to run Mercurial.
The output of this package is an "hg" binary.
Our Rust `hg` creates an embedded CPython interpreter and attempts
to mimim the current functionality in the `hg` Python script. An
immediate goal of the Rust implementation of `hg` is to serve as a
drop-in replacement for the `hg` script. Longer term, we can discuss
removing the Python script so `hg` is a native executable on all
platforms. This will allow us to:
* Merge `chg` functionality into `hg`
* Start implementing very early `hg` logic in Rust (config parsing,
argument parsing, repository opening, etc)
* Possibly have some commands implemented in 100% Rust, allowing us
to avoid Python interpreter startup overhead
The added code for "standalone Mercurial" is a proof-of-concept
demonstrating the direction I'd like to take Mercurial packaging
and distribution in the near future. Today, we generally treat
Mercurial as a Python application. This by-and-large works. But
as we start writing things in Rust, that will pull us away from
being Python centric. In addition, the current distribution model
is fragile because it often relies on the Python that end-users
have installed. We constantly run into problems with:
* Users running old Python versions with known bugs and lacking
features (such as modern SSL/TLS support).
* Missing optional packages that improve the Mercurial experience
(such as re2, pygments, and even the curses module on Windows).
* Python packaging fragility.
While Python is still a critical component of Mercurial and will be
for the indefinite future, I'd like Mercurial to pivot away from
being pictured as a "Python application" and move towards being
a "generic/system application." In other words, Python is just
an implementation detail.
The added Python script produces a standalone Mercurial
installation. It currently only works on Linux, MacOS, and similar
*NIX variants (although I haven't tested it thoroughly and it may
be broken on MacOS after recent changes). Essentially, the script
downloads the Python 2.7 source code, builds Python from source,
builds the Mercurial Rust and Python components, and produces a
tarball containing the Rust `hg` binary and all the support files.
The rpath of the `hg` binary is relative. So you should be able
to drop the files into any system and things "just work." That's
the goal anyway: we don't come close to fulfilling this end state.
This patch should be considered early alpha and RFC quality. The
following known issues and open investigation items exist:
* This is the first Rust code I've written. I'm almost certainly
doing many Rust things poorly. I'm pretty sure I'm a bit too
liberal with my .unwrap() usage.
* I haven't verified that the produced standalone Mercurial is
actually portable.
* HGUNICODEPEDANTRY likely doesn't work.
* rpath values are being hacked in post build. We should set the rpath
properly at link time.
* Windows support isn't fully implemented.
* The cpython crate seems to be optimized for writing Python
extensions, not embedding Python. As a result, the interaction
between the Rust `hg` and this crate is a bit wonky. For example,
I'm pretty sure we don't need to call `PyEval_InitThreads()`. But
the cpython crate insists a thread already exist if the Python
interpreter is already initialized. So... ???
* Python may not be receiving OS signals.
* Various inline TODOsThis commit provides a mostly-working implementation of the
`hg` script in Rust along with scaffolding to support Rust in
the repository.
If you are familiar with Rust, the contents of the added rust/
directory should be pretty straightforward. We create an "hgcli"
package that implements a binary application to run Mercurial.
The output of this package is an "hg" binary.
Our Rust `hg` (henceforth "rhg") essentially is a port of the existing
`hg` Python script. The main difference is the creation of the embedded
CPython interpreter is handled by the binary itself instead of relying
on the shebang. In that sense, rhg is more similar to the "exe wrapper"
we currently use on Windows. However, unlike the exe wrapper, rhg does
not call the `hg` Python script. Instead, it uses the CPython APIs to
import mercurial modules and call appropriate functions. The amount of
code here is surprisingly small.
It is my intent to replace the existing C-based exe wrapper with rhg.
Preferably in the next Mercurial release. This should be achievable -
at least for some Mercurial distributions. The future/timeline for
rhg on other platforms is less clear. We already ship a hg.exe on
Windows. So if we get the quirks with Rust worked out, shipping a
Rust-based hg.exe should hopefully not be too contentious.
Now onto the implementation.
We're using python27-sys and the cpython crates for talking to the
CPython API. We currently don't use too much functionality of the
cpython crate and could have probably cut it out. However, it does
provide a reasonable abstraction over unsafe {} CPython function
calls. While we still have our fair share of those, at least we're
not dealing with too much refcounting, error checking, etc. So I
think the use of the cpython crate is justified. Plus, there is
not-yet-implemented functionality that could benefit from cpython. I
see our use of this crate only increasing.
The cpython and python27-sys crates are not without their issues.
The cpython crate didn't seem to account for the embedding use case
in its design. Instead, it seems to assume that you are building
a Python extension. It is making some questionable decisions around
certain CPython APIs. For example, it insists that
PyEval_ThreadsInitialized() is called and that the Python code
likely isn't the main thread in the underlying application. It
is also missing some functionality that is important for embedded
use cases (such as exporting the path to the Python interpreter
from its build script). After spending several hours trying to
wrangle python27-sys and cpython, I gave up and forked the project
on GitHub. Our Cargo.toml tracks this fork. I'm optimistic that
the upstream project will accept our contributions and we can
eventually unfork.
There is a non-trivial amount of code in our custom Cargo build
script. Our build.rs (which is called as part of building the hgcli
crate):
* Validates that the Python interpreter that was detected by the
python27-sys crate provides a shared library (we only support
shared library linking at this time - although this restriction
could be loosened).
* Validates that the Python is built with UCS-4 support. This ensures
maximum Unicode compatibility.
* Exports variables to the crate build allowing the built crate to e.g.
find the path to the Python interpreter.
The produced rhg should be considered alpha quality. There are several
known deficiencies. Many of these are documented with inline TODOs.
Probably the biggest limitation of rhg is that it assumes it is
running from the ./rust/target/<target> directory of a source
distribution. So, rhg is currently not very practical for real-world
use. But, if you can `cargo build` it, running the binary *should*
yield a working Mercurial CLI.
In order to support using rhg with the test harness, we needed to hack
up run-tests.py so the path to Mercurial's Python files is set properly.
The change is extremely hacky and is only intended to be a stop-gap
until the test harness gains first-class support for installing rhg.
This will likely occur after we support running rhg outside the
source directory.
Despite its officially alpha quality, rhg copes extremely well with
the test harness (at least on Linux). Using
`run-tests.py --with-hg ../rust/target/debug/hg`, I only encounter
the following failures:
* test-run-tests.t -- Warnings emitted about using an unexpected
Mercurial library. This is due to the hacky nature of setting the
Python directory when run-tests.py detected rhg.
* test-devel-warnings.t -- Expected stack trace missing frame for `hg`
(This is expected since we no longer have an `hg` script!)
* test-convert.t -- Test running `$PYTHON "$BINDIR"/hg`, which obviously
assumes `hg` is a Python script.
* test-merge-tools.t -- Same assumption about `hg` being executable with
Python.
* test-http-bad-server.t -- Seeing exit code 255 instead of 1 around
line 358.
* test-blackbox.t -- Exit code 255 instead of 1.
* test-basic.t -- Exit code 255 instead of 1.
It certainly looks like we have a bug around exit code handling. I
don't think it is severe enough to hold up review and landing of this
initial implementation. Perfect is the enemy of good.