procutil: make stream detection in make_line_buffered more correct and strict
In make_line_buffered(), we don’t want to wrap the stream if we know that lines
get flushed to the underlying raw stream already.
Previously, the heuristic was too optimistic. It assumed that any stream which
is not an instance of io.BufferedIOBase doesn’t need wrapping. However, there
are buffered streams that aren’t instances of io.BufferedIOBase, like
Mercurial’s own winstdout.
The new logic is different in two ways:
First, only for the check, if unwraps any combination of WriteAllWrapper and
winstdout.
Second, it skips wrapping the stream only if it is an instance of io.RawIOBase
(or already wrapped). If it is an instance of io.BufferedIOBase, it gets
wrapped. In any other case, the function raises an exception. This ensures
that, if an unknown stream is passed or we add another wrapper in the future,
we don’t wrap the stream if it’s already line buffered or not wrap the stream
if it’s not line buffered. In fact, this was already helpful during development
of this change. Without it, I possibly would have forgot that WriteAllWrapper
needs to be ignored for the check, leading to unnecessary wrapping if stdout is
unbuffered.
The alternative would have been to always wrap unknown streams. However, I
don’t think that anyone would benefit from being less strict. We can expect
streams from the standard library to be subclassing either io.RawIOBase or
io.BufferedIOBase, so running Mercurial in the standard way should not regress
by this change. Py2exe might replace sys.stdout and sys.stderr, but that
currently breaks Mercurial anyway and also these streams don’t claim to be
interactive, so this function is not called for them.
===================
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
================
In the future, compile-time opt-ins may be added
to the `features` section in ``hg-cpython/Cargo.toml``.
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.