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 can be augmented with Rust extensions for speeding up certain
operations.
Compatibility
=============
Though the Rust extensions are only tested by the project under Linux, users of
MacOS, FreeBSD and other UNIX-likes have been using the Rust extensions. Your
mileage may vary, but by all means do give us feedback or signal your interest
for better support.
No Rust extensions are available for Windows at this time.
Features
========
The following operations are sped up when using Rust:
- discovery of differences between repositories (pull/push)
- nodemap (see :hg:`help config.format.use-persistent-nodemap`)
- all commands using the dirstate (status, commit, diff, add, update, etc.)
- dirstate-v2 (see :hg:`help config.format.use-dirstate-v2`)
- iteration over ancestors in a graph
More features are in the works, and improvements on the above listed are still
in progress. For more experimental work see the "rhg" section.
Checking for Rust
=================
You may already have the Rust extensions depending on how you install
Mercurial::
$ hg debuginstall | grep -i rust
checking Rust extensions (installed)
checking module policy (rust+c-allow)
If those lines don't even exist, you're using an old version of `hg` which does
not have any Rust extensions yet.
Installing
==========
You will need `cargo` to be in your `$PATH`. See the "MSRV" section for which
version to use.
Using pip
---------
Users of `pip` can install the Rust extensions with the following command::
$ pip install mercurial --global-option --rust --no-use-pep517
`--no-use-pep517` is here to tell `pip` to preserve backwards compatibility with
the legacy `setup.py` system. Mercurial has not yet migrated its complex setup
to the new system, so we still need this to add compiled extensions.
This might take a couple of minutes because you're compiling everything.
See the "Checking for Rust" section to see if the install succeeded.
From your distribution
----------------------
Some distributions are shipping Mercurial with Rust extensions enabled and
pre-compiled (meaning you won't have to install `cargo`), or allow you to
specify an install flag. Check with your specific distribution for how to do
that, or ask their team to add support for hg+Rust!
From source
-----------
Please refer to the `rust/README.rst` file in the Mercurial repository for
instructions on how to install from source.
MSRV
====
The minimum supported Rust version is currently 1.48.0. The project's policy is
to follow the version from Debian stable, to make the distributions' job easier.
rhg
===
There exists an experimental pure-Rust version of Mercurial called `rhg` with a
fallback mechanism for unsupported invocations. It allows for much faster
execution of certain commands while adding no discernable overhead for the rest.
The only way of trying it out is by building it from source. Please refer to
`rust/README.rst` in the Mercurial repository.
Contributing
============
If you would like to help the Rust endeavor, please refer to `rust/README.rst`
in the Mercurial repository.