util: check for compression engine availability before returning If a requested compression engine is registered but not available, requesting it will now abort. To be honest, I'm not sure if this is the appropriate mechanism for handling optional compression engines. I won't know until all uses of compression (bundles, wire protocol, revlogs, etc) are using the new API and zstd (our planned optional engine) is implemented. So this API could change.

util: expose an "available" API on compression engines When the zstd compression engine is introduced, it won't work in all installations, namely pure Python installs. So, we need a mechanism to declare whether a compression engine is available. We don't want to conditionally register the compression engine because it is sometimes useful to know when a compression engine name or encountered data is valid but just not available versus unknown.

setup: compile zstd C extension Now that zstd and python-zstandard are vendored, we can start compiling them as part of the install. python-zstandard provides a self-contained Python function that returns a distutils.extension.Extension, so it is really easy to add zstd to our setup.py without having to worry about defining source files, include paths, etc. The function even allows specifying the module name the extension should be compiled as. This conveniently allows us to compile the module into the "mercurial" package so "our" version won't collide with a version installed under the canonical "zstd" module name.

zstd: vendor python-zstandard 0.5.0 As the commit message for the previous changeset says, we wish for zstd to be a 1st class citizen in Mercurial. To make that happen, we need to enable Python to talk to the zstd C API. And that requires bindings. This commit vendors a copy of existing Python bindings. Why do we need to vendor? As the commit message of the previous commit says, relying on systems in the wild to have the bindings or zstd present is a losing proposition. By distributing the zstd and bindings with Mercurial, we significantly increase our chances that zstd will work. Since zstd will deliver a better end-user experience by achieving better performance, this benefits our users. Another reason is that the Python bindings still aren't stable and the API is somewhat fluid. While Mercurial could be coded to target multiple versions of the Python bindings, it is safer to bundle an explicit, known working version. The added Python bindings are mostly a fully-featured interface to the zstd C API. They allow one-shot operations, streaming, reading and writing from objects implements the file object protocol, dictionary compression, control over low-level compression parameters, and more. The Python bindings work on Python 2.6, 2.7, and 3.3+ and have been tested on Linux and Windows. There are CFFI bindings, but they are lacking compared to the C extension. Upstream work will be needed before we can support zstd with PyPy. But it will be possible. The files added in this commit come from Git commit e637c1b214d5f869cf8116c550dcae23ec13b677 from https://github.com/indygreg/python-zstandard and are added without modifications. Some files from the upstream repository have been omitted, namely files related to continuous integration. In the spirit of full disclosure, I'm the maintainer of the "python-zstandard" project and have authored 100% of the code added in this commit. Unfortunately, the Python bindings have not been formally code reviewed by anyone. While I've tested much of the code thoroughly (I even have tests that fuzz APIs), there's a good chance there are bugs, memory leaks, not well thought out APIs, etc. If someone wants to review the code and send feedback to the GitHub project, it would be greatly appreciated. Despite my involvement with both projects, my opinions of code style differ from Mercurial's. The code in this commit introduces numerous code style violations in Mercurial's linters. So, the code is excluded from most lints. However, some violations I agree with. These have been added to the known violations ignore list for now.

zstd: vendor zstd 1.1.1 zstd is a new compression format and it is awesome, yielding higher compression ratios and significantly faster compression and decompression operations compared to zlib (our current compression engine of choice) across the board. We want zstd to be a 1st class citizen in Mercurial and to eventually be the preferred compression format for various operations. This patch starts the formal process of supporting zstd by vendoring a copy of zstd. Why do we need to vendor zstd? Good question. First, zstd is relatively new and not widely available yet. If we didn't vendor zstd or distribute it with Mercurial, most users likely wouldn't have zstd installed or even available to install. What good is a feature if you can't use it? Vendoring and distributing the zstd sources gives us the highest liklihood that zstd will be available to Mercurial installs. Second, the Python bindings to zstd (which will be vendored in a separate changeset) make use of zstd APIs that are only available via static linking. One reason they are only available via static linking is that they are unstable and could change at any time. While it might be possible for the Python bindings to attempt to talk to different versions of the zstd C library, the safest thing to do is link against a specific, known-working version of zstd. This is why the Python zstd bindings themselves vendor zstd and why we must as well. This also explains why the added files are in a "python-zstandard" directory. The added files are from the 1.1.1 release of zstd (Git commit 4c0b44f8ced84c4c8edfa07b564d31e4fa3e8885 from https://github.com/facebook/zstd) and are added without modifications. Not all files from the zstd "distribution" have been added. Notably missing are files to support interacting with "legacy," pre-1.0 versions of zstd. The decision of which files to include is made by the upstream python-zstandard project (which I'm the author of). The files in this commit are a snapshot of the files from the 0.5.0 release of that project, Git commit e637c1b214d5f869cf8116c550dcae23ec13b677 from https://github.com/indygreg/python-zstandard.

bdiff: give slight preference to removing trailing lines [This change could be folded into the previous changeset to minimize the repo churn ...] Similar to the previous change, introduce an exception to the general preference for matches in the middle of bdiff ranges: If the best match on the B side starts at the beginning of the bdiff range, don't aim for the middle-most A side match but for the earliest. New (later) matches on the A side will only be considered better if the corresponding match on the B side *not* is at the beginning of the range. Thus, if the best (middle-most) match on the B side turns out to be at the beginning of the range, the earliest match on the A side will be used. The bundle size for 4.0 (hg bundle --base null -r 4.0 x.hg) happens to go from 22807275 to 22808120 bytes - a 0.004% increase.