annotate contrib/python-zstandard/zstd_cffi.py @ 30435:b86a448a2965

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.

# Copyright (c) 2016-present, Gregory Szorc
# All rights reserved.
#
# This software may be modified and distributed under the terms
# of the BSD license. See the LICENSE file for details.

"""Python interface to the Zstandard (zstd) compression library."""

from __future__ import absolute_import, unicode_literals

import io

from _zstd_cffi import (
    ffi,
    lib,
)


_CSTREAM_IN_SIZE = lib.ZSTD_CStreamInSize()
_CSTREAM_OUT_SIZE = lib.ZSTD_CStreamOutSize()


class _ZstdCompressionWriter(object):
    def __init__(self, cstream, writer):
        self._cstream = cstream
        self._writer = writer

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, exc_tb):
        if not exc_type and not exc_value and not exc_tb:
            out_buffer = ffi.new('ZSTD_outBuffer *')
            out_buffer.dst = ffi.new('char[]', _CSTREAM_OUT_SIZE)
            out_buffer.size = _CSTREAM_OUT_SIZE
            out_buffer.pos = 0

            while True:
                res = lib.ZSTD_endStream(self._cstream, out_buffer)
                if lib.ZSTD_isError(res):
                    raise Exception('error ending compression stream: %s' % lib.ZSTD_getErrorName)

                if out_buffer.pos:
                    self._writer.write(ffi.buffer(out_buffer.dst, out_buffer.pos))
                    out_buffer.pos = 0

                if res == 0:
                    break

        return False

    def write(self, data):
        out_buffer = ffi.new('ZSTD_outBuffer *')
        out_buffer.dst = ffi.new('char[]', _CSTREAM_OUT_SIZE)
        out_buffer.size = _CSTREAM_OUT_SIZE
        out_buffer.pos = 0

        # TODO can we reuse existing memory?
        in_buffer = ffi.new('ZSTD_inBuffer *')
        in_buffer.src = ffi.new('char[]', data)
        in_buffer.size = len(data)
        in_buffer.pos = 0
        while in_buffer.pos < in_buffer.size:
            res = lib.ZSTD_compressStream(self._cstream, out_buffer, in_buffer)
            if lib.ZSTD_isError(res):
                raise Exception('zstd compress error: %s' % lib.ZSTD_getErrorName(res))

            if out_buffer.pos:
                self._writer.write(ffi.buffer(out_buffer.dst, out_buffer.pos))
                out_buffer.pos = 0


class ZstdCompressor(object):
    def __init__(self, level=3, dict_data=None, compression_params=None):
        if dict_data:
            raise Exception('dict_data not yet supported')
        if compression_params:
            raise Exception('compression_params not yet supported')

        self._compression_level = level

    def compress(self, data):
        # Just use the stream API for now.
        output = io.BytesIO()
        with self.write_to(output) as compressor:
            compressor.write(data)
        return output.getvalue()

    def copy_stream(self, ifh, ofh):
        cstream = self._get_cstream()

        in_buffer = ffi.new('ZSTD_inBuffer *')
        out_buffer = ffi.new('ZSTD_outBuffer *')

        out_buffer.dst = ffi.new('char[]', _CSTREAM_OUT_SIZE)
        out_buffer.size = _CSTREAM_OUT_SIZE
        out_buffer.pos = 0

        total_read, total_write = 0, 0

        while True:
            data = ifh.read(_CSTREAM_IN_SIZE)
            if not data:
                break

            total_read += len(data)

            in_buffer.src = ffi.new('char[]', data)
            in_buffer.size = len(data)
            in_buffer.pos = 0

            while in_buffer.pos < in_buffer.size:
                res = lib.ZSTD_compressStream(cstream, out_buffer, in_buffer)
                if lib.ZSTD_isError(res):
                    raise Exception('zstd compress error: %s' %
                                    lib.ZSTD_getErrorName(res))

                if out_buffer.pos:
                    ofh.write(ffi.buffer(out_buffer.dst, out_buffer.pos))
                    total_write = out_buffer.pos
                    out_buffer.pos = 0

        # We've finished reading. Flush the compressor.
        while True:
            res = lib.ZSTD_endStream(cstream, out_buffer)
            if lib.ZSTD_isError(res):
                raise Exception('error ending compression stream: %s' %
                                lib.ZSTD_getErrorName(res))

            if out_buffer.pos:
                ofh.write(ffi.buffer(out_buffer.dst, out_buffer.pos))
                total_write += out_buffer.pos
                out_buffer.pos = 0

            if res == 0:
                break

        return total_read, total_write

    def write_to(self, writer):
        return _ZstdCompressionWriter(self._get_cstream(), writer)

    def _get_cstream(self):
        cstream = lib.ZSTD_createCStream()
        cstream = ffi.gc(cstream, lib.ZSTD_freeCStream)

        res = lib.ZSTD_initCStream(cstream, self._compression_level)
        if lib.ZSTD_isError(res):
            raise Exception('cannot init CStream: %s' %
                            lib.ZSTD_getErrorName(res))

        return cstream