mercurial/utils/compression.py

compression: introduce a `storage.revlog.zstd.level` configuration

This option control the zstd compression level used when compressing revlog
chunk. The usage of zstd for revlog compression has not graduated from
experimental yet, but we intend to fix that soon.

The option name for the compression level is more straight forward to pick, so
this changesets comes first.  Having a dedicated option for each compression
engine is useful because they don't support the same range of values.

I ran the same measurement as for the zlib compression level (in the parent
changesets). The variation in repository size is stay mostly in the same (small)
range. The "read/write" performance see smallish variation, but are overall much
better than zlib. Write performance show the same tend of having better write
performance for when reaching high-end compression.

Again, we don't intend to change the default zstd compression level (currently:
3) in this series. However this is worth investigating in the future.

The Performance comparison of zlib vs zstd is quite impressive. The repository
size stay in the same range, but the performance are much better in all
situations.

Comparison summary
==================

We are looking at:
- performance range for zlib
- performance range for zstd
- comparison of default zstd (level-3) to default zlib (level 6)
- comparison of the slowest zstd time to the fastest zlib time

Read performance:
-----------------
          |           zlib          |           zstd          | cmp | f2s
mercurial |   0.170159 -   0.189219 |   0.144127 -   0.149624 | 80% | 88%
pypy      |   2.679217 -   2.768691 |   1.532317 -   1.705044 | 60% | 63%
netbeans  | 122.477027 - 141.620281 |  72.996346 -  89.731560 | 58% | 73%
mozilla   | 147.867662 - 170.572118 |  91.700995 - 105.853099 | 56% | 71%

Write performance:
------------------
          |           zlib          |           zstd          | cmp | f2s
mercurial |  53.250304 - 56.2936129 |  40.877025 -  45.677286 | 75% | 86%
pypy      | 460.721984 - 476.589918 | 270.545409 - 301.002219 | 63% | 65%
netbeans  | 520.560316 - 715.930400 | 370.356311 - 428.329652 | 55% | 82%
mozilla   | 739.803002 - 987.056093 | 505.152906 - 591.930683 | 57% | 80%

Raw data
--------

repo      alg lvl  .hg/store size  00manifest.d read       write

mercurial zlib  1      49,402,813     5,963,475   0.170159  53.250304
mercurial zlib  6      47,197,397     5,875,730   0.182820  56.264320
mercurial zlib  9      47,121,596     5,849,781   0.189219  56.293612

mercurial zstd  1      49,737,084     5,966,355   0.144127  40.877025
mercurial zstd  3      48,961,867     5,895,208   0.146376  42.268142
mercurial zstd  5      48,200,592     5,938,676   0.149624  43.162875
mercurial zstd 10      47,833,520     5,913,353   0.145185  44.012489
mercurial zstd 15      47,314,604     5,728,679   0.147686  45.677286
mercurial zstd 20      47,330,502     5,830,539   0.145789  45.025407
mercurial zstd 22      47,330,076     5,830,539   0.143996  44.690460


pypy      zlib  1     370,830,572    28,462,425   2.679217 460.721984
pypy      zlib  6     340,112,317    27,648,747   2.768691 467.537158
pypy      zlib  9     338,360,736    27,639,003   2.763495 476.589918

pypy      zstd  1     362,377,479    27,916,214   1.532317 270.545409
pypy      zstd  3     354,137,693    27,905,988   1.686718 294.951509
pypy      zstd  5     342,640,043    27,655,774   1.705044 301.002219
pypy      zstd 10     334,224,327    27,164,493   1.567287 285.186239
pypy      zstd 15     329,000,363    26,645,965   1.637729 299.561332
pypy      zstd 20     324,534,039    26,199,547   1.526813 302.149827
pypy      zstd 22     324,530,595    26,198,932   1.525718 307.821218


netbeans  zlib  1   1,281,847,810   165,495,457 122.477027 520.560316
netbeans  zlib  6   1,205,284,353   159,161,207 139.876147 715.930400
netbeans  zlib  9   1,197,135,671   155,034,586 141.620281 678.297064

netbeans  zstd  1   1,259,581,737   160,840,613  72.996346 370.356311
netbeans  zstd  3   1,232,978,122   157,691,551  81.622317 396.733087
netbeans  zstd  5   1,208,034,075   160,246,880  83.080549 364.342626
netbeans  zstd 10   1,188,624,176   156,083,417  79.323935 403.594602
netbeans  zstd 15   1,176,973,589   153,859,477  89.731560 428.329652
netbeans  zstd 20   1,162,958,258   151,147,535  82.842667 392.335349
netbeans  zstd 22   1,162,707,029   151,150,220  82.565695 402.840655


mozilla   zlib  1   2,775,497,186   298,527,987 147.867662 751.263721
mozilla   zlib  6   2,596,856,420   286,597,671 170.572118 987.056093
mozilla   zlib  9   2,587,542,494   287,018,264 163.622338 739.803002

mozilla   zstd  1   2,723,159,348   286,617,532  91.700995 570.042751
mozilla   zstd  3   2,665,055,001   286,152,013  95.240155 561.412805
mozilla   zstd  5   2,607,819,817   288,060,030 101.978048 505.152906
mozilla   zstd 10   2,558,761,085   283,967,648 104.113481 497.771202
mozilla   zstd 15   2,526,216,060   275,581,300 105.853099 591.930683
mozilla   zstd 20   2,485,114,806   266,478,859  95.268795 576.515389
mozilla   zstd 22   2,484,869,080   266,456,505  94.429282 572.785537

# compression.py - Mercurial utility functions for compression
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.


from __future__ import absolute_import, print_function

import bz2
import collections
import zlib

from .. import (
    error,
    i18n,
    pycompat,
)
from . import (
    stringutil,
)

safehasattr = pycompat.safehasattr


_ = i18n._

# compression code

SERVERROLE = 'server'
CLIENTROLE = 'client'

compewireprotosupport = collections.namedtuple(r'compenginewireprotosupport',
                                               (r'name', r'serverpriority',
                                                r'clientpriority'))

class propertycache(object):
    def __init__(self, func):
        self.func = func
        self.name = func.__name__
    def __get__(self, obj, type=None):
        result = self.func(obj)
        self.cachevalue(obj, result)
        return result

    def cachevalue(self, obj, value):
        # __dict__ assignment required to bypass __setattr__ (eg: repoview)
        obj.__dict__[self.name] = value

class compressormanager(object):
    """Holds registrations of various compression engines.

    This class essentially abstracts the differences between compression
    engines to allow new compression formats to be added easily, possibly from
    extensions.

    Compressors are registered against the global instance by calling its
    ``register()`` method.
    """
    def __init__(self):
        self._engines = {}
        # Bundle spec human name to engine name.
        self._bundlenames = {}
        # Internal bundle identifier to engine name.
        self._bundletypes = {}
        # Revlog header to engine name.
        self._revlogheaders = {}
        # Wire proto identifier to engine name.
        self._wiretypes = {}

    def __getitem__(self, key):
        return self._engines[key]

    def __contains__(self, key):
        return key in self._engines

    def __iter__(self):
        return iter(self._engines.keys())

    def register(self, engine):
        """Register a compression engine with the manager.

        The argument must be a ``compressionengine`` instance.
        """
        if not isinstance(engine, compressionengine):
            raise ValueError(_('argument must be a compressionengine'))

        name = engine.name()

        if name in self._engines:
            raise error.Abort(_('compression engine %s already registered') %
                              name)

        bundleinfo = engine.bundletype()
        if bundleinfo:
            bundlename, bundletype = bundleinfo

            if bundlename in self._bundlenames:
                raise error.Abort(_('bundle name %s already registered') %
                                  bundlename)
            if bundletype in self._bundletypes:
                raise error.Abort(_('bundle type %s already registered by %s') %
                                  (bundletype, self._bundletypes[bundletype]))

            # No external facing name declared.
            if bundlename:
                self._bundlenames[bundlename] = name

            self._bundletypes[bundletype] = name

        wiresupport = engine.wireprotosupport()
        if wiresupport:
            wiretype = wiresupport.name
            if wiretype in self._wiretypes:
                raise error.Abort(_('wire protocol compression %s already '
                                    'registered by %s') %
                                  (wiretype, self._wiretypes[wiretype]))

            self._wiretypes[wiretype] = name

        revlogheader = engine.revlogheader()
        if revlogheader and revlogheader in self._revlogheaders:
            raise error.Abort(_('revlog header %s already registered by %s') %
                              (revlogheader, self._revlogheaders[revlogheader]))

        if revlogheader:
            self._revlogheaders[revlogheader] = name

        self._engines[name] = engine

    @property
    def supportedbundlenames(self):
        return set(self._bundlenames.keys())

    @property
    def supportedbundletypes(self):
        return set(self._bundletypes.keys())

    def forbundlename(self, bundlename):
        """Obtain a compression engine registered to a bundle name.

        Will raise KeyError if the bundle type isn't registered.

        Will abort if the engine is known but not available.
        """
        engine = self._engines[self._bundlenames[bundlename]]
        if not engine.available():
            raise error.Abort(_('compression engine %s could not be loaded') %
                              engine.name())
        return engine

    def forbundletype(self, bundletype):
        """Obtain a compression engine registered to a bundle type.

        Will raise KeyError if the bundle type isn't registered.

        Will abort if the engine is known but not available.
        """
        engine = self._engines[self._bundletypes[bundletype]]
        if not engine.available():
            raise error.Abort(_('compression engine %s could not be loaded') %
                              engine.name())
        return engine

    def supportedwireengines(self, role, onlyavailable=True):
        """Obtain compression engines that support the wire protocol.

        Returns a list of engines in prioritized order, most desired first.

        If ``onlyavailable`` is set, filter out engines that can't be
        loaded.
        """
        assert role in (SERVERROLE, CLIENTROLE)

        attr = 'serverpriority' if role == SERVERROLE else 'clientpriority'

        engines = [self._engines[e] for e in self._wiretypes.values()]
        if onlyavailable:
            engines = [e for e in engines if e.available()]

        def getkey(e):
            # Sort first by priority, highest first. In case of tie, sort
            # alphabetically. This is arbitrary, but ensures output is
            # stable.
            w = e.wireprotosupport()
            return -1 * getattr(w, attr), w.name

        return list(sorted(engines, key=getkey))

    def forwiretype(self, wiretype):
        engine = self._engines[self._wiretypes[wiretype]]
        if not engine.available():
            raise error.Abort(_('compression engine %s could not be loaded') %
                              engine.name())
        return engine

    def forrevlogheader(self, header):
        """Obtain a compression engine registered to a revlog header.

        Will raise KeyError if the revlog header value isn't registered.
        """
        return self._engines[self._revlogheaders[header]]

compengines = compressormanager()

class compressionengine(object):
    """Base class for compression engines.

    Compression engines must implement the interface defined by this class.
    """
    def name(self):
        """Returns the name of the compression engine.

        This is the key the engine is registered under.

        This method must be implemented.
        """
        raise NotImplementedError()

    def available(self):
        """Whether the compression engine is available.

        The intent of this method is to allow optional compression engines
        that may not be available in all installations (such as engines relying
        on C extensions that may not be present).
        """
        return True

    def bundletype(self):
        """Describes bundle identifiers for this engine.

        If this compression engine isn't supported for bundles, returns None.

        If this engine can be used for bundles, returns a 2-tuple of strings of
        the user-facing "bundle spec" compression name and an internal
        identifier used to denote the compression format within bundles. To
        exclude the name from external usage, set the first element to ``None``.

        If bundle compression is supported, the class must also implement
        ``compressstream`` and `decompressorreader``.

        The docstring of this method is used in the help system to tell users
        about this engine.
        """
        return None

    def wireprotosupport(self):
        """Declare support for this compression format on the wire protocol.

        If this compression engine isn't supported for compressing wire
        protocol payloads, returns None.

        Otherwise, returns ``compenginewireprotosupport`` with the following
        fields:

        * String format identifier
        * Integer priority for the server
        * Integer priority for the client

        The integer priorities are used to order the advertisement of format
        support by server and client. The highest integer is advertised
        first. Integers with non-positive values aren't advertised.

        The priority values are somewhat arbitrary and only used for default
        ordering. The relative order can be changed via config options.

        If wire protocol compression is supported, the class must also implement
        ``compressstream`` and ``decompressorreader``.
        """
        return None

    def revlogheader(self):
        """Header added to revlog chunks that identifies this engine.

        If this engine can be used to compress revlogs, this method should
        return the bytes used to identify chunks compressed with this engine.
        Else, the method should return ``None`` to indicate it does not
        participate in revlog compression.
        """
        return None

    def compressstream(self, it, opts=None):
        """Compress an iterator of chunks.

        The method receives an iterator (ideally a generator) of chunks of
        bytes to be compressed. It returns an iterator (ideally a generator)
        of bytes of chunks representing the compressed output.

        Optionally accepts an argument defining how to perform compression.
        Each engine treats this argument differently.
        """
        raise NotImplementedError()

    def decompressorreader(self, fh):
        """Perform decompression on a file object.

        Argument is an object with a ``read(size)`` method that returns
        compressed data. Return value is an object with a ``read(size)`` that
        returns uncompressed data.
        """
        raise NotImplementedError()

    def revlogcompressor(self, opts=None):
        """Obtain an object that can be used to compress revlog entries.

        The object has a ``compress(data)`` method that compresses binary
        data. This method returns compressed binary data or ``None`` if
        the data could not be compressed (too small, not compressible, etc).
        The returned data should have a header uniquely identifying this
        compression format so decompression can be routed to this engine.
        This header should be identified by the ``revlogheader()`` return
        value.

        The object has a ``decompress(data)`` method that decompresses
        data. The method will only be called if ``data`` begins with
        ``revlogheader()``. The method should return the raw, uncompressed
        data or raise a ``StorageError``.

        The object is reusable but is not thread safe.
        """
        raise NotImplementedError()

class _CompressedStreamReader(object):
    def __init__(self, fh):
        if safehasattr(fh, 'unbufferedread'):
            self._reader = fh.unbufferedread
        else:
            self._reader = fh.read
        self._pending = []
        self._pos = 0
        self._eof = False

    def _decompress(self, chunk):
        raise NotImplementedError()

    def read(self, l):
        buf = []
        while True:
            while self._pending:
                if len(self._pending[0]) > l + self._pos:
                    newbuf = self._pending[0]
                    buf.append(newbuf[self._pos:self._pos + l])
                    self._pos += l
                    return ''.join(buf)

                newbuf = self._pending.pop(0)
                if self._pos:
                    buf.append(newbuf[self._pos:])
                    l -= len(newbuf) - self._pos
                else:
                    buf.append(newbuf)
                    l -= len(newbuf)
                self._pos = 0

            if self._eof:
                return ''.join(buf)
            chunk = self._reader(65536)
            self._decompress(chunk)
            if not chunk and not self._pending and not self._eof:
                # No progress and no new data, bail out
                return ''.join(buf)

class _GzipCompressedStreamReader(_CompressedStreamReader):
    def __init__(self, fh):
        super(_GzipCompressedStreamReader, self).__init__(fh)
        self._decompobj = zlib.decompressobj()
    def _decompress(self, chunk):
        newbuf = self._decompobj.decompress(chunk)
        if newbuf:
            self._pending.append(newbuf)
        d = self._decompobj.copy()
        try:
            d.decompress('x')
            d.flush()
            if d.unused_data == 'x':
                self._eof = True
        except zlib.error:
            pass

class _BZ2CompressedStreamReader(_CompressedStreamReader):
    def __init__(self, fh):
        super(_BZ2CompressedStreamReader, self).__init__(fh)
        self._decompobj = bz2.BZ2Decompressor()
    def _decompress(self, chunk):
        newbuf = self._decompobj.decompress(chunk)
        if newbuf:
            self._pending.append(newbuf)
        try:
            while True:
                newbuf = self._decompobj.decompress('')
                if newbuf:
                    self._pending.append(newbuf)
                else:
                    break
        except EOFError:
            self._eof = True

class _TruncatedBZ2CompressedStreamReader(_BZ2CompressedStreamReader):
    def __init__(self, fh):
        super(_TruncatedBZ2CompressedStreamReader, self).__init__(fh)
        newbuf = self._decompobj.decompress('BZ')
        if newbuf:
            self._pending.append(newbuf)

class _ZstdCompressedStreamReader(_CompressedStreamReader):
    def __init__(self, fh, zstd):
        super(_ZstdCompressedStreamReader, self).__init__(fh)
        self._zstd = zstd
        self._decompobj = zstd.ZstdDecompressor().decompressobj()
    def _decompress(self, chunk):
        newbuf = self._decompobj.decompress(chunk)
        if newbuf:
            self._pending.append(newbuf)
        try:
            while True:
                newbuf = self._decompobj.decompress('')
                if newbuf:
                    self._pending.append(newbuf)
                else:
                    break
        except self._zstd.ZstdError:
            self._eof = True

class _zlibengine(compressionengine):
    def name(self):
        return 'zlib'

    def bundletype(self):
        """zlib compression using the DEFLATE algorithm.

        All Mercurial clients should support this format. The compression
        algorithm strikes a reasonable balance between compression ratio
        and size.
        """
        return 'gzip', 'GZ'

    def wireprotosupport(self):
        return compewireprotosupport('zlib', 20, 20)

    def revlogheader(self):
        return 'x'

    def compressstream(self, it, opts=None):
        opts = opts or {}

        z = zlib.compressobj(opts.get('level', -1))
        for chunk in it:
            data = z.compress(chunk)
            # Not all calls to compress emit data. It is cheaper to inspect
            # here than to feed empty chunks through generator.
            if data:
                yield data

        yield z.flush()

    def decompressorreader(self, fh):
        return _GzipCompressedStreamReader(fh)

    class zlibrevlogcompressor(object):

        def __init__(self, level=None):
            self._level = level

        def compress(self, data):
            insize = len(data)
            # Caller handles empty input case.
            assert insize > 0

            if insize < 44:
                return None

            elif insize <= 1000000:
                if self._level is None:
                    compressed = zlib.compress(data)
                else:
                    compressed = zlib.compress(data, self._level)
                if len(compressed) < insize:
                    return compressed
                return None

            # zlib makes an internal copy of the input buffer, doubling
            # memory usage for large inputs. So do streaming compression
            # on large inputs.
            else:
                if self._level is None:
                    z = zlib.compressobj()
                else:
                    z = zlib.compressobj(level=self._level)
                parts = []
                pos = 0
                while pos < insize:
                    pos2 = pos + 2**20
                    parts.append(z.compress(data[pos:pos2]))
                    pos = pos2
                parts.append(z.flush())

                if sum(map(len, parts)) < insize:
                    return ''.join(parts)
                return None

        def decompress(self, data):
            try:
                return zlib.decompress(data)
            except zlib.error as e:
                raise error.StorageError(_('revlog decompress error: %s') %
                                         stringutil.forcebytestr(e))

    def revlogcompressor(self, opts=None):
        level = None
        if opts is not None:
            level = opts.get('zlib.level')
        return self.zlibrevlogcompressor(level)

compengines.register(_zlibengine())

class _bz2engine(compressionengine):
    def name(self):
        return 'bz2'

    def bundletype(self):
        """An algorithm that produces smaller bundles than ``gzip``.

        All Mercurial clients should support this format.

        This engine will likely produce smaller bundles than ``gzip`` but
        will be significantly slower, both during compression and
        decompression.

        If available, the ``zstd`` engine can yield similar or better
        compression at much higher speeds.
        """
        return 'bzip2', 'BZ'

    # We declare a protocol name but don't advertise by default because
    # it is slow.
    def wireprotosupport(self):
        return compewireprotosupport('bzip2', 0, 0)

    def compressstream(self, it, opts=None):
        opts = opts or {}
        z = bz2.BZ2Compressor(opts.get('level', 9))
        for chunk in it:
            data = z.compress(chunk)
            if data:
                yield data

        yield z.flush()

    def decompressorreader(self, fh):
        return _BZ2CompressedStreamReader(fh)

compengines.register(_bz2engine())

class _truncatedbz2engine(compressionengine):
    def name(self):
        return 'bz2truncated'

    def bundletype(self):
        return None, '_truncatedBZ'

    # We don't implement compressstream because it is hackily handled elsewhere.

    def decompressorreader(self, fh):
        return _TruncatedBZ2CompressedStreamReader(fh)

compengines.register(_truncatedbz2engine())

class _noopengine(compressionengine):
    def name(self):
        return 'none'

    def bundletype(self):
        """No compression is performed.

        Use this compression engine to explicitly disable compression.
        """
        return 'none', 'UN'

    # Clients always support uncompressed payloads. Servers don't because
    # unless you are on a fast network, uncompressed payloads can easily
    # saturate your network pipe.
    def wireprotosupport(self):
        return compewireprotosupport('none', 0, 10)

    # We don't implement revlogheader because it is handled specially
    # in the revlog class.

    def compressstream(self, it, opts=None):
        return it

    def decompressorreader(self, fh):
        return fh

    class nooprevlogcompressor(object):
        def compress(self, data):
            return None

    def revlogcompressor(self, opts=None):
        return self.nooprevlogcompressor()

compengines.register(_noopengine())

class _zstdengine(compressionengine):
    def name(self):
        return 'zstd'

    @propertycache
    def _module(self):
        # Not all installs have the zstd module available. So defer importing
        # until first access.
        try:
            from .. import zstd
            # Force delayed import.
            zstd.__version__
            return zstd
        except ImportError:
            return None

    def available(self):
        return bool(self._module)

    def bundletype(self):
        """A modern compression algorithm that is fast and highly flexible.

        Only supported by Mercurial 4.1 and newer clients.

        With the default settings, zstd compression is both faster and yields
        better compression than ``gzip``. It also frequently yields better
        compression than ``bzip2`` while operating at much higher speeds.

        If this engine is available and backwards compatibility is not a
        concern, it is likely the best available engine.
        """
        return 'zstd', 'ZS'

    def wireprotosupport(self):
        return compewireprotosupport('zstd', 50, 50)

    def revlogheader(self):
        return '\x28'

    def compressstream(self, it, opts=None):
        opts = opts or {}
        # zstd level 3 is almost always significantly faster than zlib
        # while providing no worse compression. It strikes a good balance
        # between speed and compression.
        level = opts.get('level', 3)

        zstd = self._module
        z = zstd.ZstdCompressor(level=level).compressobj()
        for chunk in it:
            data = z.compress(chunk)
            if data:
                yield data

        yield z.flush()

    def decompressorreader(self, fh):
        return _ZstdCompressedStreamReader(fh, self._module)

    class zstdrevlogcompressor(object):
        def __init__(self, zstd, level=3):
            # TODO consider omitting frame magic to save 4 bytes.
            # This writes content sizes into the frame header. That is
            # extra storage. But it allows a correct size memory allocation
            # to hold the result.
            self._cctx = zstd.ZstdCompressor(level=level)
            self._dctx = zstd.ZstdDecompressor()
            self._compinsize = zstd.COMPRESSION_RECOMMENDED_INPUT_SIZE
            self._decompinsize = zstd.DECOMPRESSION_RECOMMENDED_INPUT_SIZE

        def compress(self, data):
            insize = len(data)
            # Caller handles empty input case.
            assert insize > 0

            if insize < 50:
                return None

            elif insize <= 1000000:
                compressed = self._cctx.compress(data)
                if len(compressed) < insize:
                    return compressed
                return None
            else:
                z = self._cctx.compressobj()
                chunks = []
                pos = 0
                while pos < insize:
                    pos2 = pos + self._compinsize
                    chunk = z.compress(data[pos:pos2])
                    if chunk:
                        chunks.append(chunk)
                    pos = pos2
                chunks.append(z.flush())

                if sum(map(len, chunks)) < insize:
                    return ''.join(chunks)
                return None

        def decompress(self, data):
            insize = len(data)

            try:
                # This was measured to be faster than other streaming
                # decompressors.
                dobj = self._dctx.decompressobj()
                chunks = []
                pos = 0
                while pos < insize:
                    pos2 = pos + self._decompinsize
                    chunk = dobj.decompress(data[pos:pos2])
                    if chunk:
                        chunks.append(chunk)
                    pos = pos2
                # Frame should be exhausted, so no finish() API.

                return ''.join(chunks)
            except Exception as e:
                raise error.StorageError(_('revlog decompress error: %s') %
                                         stringutil.forcebytestr(e))

    def revlogcompressor(self, opts=None):
        opts = opts or {}
        level = opts.get('zstd.level')
        if level is None:
            level = opts.get('level')
        if level is None:
            level = 3
        return self.zstdrevlogcompressor(self._module, level=level)

compengines.register(_zstdengine())

def bundlecompressiontopics():
    """Obtains a list of available bundle compressions for use in help."""
    # help.makeitemsdocs() expects a dict of names to items with a .__doc__.
    items = {}

    # We need to format the docstring. So use a dummy object/type to hold it
    # rather than mutating the original.
    class docobject(object):
        pass

    for name in compengines:
        engine = compengines[name]

        if not engine.available():
            continue

        bt = engine.bundletype()
        if not bt or not bt[0]:
            continue

        doc = b'``%s``\n    %s' % (bt[0], pycompat.getdoc(engine.bundletype))

        value = docobject()
        value.__doc__ = pycompat.sysstr(doc)
        value._origdoc = engine.bundletype.__doc__
        value._origfunc = engine.bundletype

        items[bt[0]] = value

    return items

i18nfunctions = bundlecompressiontopics().values()