mercurial/revlogutils/constants.py

revlog: implement a "default compression" mode

The revlog docker is now storing a default compression engine. When a chunk use
that compression, a dedicated mode is used in the revlog entry and we can
directly route it to the right decompressor.

We should probably make PLAIN and DEFAULT mode the only available mode for
revlogv2, but this is something for later.

Differential Revision: https://phab.mercurial-scm.org/D10652

# revlogdeltas.py - constant used for revlog logic.
#
# Copyright 2005-2007 Olivia Mackall <[e-mail]>
# Copyright 2018 Octobus <[e-mail]>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
"""Helper class to compute deltas stored inside revlogs"""

from __future__ import absolute_import

import struct

from ..interfaces import repository

### Internal utily constants

KIND_CHANGELOG = 1001  # over 256 to not be comparable with a bytes
KIND_MANIFESTLOG = 1002
KIND_FILELOG = 1003
KIND_OTHER = 1004

ALL_KINDS = {
    KIND_CHANGELOG,
    KIND_MANIFESTLOG,
    KIND_FILELOG,
    KIND_OTHER,
}

### main revlog header

INDEX_HEADER = struct.Struct(b">I")

## revlog version
REVLOGV0 = 0
REVLOGV1 = 1
# Dummy value until file format is finalized.
REVLOGV2 = 0xDEAD

##  global revlog header flags
# Shared across v1 and v2.
FLAG_INLINE_DATA = 1 << 16
# Only used by v1, implied by v2.
FLAG_GENERALDELTA = 1 << 17
REVLOG_DEFAULT_FLAGS = FLAG_INLINE_DATA
REVLOG_DEFAULT_FORMAT = REVLOGV1
REVLOG_DEFAULT_VERSION = REVLOG_DEFAULT_FORMAT | REVLOG_DEFAULT_FLAGS
REVLOGV0_FLAGS = 0
REVLOGV1_FLAGS = FLAG_INLINE_DATA | FLAG_GENERALDELTA
REVLOGV2_FLAGS = FLAG_INLINE_DATA

### individual entry

## index v0:
#  4 bytes: offset
#  4 bytes: compressed length
#  4 bytes: base rev
#  4 bytes: link rev
# 20 bytes: parent 1 nodeid
# 20 bytes: parent 2 nodeid
# 20 bytes: nodeid
INDEX_ENTRY_V0 = struct.Struct(b">4l20s20s20s")

## index v1
#  6 bytes: offset
#  2 bytes: flags
#  4 bytes: compressed length
#  4 bytes: uncompressed length
#  4 bytes: base rev
#  4 bytes: link rev
#  4 bytes: parent 1 rev
#  4 bytes: parent 2 rev
# 32 bytes: nodeid
INDEX_ENTRY_V1 = struct.Struct(b">Qiiiiii20s12x")
assert INDEX_ENTRY_V1.size == 32 * 2

#  6 bytes: offset
#  2 bytes: flags
#  4 bytes: compressed length
#  4 bytes: uncompressed length
#  4 bytes: base rev
#  4 bytes: link rev
#  4 bytes: parent 1 rev
#  4 bytes: parent 2 rev
# 32 bytes: nodeid
#  8 bytes: sidedata offset
#  4 bytes: sidedata compressed length
#  1 bytes: compression mode (2 lower bit are data_compression_mode)
#  19 bytes: Padding to align to 96 bytes (see RevlogV2Plan wiki page)
INDEX_ENTRY_V2 = struct.Struct(b">Qiiiiii20s12xQiB19x")
assert INDEX_ENTRY_V2.size == 32 * 3, INDEX_ENTRY_V2.size

# revlog index flags

# For historical reasons, revlog's internal flags were exposed via the
# wire protocol and are even exposed in parts of the storage APIs.

# revision has censor metadata, must be verified
REVIDX_ISCENSORED = repository.REVISION_FLAG_CENSORED
# revision hash does not match data (narrowhg)
REVIDX_ELLIPSIS = repository.REVISION_FLAG_ELLIPSIS
# revision data is stored externally
REVIDX_EXTSTORED = repository.REVISION_FLAG_EXTSTORED
# revision changes files in a way that could affect copy tracing.
REVIDX_HASCOPIESINFO = repository.REVISION_FLAG_HASCOPIESINFO
REVIDX_DEFAULT_FLAGS = 0
# stable order in which flags need to be processed and their processors applied
REVIDX_FLAGS_ORDER = [
    REVIDX_ISCENSORED,
    REVIDX_ELLIPSIS,
    REVIDX_EXTSTORED,
    REVIDX_HASCOPIESINFO,
]

# bitmark for flags that could cause rawdata content change
REVIDX_RAWTEXT_CHANGING_FLAGS = REVIDX_ISCENSORED | REVIDX_EXTSTORED

## chunk compression mode constants:
# These constants are used in revlog version >=2 to denote the compression used
# for a chunk.

# Chunk use no compression, the data stored on disk can be directly use as
# chunk value. Without any header information prefixed.
COMP_MODE_PLAIN = 0

# Chunk use the "default compression" for the revlog (usually defined in the
# revlog docket). A header is still used.
#
# XXX: keeping a header is probably not useful and we should probably drop it.
#
# XXX: The value of allow mixed type of compression in the revlog is unclear
#      and we should consider making PLAIN/DEFAULT the only available mode for
#      revlog v2, disallowing INLINE mode.
COMP_MODE_DEFAULT = 1

# Chunk use a compression mode stored "inline" at the start of the chunk
# itself.  This is the mode always used for revlog version "0" and "1"
COMP_MODE_INLINE = 2

SUPPORTED_FLAGS = {
    REVLOGV0: REVLOGV0_FLAGS,
    REVLOGV1: REVLOGV1_FLAGS,
    REVLOGV2: REVLOGV2_FLAGS,
}

_no = lambda flags: False
_yes = lambda flags: True


def _from_flag(flag):
    return lambda flags: bool(flags & flag)


FEATURES_BY_VERSION = {
    REVLOGV0: {
        b'inline': _no,
        b'generaldelta': _no,
        b'sidedata': False,
        b'docket': False,
    },
    REVLOGV1: {
        b'inline': _from_flag(FLAG_INLINE_DATA),
        b'generaldelta': _from_flag(FLAG_GENERALDELTA),
        b'sidedata': False,
        b'docket': False,
    },
    REVLOGV2: {
        # The point of inline-revlog is to reduce the number of files used in
        # the store. Using a docket defeat this purpose. So we needs other
        # means to reduce the number of files for revlogv2.
        b'inline': _no,
        b'generaldelta': _yes,
        b'sidedata': True,
        b'docket': True,
    },
}


SPARSE_REVLOG_MAX_CHAIN_LENGTH = 1000