mercurial/requirements.py

tracked-key: remove the dual write and rename to tracked-hint

The dual-write approach was mostly useless. As explained in the previous version
of the help, the key had to be read twice before we could cache a value.

However this "read twice" limitation actually also apply to any usage of the
key. If some operation wants to rely of the "same value == same tracked set"
property it would need to read the value before, and after running that
operation (or at least, after, in all cases). So it cannot be sure the operation
it did is "valid" until checking the key after the operation. As a resultat such
operation can only be read-only or rollbackable.

This reduce the utility of the "same value == same tracked set" a lot.

So it seems simpler to drop the double write and to update the documentation to
highlight that this file does not garantee race-free operation. As a result the
"key" is demoted to a "hint".

Documentation is updated accordingly.

Differential Revision: https://phab.mercurial-scm.org/D12201

# requirements.py - objects and functions related to repository requirements
#
# Copyright 2005-2007 Olivia Mackall <[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.

from __future__ import absolute_import

# obsolete experimental requirements:
#  - manifestv2: An experimental new manifest format that allowed
#    for stem compression of long paths. Experiment ended up not
#    being successful (repository sizes went up due to worse delta
#    chains), and the code was deleted in 4.6.

GENERALDELTA_REQUIREMENT = b'generaldelta'
DOTENCODE_REQUIREMENT = b'dotencode'
STORE_REQUIREMENT = b'store'
FNCACHE_REQUIREMENT = b'fncache'

DIRSTATE_TRACKED_HINT_V1 = b'dirstate-tracked-key-v1'
DIRSTATE_V2_REQUIREMENT = b'dirstate-v2'

# When narrowing is finalized and no longer subject to format changes,
# we should move this to just "narrow" or similar.
NARROW_REQUIREMENT = b'narrowhg-experimental'

# Enables sparse working directory usage
SPARSE_REQUIREMENT = b'exp-sparse'

# Enables the internal phase which is used to hide changesets instead
# of stripping them
INTERNAL_PHASE_REQUIREMENT = b'internal-phase'

# Stores manifest in Tree structure
TREEMANIFEST_REQUIREMENT = b'treemanifest'

REVLOGV1_REQUIREMENT = b'revlogv1'

# allow using ZSTD as compression engine for revlog content
REVLOG_COMPRESSION_ZSTD = b'revlog-compression-zstd'

# Increment the sub-version when the revlog v2 format changes to lock out old
# clients.
CHANGELOGV2_REQUIREMENT = b'exp-changelog-v2'

# Increment the sub-version when the revlog v2 format changes to lock out old
# clients.
REVLOGV2_REQUIREMENT = b'exp-revlogv2.2'

# A repository with the sparserevlog feature will have delta chains that
# can spread over a larger span. Sparse reading cuts these large spans into
# pieces, so that each piece isn't too big.
# Without the sparserevlog capability, reading from the repository could use
# huge amounts of memory, because the whole span would be read at once,
# including all the intermediate revisions that aren't pertinent for the chain.
# This is why once a repository has enabled sparse-read, it becomes required.
SPARSEREVLOG_REQUIREMENT = b'sparserevlog'

# A repository with the the copies-sidedata-changeset requirement will store
# copies related information in changeset's sidedata.
COPIESSDC_REQUIREMENT = b'exp-copies-sidedata-changeset'

# The repository use persistent nodemap for the changelog and the manifest.
NODEMAP_REQUIREMENT = b'persistent-nodemap'

# Denotes that the current repository is a share
SHARED_REQUIREMENT = b'shared'

# Denotes that current repository is a share and the shared source path is
# relative to the current repository root path
RELATIVE_SHARED_REQUIREMENT = b'relshared'

# A repository with share implemented safely. The repository has different
# store and working copy requirements i.e. both `.hg/requires` and
# `.hg/store/requires` are present.
SHARESAFE_REQUIREMENT = b'share-safe'

# Bookmarks must be stored in the `store` part of the repository and will be
# share accross shares
BOOKMARKS_IN_STORE_REQUIREMENT = b'bookmarksinstore'

# List of requirements which are working directory specific
# These requirements cannot be shared between repositories if they
# share the same store
# * sparse is a working directory specific functionality and hence working
#   directory specific requirement
# * SHARED_REQUIREMENT and RELATIVE_SHARED_REQUIREMENT are requirements which
#   represents that the current working copy/repository shares store of another
#   repo. Hence both of them should be stored in working copy
# * SHARESAFE_REQUIREMENT needs to be stored in working dir to mark that rest of
#   the requirements are stored in store's requires
# * DIRSTATE_V2_REQUIREMENT affects .hg/dirstate, of which there is one per
#   working directory.
WORKING_DIR_REQUIREMENTS = {
    SPARSE_REQUIREMENT,
    SHARED_REQUIREMENT,
    RELATIVE_SHARED_REQUIREMENT,
    SHARESAFE_REQUIREMENT,
    DIRSTATE_TRACKED_HINT_V1,
    DIRSTATE_V2_REQUIREMENT,
}

# List of requirement that impact "stream-clone" (and hardlink clone) and
# cannot be changed in such cases.
#
# requirements not in this list are safe to be altered during stream-clone.
#
# note: the list is currently inherited from previous code and miss some relevant requirement while containing some irrelevant ones.
STREAM_FIXED_REQUIREMENTS = {
    BOOKMARKS_IN_STORE_REQUIREMENT,
    CHANGELOGV2_REQUIREMENT,
    COPIESSDC_REQUIREMENT,
    GENERALDELTA_REQUIREMENT,
    INTERNAL_PHASE_REQUIREMENT,
    REVLOG_COMPRESSION_ZSTD,
    REVLOGV1_REQUIREMENT,
    REVLOGV2_REQUIREMENT,
    SPARSEREVLOG_REQUIREMENT,
    TREEMANIFEST_REQUIREMENT,
}