9
In the past when new features were added that required a change to the
10
on-disk format, Bazaar has introduced a new format (in other words,
11
a different string in .bzr/branch-format, .bzr/branch/format,
12
.bzr/repository/format or .bzr/checkout/format). The main reason for this
13
was that it made old versions of Bazaar give a sensible error message
14
when they encountered on-disk data that they could not understand.
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There are also several disadvantages to such an approach:
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* Once upgraded, a newer version of Bazaar is required to access the data
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(it is often possible to downgrade the format later on)
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* Upgrading requires an explicit action by the user. It could be
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done automatically, but then accessing a repository with a newer version
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of Bazaar might accidentally render it inaccessible by older.
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Not all format changes should necessarily render the data inaccessible
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to older versions of Bazaar.
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There are also various plugins that store extra metadata in the Bazaar
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version control directory. They currently have no way of indicating that
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e.g. writing to the repository requires a particular plugin to be installed
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(so the metadata can be kept up to date, for example).
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To allow for more granular changes to the format, this spec proposes
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to add feature flags to the Bazaar formats, indicating
38
what kind of data is present in that repository. Each feature has
39
a name and some sort of indicator that tells the bzr client its
40
"necessity" - optional, required, ...
42
bzr clients would be able to open data with features that are
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set as "optional" but that they do not support. If there are features
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that aren't supported which are marked "required" in the repository they
45
would refuse to open the repository.
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Various kinds of metadata, e.g. ones that are generated from the
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repository itself and can be discarded without losing data (caches)
49
would fall in the optional category.
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The initial implementation will feature the following set of possible
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settings for feature "necessity". Any format necessity that can't
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be understood should be interpreted as "required", and an appropriate
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- optional: the feature is present, but writing/reading of the
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repository/branch/checkout is possible without support for the
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feature. Useful for things like caches (e.g. bzr-search index,
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- required: read and write access is only possible if the feature
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is supported. Useful for things like nested trees.
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- write-required: read access is possible if the feature is not supported,
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but write access requires it
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The feature information would be included in the appropriate ``format`` file
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(``.bzr/branch-format``, ``.bzr/branch/format``, ``.bzr/repository/format`` or
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``.bzr/checkout/format``). This file currently always contains a single
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line with the format name. Older versions of bzr read the full file.
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By using the other lines for feature information it is possible to add feature
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flags in a backwards compatible manner; older clients will simply fail to open
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repositories with feature flags set, giving a unknown format error.
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The other advantage of doing this is that we don't need any additional
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roundtrips when opening a remote format. An example .bzr/repository/format
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file could then look like this::
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Bazaar repository format 2a (needs bzr 1.16 or later)
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In other words, this is a "2a" bzr format which also stores a cache of
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Git Tree/Commit objects, a bzr-search index, and a reflog. It also contains
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revisions with nested trees.
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Class methods will be added to ``BzrDirComponentFormat`` to allow registering
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and deregistering the presence of particular features. This class is inherited
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by ``BzrBranchFormat``, ``VersionedFileRepositoryFormat`` and
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``InventoryWorkingTreeFormat``.
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* BzrDirComponentFormat.register_feature(name)
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* BzrDirComponentFormat.unregister_feature(name)
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Upon opening, BzrDirComponentFormat will be responsible for checking that the
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required features are present. lock_write will raise an exception
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when there is an un unsupported mandatory feature required for write access.
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Methods will also be added to BzrDirComponentFormat to allow plugins, etc,
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to check whether a feature is present and adding new features:
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* BzrDirComponentFormat.set_feature(name, necessity)
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* BzrDirComponentFormat.get_feature(name) -> necessity
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Mercurial has a similar feature, using its `.hg/requires`_ file.
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.. _.hg/requires: http://mercurial.selenic.com/wiki/RequiresFile
added
removed
doc/developers/feature-flags.txt
