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Requirements
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============
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Repositories contain a file (``.hg/requires``) containing a list of
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features/capabilities that are *required* for clients to interface
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with the repository. This file has been present in Mercurial since
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version 0.9.2 (released December 2006).
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One of the first things clients do when opening a repository is read
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``.hg/requires`` and verify that all listed requirements are supported,
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aborting if not. Requirements are therefore a strong mechanism to
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prevent incompatible clients from reading from unknown repository
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formats or even corrupting them by writing to them.
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Extensions may add requirements. When they do this, clients not running
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an extension will be unable to read from repositories.
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The following sections describe the requirements defined by the
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Mercurial core distribution.
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revlogv1
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--------
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When present, revlogs are version 1 (RevlogNG). RevlogNG was introduced
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in 2006. The ``revlogv1`` requirement has been enabled by default
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since the ``requires`` file was introduced in Mercurial 0.9.2.
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If this requirement is not present, version 0 revlogs are assumed.
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store
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-----
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The *store* repository layout should be used.
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This requirement has been enabled by default since the ``requires`` file
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was introduced in Mercurial 0.9.2.
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fncache
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-------
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The *fncache* repository layout should be used.
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The *fncache* layout hash encodes filenames with long paths and
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encodes reserved filenames.
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This requirement is enabled by default when the *store* requirement is
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enabled (which is the default behavior). It was introduced in Mercurial
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1.1 (released December 2008).
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shared
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------
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Denotes that the store for a repository is shared from another location
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(defined by the ``.hg/sharedpath`` file).
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This requirement is set when a repository is created via :hg:`share`.
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The requirement was added in Mercurial 1.3 (released July 2009).
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dotencode
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---------
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The *dotencode* repository layout should be used.
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The *dotencode* layout encodes the first period or space in filenames
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to prevent issues on OS X and Windows.
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This requirement is enabled by default when the *store* requirement
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is enabled (which is the default behavior). It was introduced in
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Mercurial 1.7 (released November 2010).
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parentdelta
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-----------
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Denotes a revlog delta encoding format that was experimental and
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replaced by *generaldelta*. It should not be seen in the wild because
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it was never enabled by default.
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This requirement was added in Mercurial 1.7 and removed in Mercurial
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1.9.
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generaldelta
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------------
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Revlogs should be created with the *generaldelta* flag enabled. The
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generaldelta flag will cause deltas to be encoded against a parent
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revision instead of the previous revision in the revlog.
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Support for this requirement was added in Mercurial 1.9 (released
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July 2011). The requirement was disabled on new repositories by
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default until Mercurial 3.7 (released February 2016).
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manifestv2
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----------
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Denotes that version 2 of manifests are being used.
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Support for this requirement was added in Mercurial 3.4 (released
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May 2015). The requirement is currently experimental and is disabled
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by default.
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treemanifest
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------------
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Denotes that tree manifests are being used. Tree manifests are
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one manifest per directory (as opposed to a single flat manifest).
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Support for this requirement was added in Mercurial 3.4 (released
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August 2015). The requirement is currently experimental and is
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disabled by default.
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