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A bundle is a container for repository data.
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Bundles are used as standalone files as well as the interchange format
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over the wire protocol used when two Mercurial peers communicate with
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each other.
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Headers
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=======
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Bundles produced since Mercurial 0.7 (September 2005) have a 4 byte
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header identifying the major bundle type. The header always begins with
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``HG`` and the follow 2 bytes indicate the bundle type/version. Some
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bundle types have additional data after this 4 byte header.
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The following sections describe each bundle header/type.
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HG10
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----
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``HG10`` headers indicate a *changegroup bundle*. This is the original
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bundle format, so it is sometimes referred to as *bundle1*. It has been
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present since version 0.7 (released September 2005).
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This header is followed by 2 bytes indicating the compression algorithm
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used for data that follows. All subsequent data following this
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compression identifier is compressed according to the algorithm/method
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specified.
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Supported algorithms include the following.
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``BZ``
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*bzip2* compression.
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Bzip2 compressors emit a leading ``BZ`` header. Mercurial uses this
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leading ``BZ`` as part of the bundle header. Therefore consumers
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of bzip2 bundles need to *seed* the bzip2 decompressor with ``BZ`` or
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seek the input stream back to the beginning of the algorithm component
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of the bundle header so that decompressor input is valid. This behavior
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is unique among supported compression algorithms.
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Supported since version 0.7 (released December 2006).
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``GZ``
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*zlib* compression.
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Supported since version 0.9.2 (released December 2006).
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``UN``
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*Uncompressed* or no compression. Unmodified changegroup data follows.
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Supported since version 0.9.2 (released December 2006).
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3rd party extensions may implement their own compression. However, no
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authority reserves values for their compression algorithm identifiers.
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HG2X
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----
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``HG2X`` headers (where ``X`` is any value) denote a *bundle2* bundle.
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Bundle2 bundles are a container format for various kinds of repository
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data and capabilities, beyond changegroup data (which was the only data
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supported by ``HG10`` bundles.
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``HG20`` is currently the only defined bundle2 version.
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The ``HG20`` format is not yet documented here. See the inline comments
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in ``mercurial/exchange.py`` for now.
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Initial ``HG20`` support was added in Mercurial 3.0 (released May
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2014). However, bundle2 bundles were hidden behind an experimental flag
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until version 3.5 (released August 2015), when they were enabled in the
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wire protocol. Various commands (including ``hg bundle``) did not
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support generating bundle2 files until Mercurial 3.6 (released November
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2015).
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HGS1
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----
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*Experimental*
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A ``HGS1`` header indicates a *streaming clone bundle*. This is a bundle
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that contains raw revlog data from a repository store. (Typically revlog
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data is exchanged in the form of changegroups.)
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The purpose of *streaming clone bundles* are to *clone* repository data
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very efficiently.
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The ``HGS1`` header is always followed by 2 bytes indicating a
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compression algorithm of the data that follows. Only ``UN``
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(uncompressed data) is currently allowed.
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``HGS1UN`` support was added as an experimental feature in version 3.6
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(released November 2015) as part of the initial offering of the *clone
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bundles* feature.
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