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Changegroups are representations of repository revlog data, specifically
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the changelog, manifest, and filelogs.
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There are 3 versions of changegroups: ``1``, ``2``, and ``3``. From a
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high-level, versions ``1`` and ``2`` are almost exactly the same, with
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the only difference being a header on entries in the changeset
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segment. Version ``3`` adds support for exchanging treemanifests and
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includes revlog flags in the delta header.
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Changegroups consists of 3 logical segments::
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+---------------------------------+
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| changeset | manifest | filelogs |
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+---------------------------------+
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The principle building block of each segment is a *chunk*. A *chunk*
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is a framed piece of data::
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+---------------------------------------+
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| length | data |
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| (32 bits) | <length> bytes |
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+---------------------------------------+
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Each chunk starts with a 32-bit big-endian signed integer indicating
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the length of the raw data that follows.
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There is a special case chunk that has 0 length (``0x00000000``). We
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call this an *empty chunk*.
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Delta Groups
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============
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A *delta group* expresses the content of a revlog as a series of deltas,
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or patches against previous revisions.
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Delta groups consist of 0 or more *chunks* followed by the *empty chunk*
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to signal the end of the delta group::
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+------------------------------------------------------------------------+
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| chunk0 length | chunk0 data | chunk1 length | chunk1 data | 0x0 |
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| (32 bits) | (various) | (32 bits) | (various) | (32 bits) |
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+------------------------------------------------------------+-----------+
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Each *chunk*'s data consists of the following::
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+-----------------------------------------+
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| delta header | mdiff header | delta |
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| (various) | (12 bytes) | (various) |
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+-----------------------------------------+
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The *length* field is the byte length of the remaining 3 logical pieces
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of data. The *delta* is a diff from an existing entry in the changelog.
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The *delta header* is different between versions ``1``, ``2``, and
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``3`` of the changegroup format.
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Version 1::
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+------------------------------------------------------+
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| node | p1 node | p2 node | link node |
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| (20 bytes) | (20 bytes) | (20 bytes) | (20 bytes) |
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+------------------------------------------------------+
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Version 2::
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+------------------------------------------------------------------+
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| node | p1 node | p2 node | base node | link node |
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| (20 bytes) | (20 bytes) | (20 bytes) | (20 bytes) | (20 bytes) |
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+------------------------------------------------------------------+
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Version 3::
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+------------------------------------------------------------------------------+
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| node | p1 node | p2 node | base node | link node | flags |
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| (20 bytes) | (20 bytes) | (20 bytes) | (20 bytes) | (20 bytes) | (2 bytes) |
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+------------------------------------------------------------------------------+
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The *mdiff header* consists of 3 32-bit big-endian signed integers
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describing offsets at which to apply the following delta content::
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+-------------------------------------+
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| offset | old length | new length |
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| (32 bits) | (32 bits) | (32 bits) |
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+-------------------------------------+
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In version 1, the delta is always applied against the previous node from
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the changegroup or the first parent if this is the first entry in the
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changegroup.
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In version 2, the delta base node is encoded in the entry in the
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changegroup. This allows the delta to be expressed against any parent,
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which can result in smaller deltas and more efficient encoding of data.
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Changeset Segment
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=================
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The *changeset segment* consists of a single *delta group* holding
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changelog data. It is followed by an *empty chunk* to denote the
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boundary to the *manifests segment*.
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Manifest Segment
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================
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The *manifest segment* consists of a single *delta group* holding
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manifest data. It is followed by an *empty chunk* to denote the boundary
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to the *filelogs segment*.
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Filelogs Segment
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================
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The *filelogs* segment consists of multiple sub-segments, each
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corresponding to an individual file whose data is being described::
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+--------------------------------------+
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| filelog0 | filelog1 | filelog2 | ... |
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+--------------------------------------+
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In version ``3`` of the changegroup format, filelogs may include
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directory logs when treemanifests are in use. directory logs are
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identified by having a trailing '/' on their filename (see below).
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The final filelog sub-segment is followed by an *empty chunk* to denote
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the end of the segment and the overall changegroup.
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Each filelog sub-segment consists of the following::
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+------------------------------------------+
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| filename size | filename | delta group |
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| (32 bits) | (various) | (various) |
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+------------------------------------------+
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That is, a *chunk* consisting of the filename (not terminated or padded)
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followed by N chunks constituting the *delta group* for this file.
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