Changegroups

============

Changegroups are representations of repository revlog data, specifically

the changelog, manifest, and filelogs.

There are 2 versions of changegroups: ``1`` and ``2``. From a

high-level, they are almost exactly the same, with the only difference

being a header on entries in the changeset segment.

Changegroups consists of 3 logical segments::

   +---------------------------------+

   |           |          |          |

   | changeset | manifest | filelogs |

   |           |          |          |

   +---------------------------------+

The principle building block of each segment is a *chunk*. A *chunk*

is a framed piece of data::

   +---------------------------------------+

   |           |                           |

   |  length   |           data            |

   | (32 bits) |       <length> bytes      |

   |           |                           |

   +---------------------------------------+

Each chunk starts with a 32-bit big-endian signed integer indicating

the length of the raw data that follows.

There is a special case chunk that has 0 length (``0x00000000``). We

call this an *empty chunk*.

Delta Groups

------------

A *delta group* expresses the content of a revlog as a series of deltas,

or patches against previous revisions.

Delta groups consist of 0 or more *chunks* followed by the *empty chunk*

to signal the end of the delta group::

  +------------------------------------------------------------------------+

  |                |             |               |             |           |

  | chunk0 length  | chunk0 data | chunk1 length | chunk1 data |    0x0    |

  |   (32 bits)    |  (various)  |   (32 bits)   |  (various)  | (32 bits) |

  |                |             |               |             |           |

  +------------------------------------------------------------+-----------+

Each *chunk*'s data consists of the following::

  +-----------------------------------------+

  |              |              |           |

  | delta header | mdiff header |   delta   |

  |  (various)   |  (12 bytes)  | (various) |

  |              |              |           |

  +-----------------------------------------+

The *length* field is the byte length of the remaining 3 logical pieces

of data. The *delta* is a diff from an existing entry in the changelog.

The *delta header* is different between versions ``1`` and ``2`` of the

changegroup format.

Version 1::

   +------------------------------------------------------+

   |            |             |             |             |

   |    node    |   p1 node   |   p2 node   |  link node  |

   | (20 bytes) |  (20 bytes) |  (20 bytes) |  (20 bytes) |

   |            |             |             |             |

   +------------------------------------------------------+

Version 2::

   +------------------------------------------------------------------+

   |            |             |             |            |            |

   |    node    |   p1 node   |   p2 node   | base node  | link node  |

   | (20 bytes) |  (20 bytes) |  (20 bytes) | (20 bytes) | (20 bytes) |

   |            |             |             |            |            |

   +------------------------------------------------------------------+

The *mdiff header* consists of 3 32-bit big-endian signed integers

describing offsets at which to apply the following delta content::

   +-------------------------------------+

   |           |            |            |

   |  offset   | old length | new length |

   | (32 bits) |  (32 bits) |  (32 bits) |

   |           |            |            |

   +-------------------------------------+

In version 1, the delta is always applied against the previous node from

the changegroup or the first parent if this is the first entry in the

changegroup.

In version 2, the delta base node is encoded in the entry in the

changegroup. This allows the delta to be expressed against any parent,

which can result in smaller deltas and more efficient encoding of data.

Changeset Segment

-----------------

The *changeset segment* consists of a single *delta group* holding

changelog data. It is followed by an *empty chunk* to denote the

boundary to the *manifests segment*.

Manifest Segment

----------------

The *manifest segment* consists of a single *delta group* holding

manifest data. It is followed by an *empty chunk* to denote the boundary

to the *filelogs segment*.

Filelogs Segment

----------------

The *filelogs* segment consists of multiple sub-segments, each

corresponding to an individual file whose data is being described::

   +--------------------------------------+

   |          |          |          |     |

   | filelog0 | filelog1 | filelog2 | ... |

   |          |          |          |     |

   +--------------------------------------+

The final filelog sub-segment is followed by an *empty chunk* to denote

the end of the segment and the overall changegroup.

Each filelog sub-segment consists of the following::

   +------------------------------------------+

   |               |            |             |

   | filename size |  filename  | delta group |

   |   (32 bits)   |  (various) |  (various)  |

   |               |            |             |

   +------------------------------------------+

That is, a *chunk* consisting of the filename (not terminated or padded)

followed by N chunks constituting the *delta group* for this file.