mercurial/help/internals/bundles.txt

internals: document bundle2 format

It seems like a good idea to have thorough documentation of the bundle2
data format, including the format of each part and the capabilities.

The added documentation is far from complete. For example, we don't
fully capture the semantics of each capability and part. But a start
is better than nothing, which was pretty much where we were before.

Differential Revision: https://phab.mercurial-scm.org/D2298

A bundle is a container for repository data.

Bundles are used as standalone files as well as the interchange format
over the wire protocol used when two Mercurial peers communicate with
each other.

Headers
=======

Bundles produced since Mercurial 0.7 (September 2005) have a 4 byte
header identifying the major bundle type. The header always begins with
``HG`` and the follow 2 bytes indicate the bundle type/version. Some
bundle types have additional data after this 4 byte header.

The following sections describe each bundle header/type.

HG10
----

``HG10`` headers indicate a *changegroup bundle*. This is the original
bundle format, so it is sometimes referred to as *bundle1*. It has been
present since version 0.7 (released September 2005).

This header is followed by 2 bytes indicating the compression algorithm
used for data that follows. All subsequent data following this
compression identifier is compressed according to the algorithm/method
specified.

Supported algorithms include the following.

``BZ``
   *bzip2* compression.

   Bzip2 compressors emit a leading ``BZ`` header. Mercurial uses this
   leading ``BZ`` as part of the bundle header. Therefore consumers
   of bzip2 bundles need to *seed* the bzip2 decompressor with ``BZ`` or
   seek the input stream back to the beginning of the algorithm component
   of the bundle header so that decompressor input is valid. This behavior
   is unique among supported compression algorithms.

   Supported since version 0.7 (released December 2006).

``GZ``
  *zlib* compression.

   Supported since version 0.9.2 (released December 2006).

``UN``
  *Uncompressed* or no compression. Unmodified changegroup data follows.

  Supported since version 0.9.2 (released December 2006).

3rd party extensions may implement their own compression. However, no
authority reserves values for their compression algorithm identifiers.

HG2X
----

``HG2X`` headers (where ``X`` is any value) denote a *bundle2* bundle.
Bundle2 bundles are a container format for various kinds of repository
data and capabilities, beyond changegroup data (which was the only data
supported by ``HG10`` bundles.

``HG20`` is currently the only defined bundle2 version.

The ``HG20`` format is documented at :hg:`help internals.bundle2`.

Initial ``HG20`` support was added in Mercurial 3.0 (released May
2014). However, bundle2 bundles were hidden behind an experimental flag
until version 3.5 (released August 2015), when they were enabled in the
wire protocol. Various commands (including ``hg bundle``) did not
support generating bundle2 files until Mercurial 3.6 (released November
2015).

HGS1
----

*Experimental*

A ``HGS1`` header indicates a *streaming clone bundle*. This is a bundle
that contains raw revlog data from a repository store. (Typically revlog
data is exchanged in the form of changegroups.)

The purpose of *streaming clone bundles* are to *clone* repository data
very efficiently.

The ``HGS1`` header is always followed by 2 bytes indicating a
compression algorithm of the data that follows. Only ``UN``
(uncompressed data) is currently allowed.

``HGS1UN`` support was added as an experimental feature in version 3.6
(released November 2015) as part of the initial offering of the *clone
bundles* feature.