state: write the version number in plain text on top of state files
We will soon be using CBOR format to write the data in state files. But we
should not write the version number of the state files in CBOR format and we
should rather write it in plain text because in future we can change the format
of state files and we should be able to parse the version number of state file
without requiring to understand a certain format.
This will help us in making sure we have a good compatibility story with other
versions of state files.
Differential Revision: https://phab.mercurial-scm.org/D3579
Mercurial supports generating standalone "bundle" files that hold repository
data. These "bundles" are typically saved locally and used later or exchanged
between different repositories, possibly on different machines. Example
commands using bundles are :hg:`bundle` and :hg:`unbundle`.
Generation of bundle files is controlled by a "bundle specification"
("bundlespec") string. This string tells the bundle generation process how
to create the bundle.
A "bundlespec" string is composed of the following elements:
type
A string denoting the bundle format to use.
compression
Denotes the compression engine to use compressing the raw bundle data.
parameters
Arbitrary key-value parameters to further control bundle generation.
A "bundlespec" string has the following formats:
<type>
The literal bundle format string is used.
<compression>-<type>
The compression engine and format are delimited by a hyphen (``-``).
Optional parameters follow the ``<type>``. Parameters are URI escaped
``key=value`` pairs. Each pair is delimited by a semicolon (``;``). The
first parameter begins after a ``;`` immediately following the ``<type>``
value.
Available Types
===============
The following bundle <type> strings are available:
v1
Produces a legacy "changegroup" version 1 bundle.
This format is compatible with nearly all Mercurial clients because it is
the oldest. However, it has some limitations, which is why it is no longer
the default for new repositories.
``v1`` bundles can be used with modern repositories using the "generaldelta"
storage format. However, it may take longer to produce the bundle and the
resulting bundle may be significantly larger than a ``v2`` bundle.
``v1`` bundles can only use the ``gzip``, ``bzip2``, and ``none`` compression
formats.
v2
Produces a version 2 bundle.
Version 2 bundles are an extensible format that can store additional
repository data (such as bookmarks and phases information) and they can
store data more efficiently, resulting in smaller bundles.
Version 2 bundles can also use modern compression engines, such as
``zstd``, making them faster to compress and often smaller.
Available Compression Engines
=============================
The following bundle <compression> engines can be used:
.. bundlecompressionmarker
Examples
========
``v2``
Produce a ``v2`` bundle using default options, including compression.
``none-v1``
Produce a ``v1`` bundle with no compression.
``zstd-v2``
Produce a ``v2`` bundle with zstandard compression using default
settings.
``zstd-v1``
This errors because ``zstd`` is not supported for ``v1`` types.