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Commit 2435ba6c82e6 authored by Gregory Szorc's avatar Gregory Szorc
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help: document wire protocol capabilities

All capabilities from the history of the project are now documented.
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response.
The server terminates if it receives an empty command (a ``\n`` character).
Capabilities
============
Servers advertise supported wire protocol features. This allows clients to
probe for server features before blindly calling a command or passing a
specific argument.
The server's features are exposed via a *capabilities* string. This is a
space-delimited string of tokens/features. Some features are single words
like ``lookup`` or ``batch``. Others are complicated key-value pairs
advertising sub-features. e.g. ``httpheader=2048``. When complex, non-word
values are used, each feature name can define its own encoding of sub-values.
Comma-delimited and ``x-www-form-urlencoded`` values are common.
The following document capabilities defined by the canonical Mercurial server
implementation.
batch
-----
Whether the server supports the ``batch`` command.
This capability/command was introduced in Mercurial 1.9 (released July 2011).
branchmap
---------
Whether the server supports the ``branchmap`` command.
This capability/command was introduced in Mercurial 1.3 (released July 2009).
bundle2-exp
-----------
Precursor to ``bundle2`` capability that was used before bundle2 was a
stable feature.
This capability was introduced in Mercurial 3.0 behind an experimental
flag. This capability should not be observed in the wild.
bundle2
-------
Indicates whether the server supports the ``bundle2`` data exchange format.
The value of the capability is a URL quoted, newline (``\n``) delimited
list of keys or key-value pairs.
A key is simply a URL encoded string.
A key-value pair is a URL encoded key separated from a URL encoded value by
an ``=``. If the value is a list, elements are delimited by a ``,`` after
URL encoding.
For example, say we have the values::
{'HG20': [], 'changegroup': ['01', '02'], 'digests': ['sha1', 'sha512']}
We would first construct a string::
HG20\nchangegroup=01,02\ndigests=sha1,sha512
We would then URL quote this string::
HG20%0Achangegroup%3D01%2C02%0Adigests%3Dsha1%2Csha512
This capability was introduced in Mercurial 3.4 (released May 2015).
changegroupsubset
-----------------
Whether the server supports the ``changegroupsubset`` command.
This capability was introduced in Mercurial 0.9.2 (released December
2006).
This capability was introduced at the same time as the ``lookup``
capability/command.
getbundle
---------
Whether the server supports the ``getbundle`` command.
This capability was introduced in Mercurial 1.9 (released July 2011).
httpheader
----------
Whether the server supports receiving command arguments via HTTP request
headers.
The value of the capability is an integer describing the max header
length that clients should send. Clients should ignore any content after a
comma in the value, as this is reserved for future use.
This capability was introduced in Mercurial 1.9 (released July 2011).
httppostargs
------------
**Experimental**
Indicates that the server supports and prefers clients send command arguments
via a HTTP POST request as part of the request body.
This capability was introduced in Mercurial 3.8 (released May 2016).
known
-----
Whether the server supports the ``known`` command.
This capability/command was introduced in Mercurial 1.9 (released July 2011).
lookup
------
Whether the server supports the ``lookup`` command.
This capability was introduced in Mercurial 0.9.2 (released December
2006).
This capability was introduced at the same time as the ``changegroupsubset``
capability/command.
pushkey
-------
Whether the server supports the ``pushkey`` and ``listkeys`` commands.
This capability was introduced in Mercurial 1.6 (released July 2010).
standardbundle
--------------
**Unsupported**
This capability was introduced during the Mercurial 0.9.2 development cycle in
2006. It was never present in a release, as it was replaced by the ``unbundle``
capability. This capability should not be encountered in the wild.
stream-preferred
----------------
If present the server prefers that clients clone using the streaming clone
protocol (``hg clone --uncompressed``) rather than the standard
changegroup/bundle based protocol.
This capability was introduced in Mercurial 2.2 (released May 2012).
streamreqs
----------
Indicates whether the server supports *streaming clones* and the *requirements*
that clients must support to receive it.
If present, the server supports the ``stream_out`` command, which transmits
raw revlogs from the repository instead of changegroups. This provides a faster
cloning mechanism at the expense of more bandwidth used.
The value of this capability is a comma-delimited list of repo format
*requirements*. These are requirements that impact the reading of data in
the ``.hg/store`` directory. An example value is
``streamreqs=generaldelta,revlogv1`` indicating the server repo requires
the ``revlogv1`` and ``generaldelta`` requirements.
If the only format requirement is ``revlogv1``, the server may expose the
``stream`` capability instead of the ``streamreqs`` capability.
This capability was introduced in Mercurial 1.7 (released November 2010).
stream
------
Whether the server supports *streaming clones* from ``revlogv1`` repos.
If present, the server supports the ``stream_out`` command, which transmits
raw revlogs from the repository instead of changegroups. This provides a faster
cloning mechanism at the expense of more bandwidth used.
This capability was introduced in Mercurial 0.9.1 (released July 2006).
When initially introduced, the value of the capability was the numeric
revlog revision. e.g. ``stream=1``. This indicates the changegroup is using
``revlogv1``. This simple integer value wasn't powerful enough, so the
``streamreqs`` capability was invented to handle cases where the repo
requirements have more than just ``revlogv1``. Newer servers omit the
``=1`` since it was the only value supported and the value of ``1`` can
be implied by clients.
unbundlehash
------------
Whether the ``unbundle`` commands supports receiving a hash of all the
heads instead of a list.
For more, see the documentation for the ``unbundle`` command.
This capability was introduced in Mercurial 1.9 (released July 2011).
unbundle
--------
Whether the server supports pushing via the ``unbundle`` command.
This capability/command has been present since Mercurial 0.9.1 (released
July 2006).
Mercurial 0.9.2 (released December 2006) added values to the capability
indicating which bundle types the server supports receiving. This value is a
comma-delimited list. e.g. ``HG10GZ,HG10BZ,HG10UN``. The order of values
reflects the priority/preference of that type, where the first value is the
most preferred type.
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