mercurial/help/internals/wireprotocol.txt

wireproto: provide accessors for client capabilities

For HTTP, this refactors the existing logic, including the parsing of
the compression engine capability.

For SSH, this adds a ssh-only capability "protocaps" and a command for
informing the server on what the client supports. Since SSH is stateful,
keep track of the capabilities in the server instance.

Differential Revision: https://phab.mercurial-scm.org/D1944

The Mercurial wire protocol is a request-response based protocol
with multiple wire representations.

Each request is modeled as a command name, a dictionary of arguments, and
optional raw input. Command arguments and their types are intrinsic
properties of commands. So is the response type of the command. This means
clients can't always send arbitrary arguments to servers and servers can't
return multiple response types.

The protocol is synchronous and does not support multiplexing (concurrent
commands).

Handshake
=========

It is required or common for clients to perform a *handshake* when connecting
to a server. The handshake serves the following purposes:

* Negotiating protocol/transport level options
* Allows the client to learn about server capabilities to influence
  future requests
* Ensures the underlying transport channel is in a *clean* state

An important goal of the handshake is to allow clients to use more modern
wire protocol features. By default, clients must assume they are talking
to an old version of Mercurial server (possibly even the very first
implementation). So, clients should not attempt to call or utilize modern
wire protocol features until they have confirmation that the server
supports them. The handshake implementation is designed to allow both
ends to utilize the latest set of features and capabilities with as
few round trips as possible.

The handshake mechanism varies by transport and protocol and is documented
in the sections below.

HTTP Protocol
=============

Handshake
---------

The client sends a ``capabilities`` command request (``?cmd=capabilities``)
as soon as HTTP requests may be issued.

The server responds with a capabilities string, which the client parses to
learn about the server's abilities.

HTTP Version 1 Transport
------------------------

Commands are issued as HTTP/1.0 or HTTP/1.1 requests. Commands are
sent to the base URL of the repository with the command name sent in
the ``cmd`` query string parameter. e.g.
``https://example.com/repo?cmd=capabilities``. The HTTP method is ``GET``
or ``POST`` depending on the command and whether there is a request
body.

Command arguments can be sent multiple ways.

The simplest is part of the URL query string using ``x-www-form-urlencoded``
encoding (see Python's ``urllib.urlencode()``. However, many servers impose
length limitations on the URL. So this mechanism is typically only used if
the server doesn't support other mechanisms.

If the server supports the ``httpheader`` capability, command arguments can
be sent in HTTP request headers named ``X-HgArg-<N>`` where ``<N>`` is an
integer starting at 1. A ``x-www-form-urlencoded`` representation of the
arguments is obtained. This full string is then split into chunks and sent
in numbered ``X-HgArg-<N>`` headers. The maximum length of each HTTP header
is defined by the server in the ``httpheader`` capability value, which defaults
to ``1024``. The server reassembles the encoded arguments string by
concatenating the ``X-HgArg-<N>`` headers then URL decodes them into a
dictionary.

The list of ``X-HgArg-<N>`` headers should be added to the ``Vary`` request
header to instruct caches to take these headers into consideration when caching
requests.

If the server supports the ``httppostargs`` capability, the client
may send command arguments in the HTTP request body as part of an
HTTP POST request. The command arguments will be URL encoded just like
they would for sending them via HTTP headers. However, no splitting is
performed: the raw arguments are included in the HTTP request body.

The client sends a ``X-HgArgs-Post`` header with the string length of the
encoded arguments data. Additional data may be included in the HTTP
request body immediately following the argument data. The offset of the
non-argument data is defined by the ``X-HgArgs-Post`` header. The
``X-HgArgs-Post`` header is not required if there is no argument data.

Additional command data can be sent as part of the HTTP request body. The
default ``Content-Type`` when sending data is ``application/mercurial-0.1``.
A ``Content-Length`` header is currently always sent.

Example HTTP requests::

    GET /repo?cmd=capabilities
    X-HgArg-1: foo=bar&baz=hello%20world

The request media type should be chosen based on server support. If the
``httpmediatype`` server capability is present, the client should send
the newest mutually supported media type. If this capability is absent,
the client must assume the server only supports the
``application/mercurial-0.1`` media type.

The ``Content-Type`` HTTP response header identifies the response as coming
from Mercurial and can also be used to signal an error has occurred.

The ``application/mercurial-*`` media types indicate a generic Mercurial
data type.

The ``application/mercurial-0.1`` media type is raw Mercurial data. It is the
predecessor of the format below.

The ``application/mercurial-0.2`` media type is compression framed Mercurial
data. The first byte of the payload indicates the length of the compression
format identifier that follows. Next are N bytes indicating the compression
format. e.g. ``zlib``. The remaining bytes are compressed according to that
compression format. The decompressed data behaves the same as with
``application/mercurial-0.1``.

The ``application/hg-error`` media type indicates a generic error occurred.
The content of the HTTP response body typically holds text describing the
error.

The ``application/hg-changegroup`` media type indicates a changegroup response
type.

Clients also accept the ``text/plain`` media type. All other media
types should cause the client to error.

Behavior of media types is further described in the ``Content Negotiation``
section below.

Clients should issue a ``User-Agent`` request header that identifies the client.
The server should not use the ``User-Agent`` for feature detection.

A command returning a ``string`` response issues a
``application/mercurial-0.*`` media type and the HTTP response body contains
the raw string value (after compression decoding, if used). A
``Content-Length`` header is typically issued, but not required.

A command returning a ``stream`` response issues a
``application/mercurial-0.*`` media type and the HTTP response is typically
using *chunked transfer* (``Transfer-Encoding: chunked``).

HTTP Version 2 Transport
------------------------

**Experimental - feature under active development**

Version 2 of the HTTP protocol is exposed under the ``/api/*`` URL space.
It's final API name is not yet formalized.

Commands are triggered by sending HTTP POST requests against URLs of the
form ``<permission>/<command>``, where ``<permission>`` is ``ro`` or
``rw``, meaning read-only and read-write, respectively and ``<command>``
is a named wire protocol command.

Non-POST request methods MUST be rejected by the server with an HTTP
405 response.

Commands that modify repository state in meaningful ways MUST NOT be
exposed under the ``ro`` URL prefix. All available commands MUST be
available under the ``rw`` URL prefix.

Server adminstrators MAY implement blanket HTTP authentication keyed
off the URL prefix. For example, a server may require authentication
for all ``rw/*`` URLs and let unauthenticated requests to ``ro/*``
URL proceed. A server MAY issue an HTTP 401, 403, or 407 response
in accordance with RFC 7235. Clients SHOULD recognize the HTTP Basic
(RFC 7617) and Digest (RFC 7616) authentication schemes. Clients SHOULD
make an attempt to recognize unknown schemes using the
``WWW-Authenticate`` response header on a 401 response, as defined by
RFC 7235.

Read-only commands are accessible under ``rw/*`` URLs so clients can
signal the intent of the operation very early in the connection
lifecycle. For example, a ``push`` operation - which consists of
various read-only commands mixed with at least one read-write command -
can perform all commands against ``rw/*`` URLs so that any server-side
authentication requirements are discovered upon attempting the first
command - not potentially several commands into the exchange. This
allows clients to fail faster or prompt for credentials as soon as the
exchange takes place. This provides a better end-user experience.

Requests to unknown commands or URLS result in an HTTP 404.
TODO formally define response type, how error is communicated, etc.

HTTP request and response bodies use the *Unified Frame-Based Protocol*
(defined below) for media exchange. The entirety of the HTTP message
body is 0 or more frames as defined by this protocol.

Clients and servers MUST advertise the ``TBD`` media type via the
``Content-Type`` request and response headers. In addition, clients MUST
advertise this media type value in their ``Accept`` request header in all
requests.
TODO finalize the media type. For now, it is defined in wireprotoserver.py.

Servers receiving requests without an ``Accept`` header SHOULD respond with
an HTTP 406.

Servers receiving requests with an invalid ``Content-Type`` header SHOULD
respond with an HTTP 415.

The command to run is specified in the POST payload as defined by the
*Unified Frame-Based Protocol*. This is redundant with data already
encoded in the URL. This is by design, so server operators can have
better understanding about server activity from looking merely at
HTTP access logs.

In most circumstances, the command specified in the URL MUST match
the command specified in the frame-based payload or the server will
respond with an error. The exception to this is the special
``multirequest`` URL. (See below.) In addition, HTTP requests
are limited to one command invocation. The exception is the special
``multirequest`` URL.

The ``multirequest`` command endpoints (``ro/multirequest`` and
``rw/multirequest``) are special in that they allow the execution of
*any* command and allow the execution of multiple commands. If the
HTTP request issues multiple commands across multiple frames, all
issued commands will be processed by the server. Per the defined
behavior of the *Unified Frame-Based Protocol*, commands may be
issued interleaved and responses may come back in a different order
than they were issued. Clients MUST be able to deal with this.

SSH Protocol
============

Handshake
---------

For all clients, the handshake consists of the client sending 1 or more
commands to the server using version 1 of the transport. Servers respond
to commands they know how to respond to and send an empty response (``0\n``)
for unknown commands (per standard behavior of version 1 of the transport).
Clients then typically look for a response to the newest sent command to
determine which transport version to use and what the available features for
the connection and server are.

Preceding any response from client-issued commands, the server may print
non-protocol output. It is common for SSH servers to print banners, message
of the day announcements, etc when clients connect. It is assumed that any
such *banner* output will precede any Mercurial server output. So clients
must be prepared to handle server output on initial connect that isn't
in response to any client-issued command and doesn't conform to Mercurial's
wire protocol. This *banner* output should only be on stdout. However,
some servers may send output on stderr.

Pre 0.9.1 clients issue a ``between`` command with the ``pairs`` argument
having the value
``0000000000000000000000000000000000000000-0000000000000000000000000000000000000000``.

The ``between`` command has been supported since the original Mercurial
SSH server. Requesting the empty range will return a ``\n`` string response,
which will be encoded as ``1\n\n`` (value length of ``1`` followed by a newline
followed by the value, which happens to be a newline).

For pre 0.9.1 clients and all servers, the exchange looks like::

   c: between\n
   c: pairs 81\n
   c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
   s: 1\n
   s: \n

0.9.1+ clients send a ``hello`` command (with no arguments) before the
``between`` command. The response to this command allows clients to
discover server capabilities and settings.

An example exchange between 0.9.1+ clients and a ``hello`` aware server looks
like::

   c: hello\n
   c: between\n
   c: pairs 81\n
   c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
   s: 324\n
   s: capabilities: lookup changegroupsubset branchmap pushkey known getbundle ...\n
   s: 1\n
   s: \n

And a similar scenario but with servers sending a banner on connect::

   c: hello\n
   c: between\n
   c: pairs 81\n
   c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
   s: welcome to the server\n
   s: if you find any issues, email [e-mail]\n
   s: 324\n
   s: capabilities: lookup changegroupsubset branchmap pushkey known getbundle ...\n
   s: 1\n
   s: \n

Note that output from the ``hello`` command is terminated by a ``\n``. This is
part of the response payload and not part of the wire protocol adding a newline
after responses. In other words, the length of the response contains the
trailing ``\n``.

Clients supporting version 2 of the SSH transport send a line beginning
with ``upgrade`` before the ``hello`` and ``between`` commands. The line
(which isn't a well-formed command line because it doesn't consist of a
single command name) serves to both communicate the client's intent to
switch to transport version 2 (transports are version 1 by default) as
well as to advertise the client's transport-level capabilities so the
server may satisfy that request immediately.

The upgrade line has the form:

    upgrade <token> <transport capabilities>

That is the literal string ``upgrade`` followed by a space, followed by
a randomly generated string, followed by a space, followed by a string
denoting the client's transport capabilities.

The token can be anything. However, a random UUID is recommended. (Use
of version 4 UUIDs is recommended because version 1 UUIDs can leak the
client's MAC address.)

The transport capabilities string is a URL/percent encoded string
containing key-value pairs defining the client's transport-level
capabilities. The following capabilities are defined:

proto
   A comma-delimited list of transport protocol versions the client
   supports. e.g. ``ssh-v2``.

If the server does not recognize the ``upgrade`` line, it should issue
an empty response and continue processing the ``hello`` and ``between``
commands. Here is an example handshake between a version 2 aware client
and a non version 2 aware server:

   c: upgrade 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a proto=ssh-v2
   c: hello\n
   c: between\n
   c: pairs 81\n
   c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
   s: 0\n
   s: 324\n
   s: capabilities: lookup changegroupsubset branchmap pushkey known getbundle ...\n
   s: 1\n
   s: \n

(The initial ``0\n`` line from the server indicates an empty response to
the unknown ``upgrade ..`` command/line.)

If the server recognizes the ``upgrade`` line and is willing to satisfy that
upgrade request, it replies to with a payload of the following form:

   upgraded <token> <transport name>\n

This line is the literal string ``upgraded``, a space, the token that was
specified by the client in its ``upgrade ...`` request line, a space, and the
name of the transport protocol that was chosen by the server. The transport
name MUST match one of the names the client specified in the ``proto`` field
of its ``upgrade ...`` request line.

If a server issues an ``upgraded`` response, it MUST also read and ignore
the lines associated with the ``hello`` and ``between`` command requests
that were issued by the server. It is assumed that the negotiated transport
will respond with equivalent requested information following the transport
handshake.

All data following the ``\n`` terminating the ``upgraded`` line is the
domain of the negotiated transport. It is common for the data immediately
following to contain additional metadata about the state of the transport and
the server. However, this isn't strictly speaking part of the transport
handshake and isn't covered by this section.

Here is an example handshake between a version 2 aware client and a version
2 aware server:

   c:  upgrade 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a proto=ssh-v2
   c:  hello\n
   c:  between\n
   c:  pairs 81\n
   c:  0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
   s: upgraded 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a ssh-v2\n
   s: <additional transport specific data>

The client-issued token that is echoed in the response provides a more
resilient mechanism for differentiating *banner* output from Mercurial
output. In version 1, properly formatted banner output could get confused
for Mercurial server output. By submitting a randomly generated token
that is then present in the response, the client can look for that token
in response lines and have reasonable certainty that the line did not
originate from a *banner* message.

SSH Version 1 Transport
-----------------------

The SSH transport (version 1) is a custom text-based protocol suitable for
use over any bi-directional stream transport. It is most commonly used with
SSH.

A SSH transport server can be started with ``hg serve --stdio``. The stdin,
stderr, and stdout file descriptors of the started process are used to exchange
data. When Mercurial connects to a remote server over SSH, it actually starts
a ``hg serve --stdio`` process on the remote server.

Commands are issued by sending the command name followed by a trailing newline
``\n`` to the server. e.g. ``capabilities\n``.

Command arguments are sent in the following format::

    <argument> <length>\n<value>

That is, the argument string name followed by a space followed by the
integer length of the value (expressed as a string) followed by a newline
(``\n``) followed by the raw argument value.

Dictionary arguments are encoded differently::

    <argument> <# elements>\n
    <key1> <length1>\n<value1>
    <key2> <length2>\n<value2>
    ...

Non-argument data is sent immediately after the final argument value. It is
encoded in chunks::

    <length>\n<data>

Each command declares a list of supported arguments and their types. If a
client sends an unknown argument to the server, the server should abort
immediately. The special argument ``*`` in a command's definition indicates
that all argument names are allowed.

The definition of supported arguments and types is initially made when a
new command is implemented. The client and server must initially independently
agree on the arguments and their types. This initial set of arguments can be
supplemented through the presence of *capabilities* advertised by the server.

Each command has a defined expected response type.

A ``string`` response type is a length framed value. The response consists of
the string encoded integer length of a value followed by a newline (``\n``)
followed by the value. Empty values are allowed (and are represented as
``0\n``).

A ``stream`` response type consists of raw bytes of data. There is no framing.

A generic error response type is also supported. It consists of a an error
message written to ``stderr`` followed by ``\n-\n``. In addition, ``\n`` is
written to ``stdout``.

If the server receives an unknown command, it will send an empty ``string``
response.

The server terminates if it receives an empty command (a ``\n`` character).

If the server announces support for the ``protocaps`` capability, the client
should issue a ``protocaps`` command after the initial handshake to annonunce
its own capabilities. The client capabilities are persistent.

SSH Version 2 Transport
-----------------------

**Experimental and under development**

Version 2 of the SSH transport behaves identically to version 1 of the SSH
transport with the exception of handshake semantics. See above for how
version 2 of the SSH transport is negotiated.

Immediately following the ``upgraded`` line signaling a switch to version
2 of the SSH protocol, the server automatically sends additional details
about the capabilities of the remote server. This has the form:

   <integer length of value>\n
   capabilities: ...\n

e.g.

   s: upgraded 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a ssh-v2\n
   s: 240\n
   s: capabilities: known getbundle batch ...\n

Following capabilities advertisement, the peers communicate using version
1 of the SSH transport.

Unified Frame-Based Protocol
============================

**Experimental and under development**

The *Unified Frame-Based Protocol* is a communications protocol between
Mercurial peers. The protocol aims to be mostly transport agnostic
(works similarly on HTTP, SSH, etc).

To operate the protocol, a bi-directional, half-duplex pipe supporting
ordered sends and receives is required. That is, each peer has one pipe
for sending data and another for receiving.

All data is read and written in atomic units called *frames*. These
are conceptually similar to TCP packets. Higher-level functionality
is built on the exchange and processing of frames.

All frames are associated with a *stream*. A *stream* provides a
unidirectional grouping of frames. Streams facilitate two goals:
content encoding and parallelism. There is a dedicated section on
streams below.

The protocol is request-response based: the client issues requests to
the server, which issues replies to those requests. Server-initiated
messaging is not currently supported, but this specification carves
out room to implement it.

All frames are associated with a numbered request. Frames can thus
be logically grouped by their request ID.

Frames begin with an 8 octet header followed by a variable length
payload::

    +------------------------------------------------+
    |                 Length (24)                    |
    +--------------------------------+---------------+
    |         Request ID (16)        | Stream ID (8) |
    +------------------+-------------+---------------+
    | Stream Flags (8) |
    +-----------+------+
    | Type (4)  |
    +-----------+
    | Flags (4) |
    +===========+===================================================|
    |                     Frame Payload (0...)                    ...
    +---------------------------------------------------------------+

The length of the frame payload is expressed as an unsigned 24 bit
little endian integer. Values larger than 65535 MUST NOT be used unless
given permission by the server as part of the negotiated capabilities
during the handshake. The frame header is not part of the advertised
frame length. The payload length is the over-the-wire length. If there
is content encoding applied to the payload as part of the frame's stream,
the length is the output of that content encoding, not the input.

The 16-bit ``Request ID`` field denotes the integer request identifier,
stored as an unsigned little endian integer. Odd numbered requests are
client-initiated. Even numbered requests are server-initiated. This
refers to where the *request* was initiated - not where the *frame* was
initiated, so servers will send frames with odd ``Request ID`` in
response to client-initiated requests. Implementations are advised to
start ordering request identifiers at ``1`` and ``0``, increment by
``2``, and wrap around if all available numbers have been exhausted.

The 8-bit ``Stream ID`` field denotes the stream that the frame is
associated with. Frames belonging to a stream may have content
encoding applied and the receiver may need to decode the raw frame
payload to obtain the original data. Odd numbered IDs are
client-initiated. Even numbered IDs are server-initiated.

The 8-bit ``Stream Flags`` field defines stream processing semantics.
See the section on streams below.

The 4-bit ``Type`` field denotes the type of frame being sent.

The 4-bit ``Flags`` field defines special, per-type attributes for
the frame.

The sections below define the frame types and their behavior.

Command Request (``0x01``)
--------------------------

This frame contains a request to run a command.

The payload consists of a CBOR map defining the command request. The
bytestring keys of that map are:

name
   Name of the command that should be executed (bytestring).
args
   Map of bytestring keys to various value types containing the named
   arguments to this command.

   Each command defines its own set of argument names and their expected
   types.

This frame type MUST ONLY be sent from clients to servers: it is illegal
for a server to send this frame to a client.

The following flag values are defined for this type:

0x01
   New command request. When set, this frame represents the beginning
   of a new request to run a command. The ``Request ID`` attached to this
   frame MUST NOT be active.
0x02
   Command request continuation. When set, this frame is a continuation
   from a previous command request frame for its ``Request ID``. This
   flag is set when the CBOR data for a command request does not fit
   in a single frame.
0x04
   Additional frames expected. When set, the command request didn't fit
   into a single frame and additional CBOR data follows in a subsequent
   frame.
0x08
   Command data frames expected. When set, command data frames are
   expected to follow the final command request frame for this request.

``0x01`` MUST be set on the initial command request frame for a
``Request ID``.

``0x01`` or ``0x02`` MUST be set to indicate this frame's role in
a series of command request frames.

If command data frames are to be sent, ``0x10`` MUST be set on ALL
command request frames.

Command Data (``0x03``)
-----------------------

This frame contains raw data for a command.

Most commands can be executed by specifying arguments. However,
arguments have an upper bound to their length. For commands that
accept data that is beyond this length or whose length isn't known
when the command is initially sent, they will need to stream
arbitrary data to the server. This frame type facilitates the sending
of this data.

The payload of this frame type consists of a stream of raw data to be
consumed by the command handler on the server. The format of the data
is command specific.

The following flag values are defined for this type:

0x01
   Command data continuation. When set, the data for this command
   continues into a subsequent frame.

0x02
   End of data. When set, command data has been fully sent to the
   server. The command has been fully issued and no new data for this
   command will be sent. The next frame will belong to a new command.

Response Data (``0x04``)
------------------------

This frame contains raw response data to an issued command.

The following flag values are defined for this type:

0x01
   Data continuation. When set, an additional frame containing response data
   will follow.
0x02
   End of data. When set, the response data has been fully sent and
   no additional frames for this response will be sent.
0x04
   CBOR data. When set, the frame payload consists of CBOR data.

The ``0x01`` flag is mutually exclusive with the ``0x02`` flag.

Error Response (``0x05``)
-------------------------

An error occurred when processing a request. This could indicate
a protocol-level failure or an application level failure depending
on the flags for this message type.

The payload for this type is an error message that should be
displayed to the user.

The following flag values are defined for this type:

0x01
   The error occurred at the transport/protocol level. If set, the
   connection should be closed.
0x02
   The error occurred at the application level. e.g. invalid command.

Human Output Side-Channel (``0x06``)
------------------------------------

This frame contains a message that is intended to be displayed to
people. Whereas most frames communicate machine readable data, this
frame communicates textual data that is intended to be shown to
humans.

The frame consists of a series of *formatting requests*. Each formatting
request consists of a formatting string, arguments for that formatting
string, and labels to apply to that formatting string.

A formatting string is a printf()-like string that allows variable
substitution within the string. Labels allow the rendered text to be
*decorated*. Assuming use of the canonical Mercurial code base, a
formatting string can be the input to the ``i18n._`` function. This
allows messages emitted from the server to be localized. So even if
the server has different i18n settings, people could see messages in
their *native* settings. Similarly, the use of labels allows
decorations like coloring and underlining to be applied using the
client's configured rendering settings.

Formatting strings are similar to ``printf()`` strings or how
Python's ``%`` operator works. The only supported formatting sequences
are ``%s`` and ``%%``. ``%s`` will be replaced by whatever the string
at that position resolves to. ``%%`` will be replaced by ``%``. All
other 2-byte sequences beginning with ``%`` represent a literal
``%`` followed by that character. However, future versions of the
wire protocol reserve the right to allow clients to opt in to receiving
formatting strings with additional formatters, hence why ``%%`` is
required to represent the literal ``%``.

The frame payload consists of a CBOR array of CBOR maps. Each map
defines an *atom* of text data to print. Each *atom* has the following
bytestring keys:

msg
   (bytestring) The formatting string. Content MUST be ASCII.
args (optional)
   Array of bytestrings defining arguments to the formatting string.
labels (optional)
   Array of bytestrings defining labels to apply to this atom.

All data to be printed MUST be encoded into a single frame: this frame
does not support spanning data across multiple frames.

All textual data encoded in these frames is assumed to be line delimited.
The last atom in the frame SHOULD end with a newline (``\n``). If it
doesn't, clients MAY add a newline to facilitate immediate printing.

Progress Update (``0x07``)
--------------------------

This frame holds the progress of an operation on the peer. Consumption
of these frames allows clients to display progress bars, estimated
completion times, etc.

Each frame defines the progress of a single operation on the peer. The
payload consists of a CBOR map with the following bytestring keys:

topic
   Topic name (string)
pos
   Current numeric position within the topic (integer)
total
   Total/end numeric position of this topic (unsigned integer)
label (optional)
   Unit label (string)
item (optional)
   Item name (string)

Progress state is created when a frame is received referencing a
*topic* that isn't currently tracked. Progress tracking for that
*topic* is finished when a frame is received reporting the current
position of that topic as ``-1``.

Multiple *topics* may be active at any given time.

Rendering of progress information is not mandated or governed by this
specification: implementations MAY render progress information however
they see fit, including not at all.

The string data describing the topic SHOULD be static strings to
facilitate receivers localizing that string data. The emitter
MUST normalize all string data to valid UTF-8 and receivers SHOULD
validate that received data conforms to UTF-8. The topic name
SHOULD be ASCII.

Stream Encoding Settings (``0x08``)
-----------------------------------

This frame type holds information defining the content encoding
settings for a *stream*.

This frame type is likely consumed by the protocol layer and is not
passed on to applications.

This frame type MUST ONLY occur on frames having the *Beginning of Stream*
``Stream Flag`` set.

The payload of this frame defines what content encoding has (possibly)
been applied to the payloads of subsequent frames in this stream.

The payload begins with an 8-bit integer defining the length of the
encoding *profile*, followed by the string name of that profile, which
must be an ASCII string. All bytes that follow can be used by that
profile for supplemental settings definitions. See the section below
on defined encoding profiles.

Stream States and Flags
-----------------------

Streams can be in two states: *open* and *closed*. An *open* stream
is active and frames attached to that stream could arrive at any time.
A *closed* stream is not active. If a frame attached to a *closed*
stream arrives, that frame MUST have an appropriate stream flag
set indicating beginning of stream. All streams are in the *closed*
state by default.

The ``Stream Flags`` field denotes a set of bit flags for defining
the relationship of this frame within a stream. The following flags
are defined:

0x01
   Beginning of stream. The first frame in the stream MUST set this
   flag. When received, the ``Stream ID`` this frame is attached to
   becomes ``open``.

0x02
   End of stream. The last frame in a stream MUST set this flag. When
   received, the ``Stream ID`` this frame is attached to becomes
   ``closed``. Any content encoding context associated with this stream
   can be destroyed after processing the payload of this frame.

0x04
   Apply content encoding. When set, any content encoding settings
   defined by the stream should be applied when attempting to read
   the frame. When not set, the frame payload isn't encoded.

Streams
-------

Streams - along with ``Request IDs`` - facilitate grouping of frames.
But the purpose of each is quite different and the groupings they
constitute are independent.

A ``Request ID`` is essentially a tag. It tells you which logical
request a frame is associated with.

A *stream* is a sequence of frames grouped for the express purpose
of applying a stateful encoding or for denoting sub-groups of frames.

Unlike ``Request ID``s which span the request and response, a stream
is unidirectional and stream IDs are independent from client to
server.

There is no strict hierarchical relationship between ``Request IDs``
and *streams*. A stream can contain frames having multiple
``Request IDs``. Frames belonging to the same ``Request ID`` can
span multiple streams.

One goal of streams is to facilitate content encoding. A stream can
define an encoding to be applied to frame payloads. For example, the
payload transmitted over the wire may contain output from a
zstandard compression operation and the receiving end may decompress
that payload to obtain the original data.

The other goal of streams is to facilitate concurrent execution. For
example, a server could spawn 4 threads to service a request that can
be easily parallelized. Each of those 4 threads could write into its
own stream. Those streams could then in turn be delivered to 4 threads
on the receiving end, with each thread consuming its stream in near
isolation. The *main* thread on both ends merely does I/O and
encodes/decodes frame headers: the bulk of the work is done by worker
threads.

In addition, since content encoding is defined per stream, each
*worker thread* could perform potentially CPU bound work concurrently
with other threads. This approach of applying encoding at the
sub-protocol / stream level eliminates a potential resource constraint
on the protocol stream as a whole (it is common for the throughput of
a compression engine to be smaller than the throughput of a network).

Having multiple streams - each with their own encoding settings - also
facilitates the use of advanced data compression techniques. For
example, a transmitter could see that it is generating data faster
and slower than the receiving end is consuming it and adjust its
compression settings to trade CPU for compression ratio accordingly.

While streams can define a content encoding, not all frames within
that stream must use that content encoding. This can be useful when
data is being served from caches and being derived dynamically. A
cache could pre-compressed data so the server doesn't have to
recompress it. The ability to pick and choose which frames are
compressed allows servers to easily send data to the wire without
involving potentially expensive encoding overhead.

Content Encoding Profiles
-------------------------

Streams can have named content encoding *profiles* associated with
them. A profile defines a shared understanding of content encoding
settings and behavior.

The following profiles are defined:

TBD

Issuing Commands
----------------

A client can request that a remote run a command by sending it
frames defining that command. This logical stream is composed of
1 or more ``Command Request`` frames and and 0 or more ``Command Data``
frames.

All frames composing a single command request MUST be associated with
the same ``Request ID``.

Clients MAY send additional command requests without waiting on the
response to a previous command request. If they do so, they MUST ensure
that the ``Request ID`` field of outbound frames does not conflict
with that of an active ``Request ID`` whose response has not yet been
fully received.

Servers MAY respond to commands in a different order than they were
sent over the wire. Clients MUST be prepared to deal with this. Servers
also MAY start executing commands in a different order than they were
received, or MAY execute multiple commands concurrently.

If there is a dependency between commands or a race condition between
commands executing (e.g. a read-only command that depends on the results
of a command that mutates the repository), then clients MUST NOT send
frames issuing a command until a response to all dependent commands has
been received.
TODO think about whether we should express dependencies between commands
to avoid roundtrip latency.

A command is defined by a command name, 0 or more command arguments,
and optional command data.

Arguments are the recommended mechanism for transferring fixed sets of
parameters to a command. Data is appropriate for transferring variable
data. Thinking in terms of HTTP, arguments would be headers and data
would be the message body.

It is recommended for servers to delay the dispatch of a command
until all argument have been received. Servers MAY impose limits on the
maximum argument size.
TODO define failure mechanism.

Servers MAY dispatch to commands immediately once argument data
is available or delay until command data is received in full.

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.

compression
-----------

Declares support for negotiating compression formats.

Presence of this capability indicates the server supports dynamic selection
of compression formats based on the client request.

Servers advertising this capability are required to support the
``application/mercurial-0.2`` media type in response to commands returning
streams. Servers may support this media type on any command.

The value of the capability is a comma-delimited list of strings declaring
supported compression formats. The order of the compression formats is in
server-preferred order, most preferred first.

The identifiers used by the official Mercurial distribution are:

bzip2
   bzip2
none
   uncompressed / raw data
zlib
   zlib (no gzip header)
zstd
   zstd

This capability was introduced in Mercurial 4.1 (released February 2017).

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).

httpmediatype
-------------

Indicates which HTTP media types (``Content-Type`` header) the server is
capable of receiving and sending.

The value of the capability is a comma-delimited list of strings identifying
support for media type and transmission direction. The following strings may
be present:

0.1rx
   Indicates server support for receiving ``application/mercurial-0.1`` media
   types.

0.1tx
   Indicates server support for sending ``application/mercurial-0.1`` media
   types.

0.2rx
   Indicates server support for receiving ``application/mercurial-0.2`` media
   types.

0.2tx
   Indicates server support for sending ``application/mercurial-0.2`` media
   types.

minrx=X
   Minimum media type version the server is capable of receiving. Value is a
   string like ``0.2``.

   This capability can be used by servers to limit connections from legacy
   clients not using the latest supported media type. However, only clients
   with knowledge of this capability will know to consult this value. This
   capability is present so the client may issue a more user-friendly error
   when the server has locked out a legacy client.

mintx=X
   Minimum media type version the server is capable of sending. Value is a
   string like ``0.1``.

Servers advertising support for the ``application/mercurial-0.2`` media type
should also advertise the ``compression`` capability.

This capability was introduced in Mercurial 4.1 (released February 2017).

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.

protocaps
---------

Whether the server supports the ``protocaps`` command for SSH V1 transport.

This capability was introduced in Mercurial 4.6.

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 --stream``) 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.

Content Negotiation
===================

The wire protocol has some mechanisms to help peers determine what content
types and encoding the other side will accept. Historically, these mechanisms
have been built into commands themselves because most commands only send a
well-defined response type and only certain commands needed to support
functionality like compression.

Currently, only the HTTP version 1 transport supports content negotiation
at the protocol layer.

HTTP requests advertise supported response formats via the ``X-HgProto-<N>``
request header, where ``<N>`` is an integer starting at 1 allowing the logical
value to span multiple headers. This value consists of a list of
space-delimited parameters. Each parameter denotes a feature or capability.

The following parameters are defined:

0.1
   Indicates the client supports receiving ``application/mercurial-0.1``
   responses.

0.2
   Indicates the client supports receiving ``application/mercurial-0.2``
   responses.

comp
   Indicates compression formats the client can decode. Value is a list of
   comma delimited strings identifying compression formats ordered from
   most preferential to least preferential. e.g. ``comp=zstd,zlib,none``.

   This parameter does not have an effect if only the ``0.1`` parameter
   is defined, as support for ``application/mercurial-0.2`` or greater is
   required to use arbitrary compression formats.

   If this parameter is not advertised, the server interprets this as
   equivalent to ``zlib,none``.

Clients may choose to only send this header if the ``httpmediatype``
server capability is present, as currently all server-side features
consulting this header require the client to opt in to new protocol features
advertised via the ``httpmediatype`` capability.

A server that doesn't receive an ``X-HgProto-<N>`` header should infer a
value of ``0.1``. This is compatible with legacy clients.

A server receiving a request indicating support for multiple media type
versions may respond with any of the supported media types. Not all servers
may support all media types on all commands.

Commands
========

This section contains a list of all wire protocol commands implemented by
the canonical Mercurial server.

batch
-----

Issue multiple commands while sending a single command request. The purpose
of this command is to allow a client to issue multiple commands while avoiding
multiple round trips to the server therefore enabling commands to complete
quicker.

The command accepts a ``cmds`` argument that contains a list of commands to
execute.

The value of ``cmds`` is a ``;`` delimited list of strings. Each string has the
form ``<command> <arguments>``. That is, the command name followed by a space
followed by an argument string.

The argument string is a ``,`` delimited list of ``<key>=<value>`` values
corresponding to command arguments. Both the argument name and value are
escaped using a special substitution map::

   : -> :c
   , -> :o
   ; -> :s
   = -> :e

The response type for this command is ``string``. The value contains a
``;`` delimited list of responses for each requested command. Each value
in this list is escaped using the same substitution map used for arguments.

If an error occurs, the generic error response may be sent.

between
-------

(Legacy command used for discovery in old clients)

Obtain nodes between pairs of nodes.

The ``pairs`` arguments contains a space-delimited list of ``-`` delimited
hex node pairs. e.g.::

   a072279d3f7fd3a4aa7ffa1a5af8efc573e1c896-6dc58916e7c070f678682bfe404d2e2d68291a18

Return type is a ``string``. Value consists of lines corresponding to each
requested range. Each line contains a space-delimited list of hex nodes.
A newline ``\n`` terminates each line, including the last one.

branchmap
---------

Obtain heads in named branches.

Accepts no arguments. Return type is a ``string``.

Return value contains lines with URL encoded branch names followed by a space
followed by a space-delimited list of hex nodes of heads on that branch.
e.g.::

    default a072279d3f7fd3a4aa7ffa1a5af8efc573e1c896 6dc58916e7c070f678682bfe404d2e2d68291a18
    stable baae3bf31522f41dd5e6d7377d0edd8d1cf3fccc

There is no trailing newline.

branches
--------

(Legacy command used for discovery in old clients. Clients with ``getbundle``
use the ``known`` and ``heads`` commands instead.)

Obtain ancestor changesets of specific nodes back to a branch point.

Despite the name, this command has nothing to do with Mercurial named branches.
Instead, it is related to DAG branches.

The command accepts a ``nodes`` argument, which is a string of space-delimited
hex nodes.

For each node requested, the server will find the first ancestor node that is
a DAG root or is a merge.

Return type is a ``string``. Return value contains lines with result data for
each requested node. Each line contains space-delimited nodes followed by a
newline (``\n``). The 4 nodes reported on each line correspond to the requested
node, the ancestor node found, and its 2 parent nodes (which may be the null
node).

capabilities
------------

Obtain the capabilities string for the repo.

Unlike the ``hello`` command, the capabilities string is not prefixed.
There is no trailing newline.

This command does not accept any arguments. Return type is a ``string``.

This command was introduced in Mercurial 0.9.1 (released July 2006).

changegroup
-----------

(Legacy command: use ``getbundle`` instead)

Obtain a changegroup version 1 with data for changesets that are
descendants of client-specified changesets.

The ``roots`` arguments contains a list of space-delimited hex nodes.

The server responds with a changegroup version 1 containing all
changesets between the requested root/base nodes and the repo's head nodes
at the time of the request.

The return type is a ``stream``.

changegroupsubset
-----------------

(Legacy command: use ``getbundle`` instead)

Obtain a changegroup version 1 with data for changesetsets between
client specified base and head nodes.

The ``bases`` argument contains a list of space-delimited hex nodes.
The ``heads`` argument contains a list of space-delimited hex nodes.

The server responds with a changegroup version 1 containing all
changesets between the requested base and head nodes at the time of the
request.

The return type is a ``stream``.

clonebundles
------------

Obtains a manifest of bundle URLs available to seed clones.

Each returned line contains a URL followed by metadata. See the
documentation in the ``clonebundles`` extension for more.

The return type is a ``string``.

getbundle
---------

Obtain a bundle containing repository data.

This command accepts the following arguments:

heads
   List of space-delimited hex nodes of heads to retrieve.
common
   List of space-delimited hex nodes that the client has in common with the
   server.
obsmarkers
   Boolean indicating whether to include obsolescence markers as part
   of the response. Only works with bundle2.
bundlecaps
   Comma-delimited set of strings defining client bundle capabilities.
listkeys
   Comma-delimited list of strings of ``pushkey`` namespaces. For each
   namespace listed, a bundle2 part will be included with the content of
   that namespace.
cg
   Boolean indicating whether changegroup data is requested.
cbattempted
   Boolean indicating whether the client attempted to use the *clone bundles*
   feature before performing this request.
bookmarks
   Boolean indicating whether bookmark data is requested.
phases
   Boolean indicating whether phases data is requested.

The return type on success is a ``stream`` where the value is bundle.
On the HTTP version 1 transport, the response is zlib compressed.

If an error occurs, a generic error response can be sent.

Unless the client sends a false value for the ``cg`` argument, the returned
bundle contains a changegroup with the nodes between the specified ``common``
and ``heads`` nodes. Depending on the command arguments, the type and content
of the returned bundle can vary significantly.

The default behavior is for the server to send a raw changegroup version
``01`` response.

If the ``bundlecaps`` provided by the client contain a value beginning
with ``HG2``, a bundle2 will be returned. The bundle2 data may contain
additional repository data, such as ``pushkey`` namespace values.

heads
-----

Returns a list of space-delimited hex nodes of repository heads followed
by a newline. e.g.
``a9eeb3adc7ddb5006c088e9eda61791c777cbf7c 31f91a3da534dc849f0d6bfc00a395a97cf218a1\n``

This command does not accept any arguments. The return type is a ``string``.

hello
-----

Returns lines describing interesting things about the server in an RFC-822
like format.

Currently, the only line defines the server capabilities. It has the form::

    capabilities: <value>

See above for more about the capabilities string.

SSH clients typically issue this command as soon as a connection is
established.

This command does not accept any arguments. The return type is a ``string``.

This command was introduced in Mercurial 0.9.1 (released July 2006).

listkeys
--------

List values in a specified ``pushkey`` namespace.

The ``namespace`` argument defines the pushkey namespace to operate on.

The return type is a ``string``. The value is an encoded dictionary of keys.

Key-value pairs are delimited by newlines (``\n``). Within each line, keys and
values are separated by a tab (``\t``). Keys and values are both strings.

lookup
------

Try to resolve a value to a known repository revision.

The ``key`` argument is converted from bytes to an
``encoding.localstr`` instance then passed into
``localrepository.__getitem__`` in an attempt to resolve it.

The return type is a ``string``.

Upon successful resolution, returns ``1 <hex node>\n``. On failure,
returns ``0 <error string>\n``. e.g.::

   1 273ce12ad8f155317b2c078ec75a4eba507f1fba\n

   0 unknown revision 'foo'\n

known
-----

Determine whether multiple nodes are known.

The ``nodes`` argument is a list of space-delimited hex nodes to check
for existence.

The return type is ``string``.

Returns a string consisting of ``0``s and ``1``s indicating whether nodes
are known. If the Nth node specified in the ``nodes`` argument is known,
a ``1`` will be returned at byte offset N. If the node isn't known, ``0``
will be present at byte offset N.

There is no trailing newline.

protocaps
---------

Notify the server about the client capabilities in the SSH V1 transport
protocol.

The ``caps`` argument is a space-delimited list of capabilities.

The server will reply with the string ``OK``.

pushkey
-------

Set a value using the ``pushkey`` protocol.

Accepts arguments ``namespace``, ``key``, ``old``, and ``new``, which
correspond to the pushkey namespace to operate on, the key within that
namespace to change, the old value (which may be empty), and the new value.
All arguments are string types.

The return type is a ``string``. The value depends on the transport protocol.

The SSH version 1 transport sends a string encoded integer followed by a
newline (``\n``) which indicates operation result. The server may send
additional output on the ``stderr`` stream that should be displayed to the
user.

The HTTP version 1 transport sends a string encoded integer followed by a
newline followed by additional server output that should be displayed to
the user. This may include output from hooks, etc.

The integer result varies by namespace. ``0`` means an error has occurred
and there should be additional output to display to the user.

stream_out
----------

Obtain *streaming clone* data.

The return type is either a ``string`` or a ``stream``, depending on
whether the request was fulfilled properly.

A return value of ``1\n`` indicates the server is not configured to serve
this data. If this is seen by the client, they may not have verified the
``stream`` capability is set before making the request.

A return value of ``2\n`` indicates the server was unable to lock the
repository to generate data.

All other responses are a ``stream`` of bytes. The first line of this data
contains 2 space-delimited integers corresponding to the path count and
payload size, respectively::

    <path count> <payload size>\n

The ``<payload size>`` is the total size of path data: it does not include
the size of the per-path header lines.

Following that header are ``<path count>`` entries. Each entry consists of a
line with metadata followed by raw revlog data. The line consists of::

    <store path>\0<size>\n

The ``<store path>`` is the encoded store path of the data that follows.
``<size>`` is the amount of data for this store path/revlog that follows the
newline.

There is no trailer to indicate end of data. Instead, the client should stop
reading after ``<path count>`` entries are consumed.

unbundle
--------

Send a bundle containing data (usually changegroup data) to the server.

Accepts the argument ``heads``, which is a space-delimited list of hex nodes
corresponding to server repository heads observed by the client. This is used
to detect race conditions and abort push operations before a server performs
too much work or a client transfers too much data.

The request payload consists of a bundle to be applied to the repository,
similarly to as if :hg:`unbundle` were called.

In most scenarios, a special ``push response`` type is returned. This type
contains an integer describing the change in heads as a result of the
operation. A value of ``0`` indicates nothing changed. ``1`` means the number
of heads remained the same. Values ``2`` and larger indicate the number of
added heads minus 1. e.g. ``3`` means 2 heads were added. Negative values
indicate the number of fewer heads, also off by 1. e.g. ``-2`` means there
is 1 fewer head.

The encoding of the ``push response`` type varies by transport.

For the SSH version 1 transport, this type is composed of 2 ``string``
responses: an empty response (``0\n``) followed by the integer result value.
e.g. ``1\n2``. So the full response might be ``0\n1\n2``.

For the HTTP version 1 transport, the response is a ``string`` type composed
of an integer result value followed by a newline (``\n``) followed by string
content holding server output that should be displayed on the client (output
hooks, etc).

In some cases, the server may respond with a ``bundle2`` bundle. In this
case, the response type is ``stream``. For the HTTP version 1 transport, the
response is zlib compressed.

The server may also respond with a generic error type, which contains a string
indicating the failure.