view mercurial/help/internals/wireprotocolv2.txt @ 39641:aa7e312375cf

wireprotov2: let clients drive delta behavior

Previously, the "manifestdata" and "filedata" commands assumed the
receiver had all parent revisions for requested nodes. Unless the
revision had no parents, they emitted a delta instead of a fulltext.

This strategy isn't appropriate for shallow clones and for clients
that only want to access fulltext revision data for a single node
without fetching their parent revisions.

This commit adds an "haveparents" argument to the "manifestdata"
and "filedata" commands that controls delta generation behavior.

Unless "haveparents" is set, the server assumes that the client
doesn't have parent revisions unless they were previously sent
as part of the current group of revisions.

This change allows the fulltext revision data of any individual
revision to be obtained. This will facilitate shallow clones
and other data retrieval strategies that don't require all previous
revisions of an entity to be fetched.

Differential Revision: https://phab.mercurial-scm.org/D4492

**Experimental and under active development**

This section documents the wire protocol commands exposed to transports
using the frame-based protocol. The set of commands exposed through
these transports is distinct from the set of commands exposed to legacy
transports.

The frame-based protocol uses CBOR to encode command execution requests.
All command arguments must be mapped to a specific or set of CBOR data
types.

The response to many commands is also CBOR. There is no common response
format: each command defines its own response format.

TODOs
=====

* Add "node namespace" support to each command. In order to support
  SHA-1 hash transition, we want servers to be able to expose different
  "node namespaces" for the same data. Every command operating on nodes
  should specify which "node namespace" it is operating on and responses
  should encode the "node namespace" accordingly.

Commands
========

The sections below detail all commands available to wire protocol version
2.

branchmap
---------

Obtain heads in named branches.

Receives no arguments.

The response is a map with bytestring keys defining the branch name.
Values are arrays of bytestring defining raw changeset nodes.

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

Obtain the server's capabilities.

Receives no arguments.

This command is typically called only as part of the handshake during
initial connection establishment.

The response is a map with bytestring keys defining server information.

The defined keys are:

commands
   A map defining available wire protocol commands on this server.

   Keys in the map are the names of commands that can be invoked. Values
   are maps defining information about that command. The bytestring keys
   are:

      args
         A map of argument names and their expected types.

         Types are defined as a representative value for the expected type.
         e.g. an argument expecting a boolean type will have its value
         set to true. An integer type will have its value set to 42. The
         actual values are arbitrary and may not have meaning.
      permissions
         An array of permissions required to execute this command.

compression
   An array of maps defining available compression format support.

   The array is sorted from most preferred to least preferred.

   Each entry has the following bytestring keys:

      name
         Name of the compression engine. e.g. ``zstd`` or ``zlib``.

framingmediatypes
   An array of bytestrings defining the supported framing protocol
   media types. Servers will not accept media types not in this list.

rawrepoformats
   An array of storage formats the repository is using. This set of
   requirements can be used to determine whether a client can read a
   *raw* copy of file data available.

changesetdata
-------------

Obtain various data related to changesets.

The command accepts the following arguments:

noderange
   (array of arrays of bytestrings) An array of 2 elements, each being an
   array of node bytestrings. The first array denotes the changelog revisions
   that are already known to the client. The second array denotes the changelog
   revision DAG heads to fetch. The argument essentially defines a DAG range
   bounded by root and head nodes to fetch.

   The roots array may be empty. The heads array must be defined.

nodes
   (array of bytestrings) Changelog revisions to request explicitly.

fields
   (set of bytestring) Which data associated with changelog revisions to
   fetch. The following values are recognized:

   bookmarks
      Bookmarks associated with a revision.

   parents
      Parent revisions.

   phase
      The phase state of a revision.

   revision
      The raw, revision data for the changelog entry. The hash of this data
      will match the revision's node value.

The server resolves the set of revisions relevant to the request by taking
the union of the ``noderange`` and ``nodes`` arguments. At least one of these
arguments must be defined.

The response bytestream starts with a CBOR map describing the data that follows.
This map has the following bytestring keys:

totalitems
   (unsigned integer) Total number of changelog revisions whose data is being
   transferred. This maps to the set of revisions in the requested node
   range, not the total number of records that follow (see below for why).

Following the map header is a series of 0 or more CBOR values. If values
are present, the first value will always be a map describing a single changeset
revision. If revision data is requested, the raw revision data (encoded as
a CBOR bytestring) will follow the map describing it. Otherwise, another CBOR
map describing the next changeset revision will occur.

Each map has the following bytestring keys:

node
   (bytestring) The node value for this revision. This is the SHA-1 hash of
   the raw revision data.

bookmarks (optional)
   (array of bytestrings) Bookmarks attached to this revision. Only present
   if ``bookmarks`` data is being requested and the revision has bookmarks
   attached.

parents (optional)
   (array of bytestrings) The nodes representing the parent revisions of this
   revision. Only present if ``parents`` data is being requested.

phase (optional)
   (bytestring) The phase that a revision is in. Recognized values are
   ``secret``, ``draft``, and ``public``. Only present if ``phase`` data
   is being requested.

revisionsize (optional)
   (unsigned integer) Indicates the size of raw revision data that follows this
   map. The following data contains a serialized form of the changeset data,
   including the author, date, commit message, set of changed files, manifest
   node, and other metadata.

   Only present if ``revision`` data was requested and the data follows this
   map.

If nodes are requested via ``noderange``, they will be emitted in DAG order,
parents always before children.

If nodes are requested via ``nodes``, they will be emitted in requested order.

Nodes from ``nodes`` are emitted before nodes from ``noderange``.

The set of changeset revisions emitted may not match the exact set of
changesets requested. Furthermore, the set of keys present on each
map may vary. This is to facilitate emitting changeset updates as well
as new revisions.

For example, if the request wants ``phase`` and ``revision`` data,
the response may contain entries for each changeset in the common nodes
set with the ``phase`` key and without the ``revision`` key in order
to reflect a phase-only update.

TODO support different revision selection mechanisms (e.g. non-public, specific
revisions)
TODO support different hash "namespaces" for revisions (e.g. sha-1 versus other)
TODO support emitting obsolescence data
TODO support filtering based on relevant paths (narrow clone)
TODO support depth limiting
TODO support hgtagsfnodes cache / tags data
TODO support branch heads cache

filedata
--------

Obtain various data related to an individual tracked file.

The command accepts the following arguments:

fields
   (set of bytestring) Which data associated with a file to fetch.
   The following values are recognized:

   parents
      Parent nodes for the revision.

   revision
      The raw revision data for a file.

haveparents
   (bool) Whether the client has the parent revisions of all requested
   nodes. If set, the server may emit revision data as deltas against
   any parent revision. If not set, the server MUST only emit deltas for
   revisions previously emitted by this command.

   False is assumed in the absence of any value.

nodes
   (array of bytestrings) File nodes whose data to retrieve.

path
   (bytestring) Path of the tracked file whose data to retrieve.

TODO allow specifying revisions via alternate means (such as from
changeset revisions or ranges)

The response bytestream starts with a CBOR map describing the data that
follows. It has the following bytestream keys:

totalitems
   (unsigned integer) Total number of file revisions whose data is
   being returned.

Following the header map is a series of 0 or more CBOR values. The first
value is always a map describing a file revision. If this map has the
``deltasize`` or ``revisionsize`` keys, a bytestring containing the delta
or revision, respectively, will immediately follow the map. Otherwise
the next value will be a map describing the next file revision.

Each map has the following bytestring keys:

node
   (bytestring) The node of the file revision whose data is represented.

deltabasenode
   (bytestring) Node of the file revision the following delta is against.

   Only present if the ``revision`` field is requested and delta data
   follows this map.

deltasize
   (unsigned integer) The size of the delta data that follows this map.

   Only present if the ``revision`` field is requested and delta data
   follows this map.

parents
   (array of bytestring) The nodes of the parents of this file revision.

   Only present if the ``parents`` field is requested.

revisionsize
   (unsigned integer) The size of the fulltext revision data that follows
   this map.

   Only present if the ``revision`` field is requested and fulltext revision
   data follows this map.

When ``revision`` data is requested, the server chooses to emit either fulltext
revision data or a delta. What the server decides can be inferred by looking
for the presence of the ``deltasize`` or ``revisionsize`` keys in the map.
Servers MUST NOT define both keys.

heads
-----

Obtain DAG heads in the repository.

The command accepts the following arguments:

publiconly (optional)
   (boolean) If set, operate on the DAG for public phase changesets only.
   Non-public (i.e. draft) phase DAG heads will not be returned.

The response is a CBOR array of bytestrings defining changeset nodes
of DAG heads. The array can be empty if the repository is empty or no
changesets satisfied the request.

TODO consider exposing phase of heads in response

known
-----

Determine whether a series of changeset nodes is known to the server.

The command accepts the following arguments:

nodes
   (array of bytestrings) List of changeset nodes whose presence to
   query.

The response is a bytestring where each byte contains a 0 or 1 for the
corresponding requested node at the same index.

TODO use a bit array for even more compact response

listkeys
--------

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

The command receives the following arguments:

namespace
   (bytestring) Pushkey namespace to query.

The response is a map with bytestring keys and values.

TODO consider using binary to represent nodes in certain pushkey namespaces.

lookup
------

Try to resolve a value to a changeset revision.

Unlike ``known`` which operates on changeset nodes, lookup operates on
node fragments and other names that a user may use.

The command receives the following arguments:

key
   (bytestring) Value to try to resolve.

On success, returns a bytestring containing the resolved node.

manifestdata
------------

Obtain various data related to manifests (which are lists of files in
a revision).

The command accepts the following arguments:

fields
   (set of bytestring) Which data associated with manifests to fetch.
   The following values are recognized:

   parents
      Parent nodes for the manifest.

   revision
      The raw revision data for the manifest.

haveparents
   (bool) Whether the client has the parent revisions of all requested
   nodes. If set, the server may emit revision data as deltas against
   any parent revision. If not set, the server MUST only emit deltas for
   revisions previously emitted by this command.

   False is assumed in the absence of any value.

nodes
   (array of bytestring) Manifest nodes whose data to retrieve.

tree
   (bytestring) Path to manifest to retrieve. The empty bytestring represents
   the root manifest. All other values represent directories/trees within
   the repository.

TODO allow specifying revisions via alternate means (such as from changeset
revisions or ranges)
TODO consider recursive expansion of manifests (with path filtering for
narrow use cases)

The response bytestream starts with a CBOR map describing the data that
follows. It has the following bytestring keys:

totalitems
   (unsigned integer) Total number of manifest revisions whose data is
   being returned.

Following the header map is a series of 0 or more CBOR values. The first
value is always a map describing a manifest revision. If this map has the
``deltasize`` or ``revisionsize`` keys, a bytestring containing the delta
or revision, respectively, will immediately follow the map. Otherwise
the next value will be a map describing the next manifest revision.

Each map has the following bytestring keys:

node
   (bytestring) The node of the manifest revision whose data is represented.

deltabasenode
   (bytestring) The node that the delta representation of this revision is
   computed against. Only present if the ``revision`` field is requested and
   a delta is being emitted.

deltasize
   (unsigned integer) The size of the delta data that follows this map.
   Only present if the ``revision`` field is requested and a delta is
   being emitted.

parents
   (array of bytestring) The nodes of the parents of this manifest revision.
   Only present if the ``parents`` field is requested.

revisionsize
   (unsigned integer) The size of the fulltext revision data that follows
   this map. Only present if the ``revision`` field is requested and a fulltext
   revision is being emitted.

When ``revision`` data is requested, the server chooses to emit either fulltext
revision data or a delta. What the server decides can be inferred by looking
for the presence of the ``deltasize`` or ``revisionsize`` keys in the map.
Servers MUST NOT define both keys.

pushkey
-------

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

The command receives the following arguments:

namespace
   (bytestring) Pushkey namespace to operate on.
key
   (bytestring) The pushkey key to set.
old
   (bytestring) Old value for this key.
new
   (bytestring) New value for this key.

TODO consider using binary to represent nodes is certain pushkey namespaces.
TODO better define response type and meaning.